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Analysis of Endogenous Hormone Levels to Reveal the Retardation by Suppression of Phospholipase Dα1 in Arabidopsis Leaves during Hormonepromoted Senescence

分析拟南芥叶片在激素促进的衰老中内源激素变化来解析抑制磷脂酶Dα1延缓激素促进衰老的机制



全 文 :分析拟南芥叶片在激素促进的衰老中内源激素变化来解析
抑制磷脂酶 Dα1延缓激素促进衰老的机制∗
贾艳霞ꎬ 陶发清ꎬ 李唯奇∗∗
(中国科学院昆明植物研究所 中国西南野生生物种质资源库ꎬ 云南 昆明  650201)
摘要: 采后衰老进程在很大程度上受到内源和外源激素的影响ꎮ 抑制拟南芥中磷脂酶 Dα1 (phospholipase
Dα1ꎬ PLDα1) 的表达后ꎬ 使得外源脱落酸 (abscisic acidꎬ ABA) 和乙烯加速的离体叶片衰老过程在一定
程度上得到了缓解ꎮ 然而ꎬ 内源激素在这个过程中的作用尚不清楚ꎮ 本研究对比分析了野生型和 PLDα1
缺失型两种基因型拟南芥叶片在 3种不同人工老化过程中 (离体诱导的、 外源 ABA 和乙烯促进的衰老过
程)ꎬ 内源 ABAꎬ 茉莉酸甲酯 (methyl jasmonateꎬ MeJA)、 吲哚乙酸 ( indole ̄3 ̄acetic acidꎬ IAA)、 玉米素
核苷 (zeatin ribosideꎬ ZR) 和赤霉素 (gibberellic acidꎬ GA3) 的含量变化ꎮ 这 5种激素对 3种不同衰老处
理方式的响应模式表明了人工老化过程存在着两个不同阶段ꎬ 并且在衰老早期每种激素的变化模式相同ꎮ
PLDα1功能缺失使得激素加速的衰老过程得以延缓ꎬ 这与内源 ABA、 MeJA、 ZR和 IAA 的含量变化有关ꎬ
而与 GA3的含量变化无关ꎮ 同时ꎬ ZR和 IAA的变化模式也说明了这两种激素的变化可能是缺失 PLDα1延
缓激素加速的衰老过程这一事件的原因而非结果ꎮ
关键词: 叶片衰老ꎻ 采后生理ꎻ 激素ꎻ 磷脂酶 Dα1ꎻ 拟南芥
中图分类号: Q 945            文献标识码: A                文章编号: 2095-0845(2014)04-485-12
Analysis of Endogenous Hormone Levels to Reveal the Retardation
by Suppression of Phospholipase Dα1 in Arabidopsis Leaves
during Hormone ̄promoted Senescence
JIA Yan ̄Xiaꎬ TAO Fa ̄Qingꎬ LI Wei ̄Qi∗∗
(Germplasm Bank of Wild Species in Southwest Chinaꎬ Kunming Institute of Botanyꎬ
Chinese Academy of Sciencesꎬ Kunming 650201ꎬ China)
Abstract: Postharvest senescence is affected greatly by endogenous and exogenous hormones. The promotive effects
of exogenous abscisic acid (ABA) and ethylene on senescence are retarded by suppression of phospholipase Dα1
(PLDα1) in detached Arabidopsis leaves. Howeverꎬ understanding about the roles of endogenous hormones in the re ̄
tardation of postharvest senescence by the suppression of PLDα1 activity remains incomplete. Here we report changes
in the levels of endogenous ABAꎬ methyl jasmonate (MeJA)ꎬ indole ̄3 ̄acetic acid ( IAA)ꎬ zeatin riboside (ZR)ꎬ
and gibberellic acid (GA3) during leaf senescence induced by detachment and also accelerated by the application of
exogenous ABA or ethylene in wild ̄type and PLDα1 ̄deficient Arabidopsis leaves. The responses of the five hormones
to the three treatments showed the existence of two stages during the artificial senescenceꎬ andꎬ for each hormoneꎬ
the pattern during the early stage was identical. The retardation of senescence in PLDα1 ̄deficient plants was associ ̄
植 物 分 类 与 资 源 学 报  2014ꎬ 36 (4): 485~496
Plant Diversity and Resources                                    DOI: 10.7677 / ynzwyj201413220

∗∗
Funding: The National Natural Science Foundation of China (NSFC 30670474ꎬ 30870571ꎬ and 31070262)ꎬ Fund of State Key Laboratory
of Phytochemistry and Plant Resources in West China (O97C1211Z1)ꎬ 100 ̄Talents Program of CAS (54O50617101)
Author for correspondenceꎻ E ̄mail: weiqili@mail􀆰 kib􀆰 ac􀆰 cn
Received date: 2013-11-05ꎬ Accepted date: 2014-02-18
作者简介: 贾艳霞 (1982-) 女ꎬ 工程师ꎬ 主要从事植物逆境分子生理学研究ꎮ E ̄mail: jiayanxia@mail􀆰 kib􀆰 ac􀆰 cn
ated with changes in ABAꎬ MeJAꎬ ZRꎬ and IAA levelsꎬ but was independent of changes in endogenous levels of
GA3 . The profiles of ZR and IAA suggest that changes in ZR and IAA levels might causeꎬ and not simply be the con ̄
sequence ofꎬ the retardation of senescence that is caused by suppression of PLDα1.
