全 文 ：植物病理学报
ACTA PHYTOPATHOLOGICA SINICA　 42(5): 505-514(2012)
Received date: 2012-02-28; Revised date: 2012-06-26
Foundation item: This project is financially supported by the Program of New Varieties of Genetically Modified Organism Cultivation of China
(2009ZX08009-043B, 2009ZX08001-006B); ‘ 973 ’ Program ( 2012CB722504 ); ‘ 863 ’ Program ( 2008AA02Z125 );
National Natural Science Foundation of China for young scholar (31101208, 30900263); Zhejiang Provincial Foundation for
Natural Science (Y3090665, Y3100573)
Corresponding author: CHEN Jian-ping, professor, major in plant pathology and plant virology; E-mail: jpchen2001@yahoo. com. cn
Biography: YANG Yong (1978 - ), male, Shandong Province, doctor, major in plant pathology; E-mail: youngyong@yahoo. cn.
Xylem Secondary Cell-wall Thickening Involved
in Defense Responses of Oryza meyeriana
to Xanthomonas oryzae pv. oryzae
YANG Yong, XIE Li, YAN Cheng-qi, WANG Xu-ming, YU Chu-lang,
CHENG Xiao-yue, CHENG Ye, CHEN Jian-ping*
(State Key Laboratory Breeding Base for Zhejiang Sustainable Pest and Disease Control; Key Laboratory of Plant Protection and
Biotechnology, Ministry of Agriculture, China; Zhejiang Provincial Key Laboratory of Plant Virology, Institute of Virology
and Biotechnology, Zhejiang Academy of Agricultural Sciences, Hangzhou 310021, China)
Abstract:Bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most destructive
disease of rice worldwide. The wild rice species Oryzae meyeriana possesses high resistance to Xoo but the
resistance mechanism is unknown. In this study, the effects of Xoo on leaf lesion, chloroplast ultrastructure,
photosystem activity and xylem ultrastructure were examined in O. meyeriana as well as in a susceptible rice
cultivar Dalixiang. After inoculation with a variety of Xoo strains, lesion length in the wild rice was much
shorter than that in Dalixiang. In the susceptible rice, chloroplast structure was markedly destroyed and photo-
synthetic activity (gas exchange and chlorophyll fluorescence) was depressed by the pathogen but there were
no such effects in O. meyeriana. Electron microscopic observations showed that large numbers of bacteria were
present in the xylem vessels of the wild rice, which suggests that Xoo could invade and reproduce in the leaves
of O. meyeriana. Xoo infection induced the thickening of xylem secondary cell walls in O. meyeriana, which
subsequently inhibited the further invasion of the pathogen into adjacent cells through the vessel pits. This
response in the secondary cell wall could be involved in the high resistance of O. meyeriana to Xoo.
Key words: cell-wall thickening; ultrastructure; Oryzae meyeriana; Xanthomonas oryzae pv. oryzae;
resistance mechanism
木质部次生细胞壁增厚参与疣粒野生稻对黄单胞杆菌水稻变种的抗性 　 杨 勇, 谢 礼,
严成其, 王栩鸣, 余初浪, 成晓越, 程 晔, 陈剑平　 (浙江省植物有害生物防控重点实验室-省部共建国家重点
实验室培育基地;农业部植保生物技术重点实验室;浙江省植物病毒学重点实验室;浙江省农业科学院 病毒学与生物技术
研究所, 杭州 310021)
摘要:黄单胞杆菌水稻变种(Xanthomonas oryzae pv. oryzae,Xoo)引起的水稻白叶枯病是一个世界性的严
重病害。 疣粒野生稻(Oryzae meyeriana)对 Xoo具有高度抗性,但其抗性机制仍不清楚。 本文以抗病的疣
粒野生稻和感病的水稻品种大粒香为材料,研究了 Xoo 侵染对叶片病斑、叶绿体超微结构、光合系统活性
　
植物病理学报 42 卷
和木质部超微结构的影响。 结果表明,多种 Xoo生理小种导致的疣粒野生稻叶片病斑长度都明显短于大
粒香叶片的病斑长度。 Xoo病菌侵染显著破坏了大粒香的叶绿体结构,明显抑制了其光合活性,而疣粒野
生稻中的变化要轻得多。 通过电镜切片,发现疣粒野生稻叶片导管内存在大量的 Xoo 病菌,这表明 Xoo
能够侵染疣粒野生稻且能够在叶片内增殖。 病菌的侵染诱导了疣粒野生稻木质部次生细胞壁的增厚,抑
制了病菌通过导管纹孔向邻近细胞的进一步侵染,这种反应可能参与了疣粒野生稻对 Xoo的抗性。
关键词:细胞壁增厚;超微结构;疣粒野生稻;黄单胞杆菌水稻变种;抗性机制
中图分类号: S435. 111. 47　 　 　 　 　 文献标识码: A　 　 　 　 　 文章编号: 0412-0914(2012)05-0505-10
　 　 Bacterial blight of rice, caused by Xanthomonas
oryzae pv. oryzae (Xoo) is one of the most serious
diseases of the crop, decreasing rice yields by 20%
-30% and as high as 50% in some areas of Asia.