Key words: Leaf senescenceꎻ Postharvest physiologyꎻ Hormoneꎻ Phospholipase Dα1ꎻ Arabidopsis thaliana
Abbreviations: ABAꎬ abscisic acidꎻ CKꎬ cytokininꎻ GAꎬ gibberellic acidꎻ MeJAꎬ methyl jasmonateꎻ JAꎬ jasmon ̄
ic acidꎻ IAAꎬ indole ̄3 ̄acetic acidꎻ PAꎬ phosphatidic acidꎻ PLDꎬ phospholipase Dꎻ PS IIꎬ photosystem IIꎻ SAGsꎬ
senescence ̄associated genesꎻ WTꎬ wild ̄type
  Leaf senescence is the final stage of leaf devel ̄
opment when mobilisable nutrientsꎬ such as amino
acid and lipidsꎬ are usually recycled ( Lim et al.ꎬ
2007ꎻ Guiboileau et al.ꎬ 2010). Leaf senescence is
a highly regulated processꎬ and comprises a series of
ordered events that occur at the physiologicalꎬ bio ̄
chemicalꎬ and molecular levels. Typical syndromes
of senescence include yellowing of leaf marginsꎬ ter ̄
mination of photosynthesisꎬ changes in the levels of
endogenous hormonesꎬ degradation of membrane lip ̄
idsꎬ and the expression of senescence ̄associated
genes ( SAGs) ( Smartꎬ 1994ꎻ Thompson et al.ꎬ
1998ꎻ Ralph et al.ꎬ 2005). Leaf senescence can be
induced by both developmental signals and environ ̄
mental stimuli (Guo and Ganꎬ 2005). The sudden
disruption of their energyꎬ nutrientꎬ and hormone
supplies causes harvested immature vegetables to en ̄
ter into a senescent state (King et al.ꎬ 1990). Given
that the degradative processes that occur in detached
leaves show many similarities to the events that occur
during natural leaf senescenceꎬ detached leaves are
widely used as a model system to study leaf senes ̄
cence (Page et al.ꎬ 2001).
The exogenous application of hormones can in ̄
fluence the progress of leaf senescence that is induced
by detachment (Weaver et al.ꎬ 1998). For instanceꎬ
the application of abscisic acid (ABA) or ethylene
strongly promotes senescence in detached leavesꎬ and
in Arabidopsisꎬ this process can be retarded by the
suppression of phospholipase Dα1 ( PLDα1) and
PLDδ (Fan et al.ꎬ 1997ꎻ Jia et al.ꎬ 2013). PLD hy ̄
drolyzes phospholipids to yield phosphatidic acid
(PA) and a soluble head groupꎬ and thus plays criti ̄
cal roles in the metabolism of membrane phospholip ̄
ids and lipid signaling (Wangꎬ 2005). As one of the
most abundant members of the PLD family in Arabi ̄
dopsisꎬ PLDα1 regulates plant responses to abiotic
stressesꎬ such as droughtꎬ freezingꎬ high salinityꎬ
and woundingꎬ through influencing the signaling and /
or structural roles of its product PA (Bargmann and
Munnikꎬ 2006). It has been proposed that the mecha ̄
nism by which PLDα1 promotes senescence centres
around the ability of PLDα1 ̄mediated PA to destabi ̄
lize membranes and activate other lipid ̄degrading
enzymesꎬ thus causing a loss of both membrane in ̄
tegrity and functionality of membrane ̄associated pro ̄
teins ( Fan et al.ꎬ 1997). Recent reports indicate
that PLDα1 ̄mediated PA has important functions in
ABA signaling through its interaction with ABI1 and
G protein (Hong et al.ꎬ 2010)ꎬ which provides a
potential link between PLDα1 and ABA ̄promoted
senescence. Nonethelessꎬ the precise details of the
mechanism (s) involved remain unclear.
The progress of leaf senescence is regulated by
the levels of endogenous hormones (Manju et al.ꎬ
2001ꎻ Ganꎬ 2010 ). In generalꎬ ethyleneꎬ ABA
(Manzano et al.ꎬ 2006 )ꎬ and methyl jasmonate
(MeJA) or its precursor jasmonic acid ( JA) pro ̄
mote senescenceꎬ whereas cytokinins (CKs)ꎬ aux ̄
inꎬ and gibberellic acid ( GA3 ) retard senescence
(Ganꎬ 2010). In addition to molecular genetic ap ̄
proachesꎬ studies of the correlations between the
levels of endogenous and / or exogenously applied
hormones and the progress of senescence have been
used frequently to investigate the roles of hormones
in senescence (Ganꎬ 2010). Changes in endogenous
hormone levels had been examined in some plant
species in detachment ̄induced senescence. For ex ̄
684                                  植 物 分 类 与 资 源 学 报                            第 36卷
ampleꎬ changes in the levels of endogenous ABA
were measured during the senescence of detached
lettuce leaves (Aharoni and Richmondꎬ 1978)ꎬ to ̄
bacco leaves (Even ̄Chen and Itaiꎬ 1975)ꎬ and nas ̄
turtium leaves (Chin and Beeversꎬ 1970)ꎬ whereas
changes in the levels of endogenous auxin were
measured in tobacco leaves ( Evenchen et al.ꎬ
1978) and excised bean leaves ( Sheldrak and
Northcotꎬ 1968). Howeverꎬ the correlations between
hormone levels and senescence appear to differ a ̄
mong plant species (Sheldrak and Northcotꎬ 1968).
These differences underscore the need for improved
understanding of the response of endogenous hor ̄
mone levels to detachment ̄induced leaf senescence
in Arabidopsis. The correlation between hormone lev ̄
els and senescence remains undocumented in rela ̄
tion to leaf senescence that is promoted by ABA or
ethylene. Furthermoreꎬ it remains to be established
whether and how retardation of ABA ̄ and ethylene ̄
promoted senescence involves changes in endogenous
hormone levels in the leaves of Arabidopsis plants
with attenuated PLDα1 activity. Investigation of
these correlations is of biological significanceꎬ given
that the effects of PLDα1 on hormone synthesis are
unclear so far.
To address the above ̄mentioned questionsꎬ we
used enzyme ̄linked immunosorbent assays ( ELI ̄
SAs) to examine changes in the endogenous concen ̄
trations of MeJAꎬ ABAꎬ GA3ꎬ and a representative
CK ( Zeatin ribosideꎬ ZR)ꎬ and auxin ( indole ̄3 ̄
acetic acidꎬ IAA) during detachment ̄induced leaf
senescence and ABA ̄ and ethylene ̄promoted leaf
senescence. We analyzed both wild ̄type (WT) Ara ̄
bidopsis plants and transgenic plants in which ex ̄
pression of PLDα1 was attenuated using antisense ̄
mediated suppression. Here we report changes in the
patterns of endogenous accumulation of MeJAꎬ
ABAꎬ GA3ꎬ ZRꎬ and IAA and the association of
these hormone compounds with PLDα1 ̄mediated re ̄
tardation of ABA ̄ and ethylene ̄promoted senes ̄
cence. The patterns reveal the presence of two physi ̄
ological stages of senescence in detached leaves.
1  Materials and methods
1􀆰 1  Plant materialsꎬ growth conditionꎬ wounding
treatmentsꎬ and hormone treatments
Arabidopsis thalianaꎬ ecotype Columbia (wild ̄
typeꎬ WT) and PLDα1 ̄antisense ( PLDα1 ̄AS )ꎬ
which was generated from the Columbia ecotype and
in which PLDα1 is constitutively attenuated by anti ̄
sense ̄mediated suppression (Fan et al.ꎬ 1997)ꎬ were
used. The plants were grown in a controlled growth
chamber at 23 ℃ (day) and 19 ℃ (night) and 60%
relative humidity under a 12 h photoperiod with fluo ̄
rescent lighting of intensity 120 μmol􀅰m-2􀅰s-1 . For
wounding treatmentsꎬ we followed the procedures de ̄
scribed previously ( Gepstein and Thimannꎬ 1981)
with minor modification. Leaves were wounded with a
hemostat three times immediately after detachment
and then incubated in water. The leaves were sampled
at the indicated times and then stored in liquid nitro ̄
gen before MeJA extraction. The procedures that were
used for phytohormone ̄induced senescence were de ̄
scribed previously (Oh et al.ꎬ 1996). In briefꎬ fully
expanded leaves of the same age were collected from
~6 ̄week ̄old plantsꎬ and the leaves were floated on
water that contained either 50 μmol􀅰L-1 ABA (Sig ̄
maꎬ A1049) or 50 μmol􀅰L-1 ethephon ( Sigmaꎬ
C0143)ꎬ or on water without any hormone (Fan et
al.ꎬ 1997) under the normal growth conditions. Ethe ̄
phon is a water ̄soluble compound that releases ethyl ̄
ene in the cellꎬ which enables researchers to evaluate
the effects of ethylene more conveniently than treat ̄
ments that involve the administration of ethylene gas.
1􀆰 2  Measurements of photosynthetic activity and
cell death
The emission of chlorophyll fluorescence from
the upper surface of the leaves was measured using
an imaging chlorophyll fluorometerꎬ MAXI ̄Imaging
Pulse ̄Amplitude (PAM) (Walzꎬ Germany)ꎬ as de ̄
scribed previously (Yang et al.ꎬ 2012). The maxi ̄
mal photochemical quantum yield of photosystem II
(PS II) was determined in dark ̄adapted (20 mi ̄
nutes) samples on the basis of the initial level of flu ̄
orescence (F0 ) and the maximal level of fluores ̄
7844期      JIA Yan ̄Xia et al.: Analysis of Endogenous Hormone Levels to Reveal the Retardation by Suppression 􀆺     
cence (Fm)ꎬ and calculated as Fv / Fm = (Fm ̄F0) /
Fm and variable fluorescence (Fv).
Cell death was quantified spectrophotometrically
by Evan’s blue staining of detached leaves as de ̄
scribed previously with minor modifications (Rea et
al.ꎬ 2004). In briefꎬ detached leaves were incuba ̄
ted with 0􀆰 1% Evan’s blue for 2 h with shakingꎬ
and then washed extensively to remove unbound dye.
The leaves were ground into powder in liquid nitro ̄
gen. The tissue powder was incubated with 50%
methanol and 1% SDS at 60 ℃ for 30 min and then
centrifuged. As a control for complete cell deathꎬ the
leaves were heated at 100 ℃ for 5 min. The absor ̄
bance of the supernatant solution was measured at
600 nm.
1􀆰 3  Measurement of endogenous hormone levels
Plant tissue was extracted and samples were
prepared using a slight modification of a previously
described method (Lei et al.ꎬ 2007). Immediately
after samplingꎬ detached leaves ( 0􀆰 5 ~ 1 g) were
homogenized in 2 mL of 80% methanol that con ̄
tained 1 μmol􀅰L-1 butylated hydroxytoluene and the
contents were transferred to a separate containerꎻ the
procedure was repeated twice. Rinsing the mortar in
this way ensured that all plant material was included
in the extract used for analysis. After centrifugationꎬ
the extracts were dried under nitrogen gas. The lev ̄
els of endogenous hormones in the leaves were deter ̄
mined using ELISA kits that were obtained from Pro ̄
fessor Baomin Wang ( Zhao et al.ꎬ 2006ꎻ Deng et
al.ꎬ 2008ꎻ Zhao et al.ꎬ 2011). Five replicates from
each sampling time were analyzed. The data were
subjected to one ̄way analysis of variance (ANOVA)
with SPSS 16􀆰 0. Statistical significance was tested
by Fisher ’ s least significant difference ( LSD )
method.
2  Results
2􀆰 1  MeJA levels increased in an identical man ̄
ner during detachment ̄induced senescence in
WT and PLDα1 ̄AS leaves
As indicated in Figure 1ꎬ we compared changes
in Fv / Fm values (which indicate the photochemical
quantum efficiency of PS II) and cell death between
WT and PLDα1 ̄AS plants during three treatments
that induced senescence artificially. In the first
treatmentꎬ detachment ̄induced senescenceꎬ detached
Arabidopsis leaves were floated on water. In the second
and the thirdꎬ detachment ̄induced senescence was ac ̄
celerated by the addition of ABA and ethephonꎬ re ̄
spectively. A visible sign of leaf yellowing and a de ̄
crease in Fv / Fm values was characterized during the
three senescence treatments. In the absence of ABA or
ethephonꎬ detached leaves stayed green after 5 daysꎬ
leaves detached from WT plants started yellowing 1 day
after treatment and most parts turned yellow 5 days af ̄
ter incubation in 50 μmol􀅰L-1 ABA and ethephon. In
contrastꎬ most parts of the PLDα1 ̄suppressed leaves
were still green after the 5 ̄day hormone treatment
(Fig􀆰 1A). Consistent with the decreased loss of chlo ̄
rophyllꎬ the PLDα1 ̄suppressed leaves retained a
higher photosynthetic activity than that of WT leaves
(Fig􀆰 1A). After a 5 ̄day treatment with ABA and
ethephonꎬ the cell death rate in WT leaves were 25ꎬ
62 and 60% of their respective initial valuesꎬ which
obviously higher than their corresponding control
PLDα1 antisense leaves (Fig􀆰 1B).
We detached leaves from intact plants by cut ̄
ting the leafstalk. These leaves not only experience
senescence but are also subject to mechanical woun ̄
ding. To test the potential effects of woundingꎬ we
first examined levels of MeJA in the detached leaves
that were floated on water. Wounding usually causes
transient increases in levels of MeJA within 3 hoursꎬ
and this increase is suppressed in PLDα1 ̄AS Arabi ̄
dopsis plants (Wang et al.ꎬ 2000). In our control
experiments in which leaves were wounded by hemo ̄
stat twice (Fig􀆰 2A)ꎬ the MeJA levels in WT leaves
increased to 234%ꎬ 174% and 187% of its initial
levels at day 1ꎬ 2 and 3ꎬ respectively. MeJA levels
were significantly higher in WT leaves than in
PLDα1 ̄AS leaves after wounding for 1 and 2 days.
(Fig􀆰 2A). In contrastꎬ in our detachment experi ̄
ments in which leave were cut at leafstalk (Fig􀆰 2Bꎬ
884                                  植 物 分 类 与 资 源 学 报                            第 36卷
top panel)ꎬ the levels of MeJA increased 188% at
day 1 and then went back to the initial levels. The
extensity and duration of the effects in detachments
was much less than that in wounding. Most impor ̄
tantlyꎬ no difference of MeJA levels between WT
and PLDα1 ̄AS plants was observed after leaves
were cut. This indicates that the wounding effects
from cutting leafsalk was small and could be ignored.
Fig􀆰 1  Senescence of detached leaves from WT and
PLDα1 ̄AS Arabidopsis plants
Notes: Leaves that had been detached from WT and PLDα1 ̄AS plants
were treated with sterile waterꎬ 50 μmol􀅰L-1 ABAꎬ or 50 μmol􀅰L-1
ethephon (ETH) for 5 days. A. Retardation of ABA ̄ and ethylene ̄
promoted senescence in detached leaves of PLDα1 ̄AS as compared
with WT plants. Yellowing of the leaves ( top panel) or low Fv / Fm
values for variable fluorescence (bottom panel) indicated the presence
of senescence. B. Cell death was determined spectrophotometrically
using Evan’s blue staining. Values are the mean ± SD (n= 5)
Fig􀆰 2  Change in levels of endogenous MeJA in leaves from WT
and PLDα1 ̄AS plants (a) after mechanical wounding and
(b) during three different senescence treatments
Notes: A. Leaves were treated with sterile water after wounding. B.
Detached leaves were treated with sterile water ( top)ꎬ 50 μmol􀅰L-1
ABA (middle)ꎬ or 50 μmol􀅰L-1 ethephon (ETHꎻ bottom) and sam ̄
pled at the indicated time points. FWꎬ fresh weight. Values are the
mean ± SD (n= 5). Values with different letters are significantly dif ̄
ferent (P < 0􀆰 05) . “∗” indicates that the value is significantly dif ̄
ferent from that of the WT under the same condition (P < 0􀆰 05)
9844期      JIA Yan ̄Xia et al.: Analysis of Endogenous Hormone Levels to Reveal the Retardation by Suppression 􀆺     
We thus ignored the wounding effects as a result of
the procedures used in our study.
2􀆰 2   An increase in MeJA levels during hor ̄
mone ̄promoted senescence is attenuated by sup ̄
pression of PLDα1
The hormone MeJA and its precursor JA play
critical roles in the regulation of senescence. In the
present studyꎬ both the ABA and ethylene treatments
increased the levels of MeJA in detached WT Arabi ̄
dopsis leaves (Fig􀆰 2Bꎬ middle and bottom panels) .
Howeverꎬ MeJA levels were significantly lower in
PLDα1 ̄AS leaves than in WT leaves (Fig􀆰 2Bꎬ mid ̄
dle and bottom panels) . These effects differed in
terms of intensity between the ABA and ethylene
treatments. Both the increase in MeJA and the inhi ̄
bition of the increase in MeJA by the suppression of
PLDα1 were larger under ABA treatment than under
ethylene treatment. These results indicate that chan ̄
ges in MeJA levels are associated with the retarda ̄
tion of hormone ̄promoted senescenceꎬ in particular
ABA ̄promoted senescenceꎬ that is mediated by the
suppression of PLDα1 activity.
2􀆰 3  ABA levels increase at the onset of senescence
and decline during the late stage of senescence
We examined the changes in endogenous ABA
levels that occurred during the three senescence
treatments mentioned above. In WT Arabidopsis leav ̄
esꎬ ABA levels increased significantly during the
early stage of senescence and then decreased during
the late stage ( Fig􀆰 3ꎬ top panel) . The levels of
ABA in detached leaves that were incubated in water
increased 1􀆰 5 ̄fold within 2 days and then began to
decrease over subsequent daysꎬ eventually dropping
to 50% of the initial level after treatment for 5 days
(Fig􀆰 3ꎬ top panel) . The sudden large increase in
endogenous ABA after the exogenous application of
ABA ( Fig􀆰 3ꎬ middle panel ) might have been
caused by the diffusion and / or uptake of ABA from
outside. Nonethelessꎬ even though the level of ABA
increased to its highest point within 1 day of the start
of ABA treatmentꎬ it then began to decrease. Within
1 day after ethylene treatmentꎬ ABA levels increased
1􀆰 7 ̄foldꎬ and then began to decreaseꎬ eventually
reaching 25% of the initial level by day 5 (Fig􀆰 3ꎬ
bottom panel) . These results indicated that transient
increases in the levels of endogenous ABA are asso ̄
ciated with the triggering of senescenceꎬ and thatꎬ at
later stages of senescenceꎬ ABA levels decreased in
leaves treated with exogenous ABA or ethylene.
Fig􀆰 3  Change in levels of endogenous ABA in leaves from WT and
PLDα1 ̄AS plants during three different senescence treatments
Notes: Detached leaves were treated with sterile water ( top)ꎬ 50
μmol􀅰L-1 ABA (middle)ꎬ or 50 μmol􀅰L-1 ethephon (ETHꎻ bot ̄
tom) and sampled at the indicated time points. FWꎬ fresh weight.
Values are the mean ± SD (n= 5). Values with different letters are sig ̄
nificantly different (P < 0􀆰 05). “∗” indicates that the value is signifi ̄
cantly different from that of the WT under the same condition (P < 0􀆰 05)
2􀆰 4  Suppression of PLDα1 decreases levels of
endogenous ABA during ethylene ̄accelerated se ̄
nescence
To further investigate the roles of ABA in senes ̄
cenceꎬ we analyzed ABA levels in PLDα1 ̄AS
plantsꎬ in which hormone ̄accelerated senescence
was delayed relative to that in WT plants (Fig􀆰 1).
To avoid the possible complication that levels of en ̄
094                                  植 物 分 类 与 资 源 学 报                            第 36卷
dogenous ABA are affected by the uptake of exoge ̄
nously applied ABA during ABA ̄induced senes ̄
cenceꎬ we focused on the levels of endogenous ABA
in detached leaves under water and ethylene treat ̄
ments ( Fig􀆰 3). Levels of ABA were identical be ̄
tween WT and PLDα1 ̄AS plants during detachment ̄
induced senescence (Fig􀆰 3ꎬ top panels) . Howeverꎬ
there were significant differences in ABA levels be ̄
tween WT and PLDα1 ̄AS plants during ethylene ̄in ̄
duced senescence. No transient increase in ABA leve ̄
ls occurred during the early stage of senescence in
PLDα1 ̄AS leavesꎬ and ABA levels were significant ̄
ly lower in PLDα1 ̄AS leaves than in WT leaves
(Fig􀆰 3ꎬ bottom panel) . The data indicated that the
delayed senescence that was associated with the sup ̄
pression of PLDα1 correlated with a decrease in
ABA levelsꎬ which meant that the levels of endoge ̄
nous ABA contributed to this delay in ethylene ̄pro ̄
moted senescence.
2􀆰 5   ZR levels decrease during detachment ̄in ̄
duced senescence and suppression of PLDα1 at ̄
tenuates the decline of ZR abundance during
hormone ̄accelerated senescence
We measured changes in the content of ZRꎬ the
major bioactive cytokininꎬ during the three senes ̄
cence treatments defined above. During detachment ̄
induced senescence (Fig􀆰 4ꎬ top panel)ꎬ ZR levels
dropped to 30% of their initial levels within 1 day
after detachment and then kept decreasing until they
reached 10% of their initial levels at day 5. We
found no differences in ZR levels between leaves
from WT and PLDα1 ̄AS plants during detachment ̄
induced senescenceꎬ except that ZR levels were
slightly lower in PLDα1 ̄AS than in WT leaves at
day 2. Howeverꎬ during ABA ̄ and ethylene ̄acceler ̄
ated senescence ( Fig􀆰 4ꎬ middle and bottom pan ̄
els)ꎬ the decreases in ZR levels were less dramatic
and were subject to greater fluctuations over the
course of the experiments. This indicates that senes ̄
cence in detached leaves is associated with a de ̄
crease in the abundance of ZR. In additionꎬ the pat ̄
tern of fluctuation in ZR levels was distinct between
ABA ̄ and ethylene treatments. Moreoverꎬ the levels
of ZR in PLDα1 ̄AS leaves were significantly higher
than those in WT leaves during the late stage of ABA
treatmentꎬ when ZR reached the highest levelsꎬ and
during the early stage of ethylene treatmentꎬ when
ZR remained at the highest levels. These results sug ̄
gest that the changes in the levels of endogenous
ZRꎬ or cytokinin ̄like compoundsꎬ are not the conse ̄
quence of hormone ̄promoted senescence but contri ̄
bute to the regulation of the attenuation of hormone ̄
accelerated senescence in PLDα1 ̄AS leaves. Results
also indicated that PLDα1 ̄AS leaves respond differ ̄
ently to ABA and ethylene ̄induced senescence.
Fig􀆰 4  Chnage in levels of endogenous ZR in leaves from WT and
PLDα1 ̄AS plants during three different senescence treatments
Notes: as the same as the fig􀆰 3
2􀆰 6  IAA levels decrease transiently during the
early stages of both detachment ̄induced and
hormone ̄accelerated senescenceꎬ and are higher
in PLDα1 ̄AS leaves than in WT leaves
We analyzed the changes that occurred in the
levels of IAAꎬ a well ̄known form of auxinꎬ during
1944期      JIA Yan ̄Xia et al.: Analysis of Endogenous Hormone Levels to Reveal the Retardation by Suppression 􀆺     
the three senescence treatments defined above. The
concentration of IAA decreased dramatically on the
first day in both water ̄ and hormone ̄treated de ̄
tached leavesꎬ and then returned to almost the initial
levels during the later stage (Fig􀆰 5). The decrease
at the onset of detachment ̄induced senescence may
be the consequence of the sudden disruption of nutri ̄
ent and energy supplies. During ABA ̄accelerated se ̄
nescenceꎬ levels of IAA were slightlyꎬ but signifi ̄
cantlyꎬ higher in PLDα1 ̄AS leaves than in WT
leaves at day 5ꎬ whereas no obvious differences were
detected at other sample time. During ethylene ̄ac ̄
celerated senescenceꎬ levels of IAA in PLDα1 ̄AS
leaves was 1􀆰 6 ̄foldꎬ 1􀆰 5 ̄fold and 1􀆰 2 ̄fold com ̄
pared to WT leaves after treatment for 1ꎬ 2ꎬ and 3
daysꎬ respectively (Fig􀆰 5ꎬ middle and bottom pan ̄
els) . These results indicated that attenuation of hor ̄
mone ̄accelerated senescence in PLDα1 ̄AS plants
was associated with an increase in IAA content. The
results also suggested that changes in the levels of
endogenous auxinꎬ at least auxin ̄like compoundsꎬ
might not just be the consequence ofꎬ but also be in ̄
volved inꎬ the regulation of ABA ̄ and ethylene ̄ac ̄
celerated senescence.
2􀆰 7  Gibberellic acid (GA3) levels decrease dur ̄
ing detachment ̄induced senescence but do not
change during ABA ̄ and ethylene ̄accelerated se ̄
nescence
In the present studyꎬ levels of GA3 decreased
during detachment ̄induced senescence (Fig􀆰 6ꎬ top
panel) . Howeverꎬ the levels of GA3 during ABA ̄
and ethylene ̄accelerated senescence were no signifi ̄
cant difference to those in the control (Fig􀆰 6ꎬ mid ̄
dle and bottom panels) . There were no differences in
GA3 levels between WT and PLDα1 ̄AS plants under
any of the conditions. The results showed that levels
of GA3ꎬ or giberellin ̄like compoundsꎬ are negatively
correlated with the progression of detachment ̄in ̄
duced senescenceꎬ but do not contribute to hormone ̄
accelerated senescence.
Fig􀆰 5  Change in levels of IAA in leaves from WT and PLDα1 ̄AS
plants during three different senescence treatments
Notes: as the same as the fig􀆰 3
Fig􀆰 6  Change in levels of endogenous GA3 in leaves from WT and
PLDα1 ̄AS plants during three different senescence treatments
Notes: as the same as the fig􀆰 3
294                                  植 物 分 类 与 资 源 学 报                            第 36卷
3  Discussion
An understanding of postharvest senescenceꎬ
and the mechanisms that can retard the processꎬ is
very important in relation to efforts to enhance crop
productivity ( Page et al.ꎬ 2001ꎻ Guo and Ganꎬ
2005). Hormones play critical roles in these proces ̄
sesꎬ although much remains to be learned about cor ̄
relations between hormone levels and senescenceꎬ
particularly during hormone ̄promoted senescence.
The effects of hormones on the retardation of senes ̄
cence in PLDα1 ̄deficient plants have never been re ̄
ported. After confirming that mechanical wounding
by cutting the leaf stalk did not cause substantial
effects in detached Arabidopsis leavesꎬ we used a de ̄
tached ̄leaf system to examine the endogenous levels
of five commonly studied hormones during three se ̄
nescence treatments. This approach enabled us to
monitor correlations between changes in the patterns
of the hormone levels and the progress of senescence
in leaves from both WT and PLDα1 ̄AS Arabidopsis
plants. The patterns and differences that emerged
from our analysis are summarized in Table 1 and Ta ̄
ble 2ꎬ respectively. In addition to enabling us to
compare and contrast the roles of hormones in de ̄
tachment ̄ꎬ ABA ̄ꎬ and ethylene ̄promoted senes ̄
cence and providing insights into the role of PLDα1
in senescenceꎬ these comprehensive data both sup ̄
port previous data and reveal new insights into the
roles of plant hormone levels during senescence. For
exampleꎬ the observed changes in the levels of en ̄
dogenous MeJAꎬ ABAꎬ and IAA during detachment ̄
induced leaf senescence in Arabidopsis (Fig􀆰 2ꎬ 3ꎬ
and 5) were similar to those reported for other spe ̄
cies ( Gepstein and Thimannꎬ 1980ꎻ He et al.ꎬ
2002ꎻ Van der Graaff et al.ꎬ 2006ꎻ Ghanem et al.ꎬ
2008). Hereꎬ we have expanded on these studies by
reporting that levels of ZR decreased during the
whole course of detachment ̄induced senescence. ZR
levels decreasedꎬ and then sightly increased eventu ̄
ally dropped again during the whole course of ABA ̄
or ethylene ̄promoted senescence (Table 1). Mean ̄
whileꎬ levels of GA3 remained unchanged throughout
all three conditionsꎬ except during the later stage of
detachment ̄induced senescenceꎬ when GA3 levels
decreased. Leaf senescence caused by detachment
resulted in the dropping in CK levels (Fig􀆰 4) which
was believed to be a key signal for the senescence
initiation (Nooden et al.ꎬ 1990) In additionꎬ acce ̄
leration of senescence by ABA and ethephon in ̄
volved a increase of ZR levels rather than keeping
dropping as it did in leaves just incubated in water
(Fig􀆰 2A). Analysis of cytokinin levels in leaves be ̄
fore and after the onset of senescence has revealed
an inverse correlation between cytokinin levels and
the progression of senescence in a variety of tissues
and plant species ( King et al.ꎬ 1990ꎻ Gan and
Amasinoꎬ 1995). The increase of ZR levels was ex ̄
amined in the progression of ABA ̄ and ethephon ̄ac ̄
celerated senescence might due to the interaction
took place between external application hormones
with endogenous ZR. The elevation of endogenous
ZR levels might be in order to antagonize the senes ̄
cence ̄promoting effect of ABA and ethephon.
Table 1  Patterns of changes in hormone levels in detached WT
Arabidopsis leaves during three senescence treatments:
waterꎬ ABAꎬ and ethephonꎬ respectively
Hormone
Change in endogenous levels in WT
Detachment ABA Ethylene
ABA km ∗ km
JA kg kg kg
ZR mg mg mg
IAA mk mk mk
GA3 gm gg gg
ZR mm mkm mkm
“k” indicates an increase in hormone levelsꎬ “g” indicates a main ̄
tenance of hormone levelsꎬ and “m” indicates a decrease in hormone
levels. “∗” indicates that the observed effect might be an artifact
caused by interference owing to the presence of exogenous ABA
Senescence of intact leaves can be divided con ̄
veniently into two stagesꎬ on the basis of their re ̄
versibility. Whereas the first stage of senescence is
reversibleꎬ the second stageꎬ which involves cell
death and even necrosisꎬ is not (Buchanan ̄Wollas ̄
ton et al.ꎬ 2003). For detachment ̄induced senes ̄
3944期      JIA Yan ̄Xia et al.: Analysis of Endogenous Hormone Levels to Reveal the Retardation by Suppression 􀆺     
cenceꎬ our data indicated that there are two stages
that are characterized by different trends in the ways
in which hormone levels change (Table 1). For ex ̄
ampleꎬ whereas ABA levels increase first before de ̄
cliningꎬ levels of IAA decrease first and then in ̄
crease. These two stages may serve distinct physio ̄
logical roles. As shown in Table 1ꎬ during the first
stageꎬ the response of each of the five hormones did
not differ among the three treatments during the arti ̄
ficial senescence: regardless of whether senescence
was induced by detachmentꎬ or accelerated by either
ABA or ethephonꎬ levels of ABA and MeJA always
increasedꎬ levels of ZR and IAA always decreasedꎬ
and levels of GA3 remained unchanged. These data
suggest that the changes in hormone levels that occur
during the first stage of all three treatments are re ̄
sponses to common factors that mediate their effects
shortly after detachment. Given that the effects of
mechanical wounding on hormone levels of detached
leaves is small and can be ignoredꎬ as already dis ̄
cussedꎬ such factors might be related to the disrup ̄
tion of energy and nutrient supplies. During the later
stageꎬ there were some differences in the responses
of the five hormones to the three treatments during
senescence. Levels of GA3 decreased during leaf seg ̄
ments kept in waterꎬ but remained unchanged in seg ̄
ments kept in ABA or ethephon (Table 1). There ̄
foreꎬ we propose thatꎬ during the initial stageꎬ the
observed changes in hormone levels are responses to
deficits in energy and nutrient suppliesꎬ whereas
during the later stageꎬ they are responses to hor ̄
mone ̄mediated effects.
It has been proposed that suppression of PLDα1
activity retards hormone ̄promoted senescence by in ̄
terfering with structural roles of PAꎬ which is largely
produced by PLDα1 (Fan et al.ꎬ 1997). Howeverꎬ
recent progress has revealed that PLDα1 and PA al ̄
so participate in hormone signallingꎬ including the
transduction of signals that are downstream of the
perception of ABA and JA (Gepstein and Thimannꎬ
1981ꎻ Ritchie and Gilroyꎬ 1998ꎻ Wang et al.ꎬ 2000).
Thereforeꎬ the effects of PLDα1 on senescence
might be mediatedꎬ at least in partꎬ through the ac ̄
tions of hormones. The results of the present study
revealed no differences in either phenotypes or hor ̄
mone levels between the leaves of WT and PLDα1 ̄
AS plants during detachment ̄induced senescence.
Howeverꎬ during hormone ̄promoted senescenceꎬ we
noted significant differences between the leaves of
WT and PLDα1 ̄AS plants in relation to both pheno ̄
types and levels of MeJAꎬ ZRꎬ and IAA (Fig􀆰 1 and
Table 2). These results indicate a significant rela ̄
tionship between the retardation of senescence fol ̄
lowing antisense ̄mediated suppression of PLDα1 ac ̄
tivity and changes in the levels of endogenous Me ̄
JAꎬ ZRꎬ and IAA. Levels of GA3 do not appear to
participate in these treatments.
Table 2  Differences in levels of hormones between WT and
PLDα1 ̄AS plants during three senescence treatments:
waterꎬ ABAꎬ and ethephonꎬ respectively
Hormone
Difference in levels of hormones between WT and
PLDα1 ̄AS plants during senescence
Detachment ABA Ethylene
ABA 0 ∗ -
JA 0 - -
ZR 0 - + + 0
IAA 0 0 + + 0
GA3 0 0 0
The symbols “+”ꎬ “0”ꎬ and “-” indicate that the levels of hormones
in PLDα1 ̄AS plants are higher thanꎬ equal toꎬ and lower than those
in WT plantsꎬ respectively. “∗” indicates that the observed effect
might be an artifact caused by interference owing to the presence of ex ̄
ogenous ABA
It remains to be established whether changes in
the levels of endogenous MeJAꎬ ZRꎬ and IAA are
causal factors or just the consequences of the PLDα1 ̄
mediated retardation of hormone ̄promoted senes ̄
cence. Membrane degradation is a major cellular
process during leaf senescence ( Thompson et al.ꎬ
1998). Considering the key roles of PLDα1 and its
product PA in the metabolism of membrane lipidsꎬ
one possibility is that the suppression of PLDα1 de ̄
lays the degradation of membranes and this delay re ̄
sults in the retardation of senescence. In this caseꎬ
494                                  植 物 分 类 与 资 源 学 报                            第 36卷
levels of MeJAꎬ ZRꎬ and IAA may be only the con ̄
sequences of the retardation of senescence. Nonethe ̄
lessꎬ our data might provide another clue to answer ̄
ing this questionꎬ given that the differences in the
levels of ZR and IAA between WT and PLDα1 ̄
plants during ABA treatment differed from those that
occurred during ethylene treatment (Table 2). Thusꎬ
higher content of ZR and IAA in PLDα1 ̄AS plants
might contribute to its retardation of hormone ̄promo ̄
ted senescence.
Acknowledgements: The authors thank Dr. Xuemin Wang
for kindly providing seeds of transgenic PLDα1 ̄antisense Ara ̄
bidopsis.
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