The most economical and environment-friendly strat-
egy to control this disease is to grow genetically re-
sistant rice varieties. Wild species of the genus Ory-
za have adapted through natural selection to survive
in harsh environments and possess many useful traits
that are not present in cultivated rice. Some wild
species with high resistance to Xoo provide a valua-
ble resistance reservoir for breeding new varieties of
rice. For example, high resistance against various
strains of Xoo has been observed in O. longistamina-
ta, O. rufipogon, O. minuta and O. officinalis, from
which four resistance genes Xa21, Xa23, Xa27 and
Xa29 have been identified, respectively[1 ~ 4] . Xa21
and Xa27 have even been isolated and characterized
by map-based cloning approaches and the molecular
genetic mechanism for disease resistance has been
widely studied[5 ~ 9] . These genes have been intro-
duced into cultivated rice in breeding programs and
widely used for sustained improvement of rice pro-
ductivity.
O. meyeriana is one of the wild relatives of rice
growing in tropical and subtropical areas of South
and Southeast Asia. In China, it is distributed in the
southwestern Yunnan Province and the southern
Hainan Province. O. meyeriana has been shown to
possess a high resistance to Xoo. Zhang et al. [10]
demonstrated that the resistance of O. meyeriana was
ranked the highest among 871 accessions of 13 wild
rice species. We had also introduced the bacterial
blight resistance trait from O. meyeriana into a ja-
ponica rice cultivar using asymmetric somatic hy-
bridization[11,12] .
However, the mechanism of resistance of
O. meyeriana to Xoo remains uncertain and has been
little studied. O. meyeriana contains the GG ge-
nome, which is distantly related to O. sativa with its
AA genome[11,13] . O. meyeriana was therefore
thought not a host of Xoo and the pathogen could not
infect and reproduce in this wild rice. It had also
been suggested that the resistance of O. meyeriana
might be related to the particular physical characte-
ristics of its leaves. Waxiness of leaf sheaths and
leaves keeps less water adhere and makes an unfa-
vorable environment for the invasion and multiplica-
tion of the pathogen[14] . However, it is also possible
that O. meyeriana possesses certain inherent cellular
and physiological mechanisms accounting for its high
Xoo resistance. In this study, the location of the
bacteria in the leaves of O. meyeriana and the effects
of pathogen invasion on xylem ultrastructure were
examined to investigate possible mechanisms in-
volved in resistance.
1　 Materials and Methods
1. 1　 Plant materials
The wild species of rice O. meyeriana L. , with
a high resistance to Xoo, was provided by Professor
Xinhua Wei, China National Rice Research Institu-
te. O. sativa L. ssp. japonica ( cv. Dalixiang), a
cultivar susceptible to Xoo[11] is from the Zhejiang
Academy of Agricultural Sciences collection. Rice
seedlings were grown in a growth chamber at 28℃ /
25℃ (day / night: 16 h / 8 h), photon flux density
600 - 800 μmol·m -2·s -1, and relative humidity
605
　
　 5 期 YANG Yong,et al.:Xylem secondary cell-wall thickening involved in defense responses of O.meyeriana to Xoo
approximately 60% -80% .
1. 2　 Pathogen inoculation
Xoo strains PXO86, PXO79, PXO71,
PXO112, PXO99, PXO280, PXO145, PXO87,
PXO124 and ZHE173 were used in this study. The
inoculation was carried out using the leaf-clipping
method described by Yu et al[15] . Briefly, the bacte-
rial inoculum was prepared from a 48 - h culture on
PSA slants and its density was adjusted to 109 cfu·
mL -1 . At the booting stage, the uppermost fully ex-
panded leaves were clipped about 1 cm from their
tips using a pair of scissors dipped in the inoculum,
and then the rice plants were incubated in the growth
room at 28℃. Controls were treated similarly but
with sterile water. Lesion length was measured 2
weeks after inoculation.
1. 3　 Electron microscopic observations
Leaf segments (1 × 3 mm) were fixed with
2． 5% (v·v -1) glutaraldehyde in 0. 1 M phosphate
buffer (pH 7. 2) for 2 h at 4℃. The samples were
then post-fixed for 1 h at room temperature in phos-
phate buffer ( pH 7. 2) containing 1% (m·v -1 )
osmium tetroxide, and then dehydrated through an
ethanol series, 15 min each step, and finally de-
hydrated twice in 100% ethanol for 20 min. The
segments were infiltrated in a mixture of acetone and
Spurr resin, 1∶ 1 (v·v -1) for 1 h and 1 ∶ 3 (v·
v -1) for 3 h at room temperature, and then in abso-
lute Spurr resin overnight. Finally, the segments
were embedded in Spurr resin at 37℃ for 12 h fol-
lowed by 78℃ for 8 h. After polymerization the
blocks were cut into 70 nm ultra-thin sections with
the ultramicrotome. After staining the sections with
uranyl acetate and lead citrate for 15 min, cell ultra-
structure was observed and photographed with an
electron microscope (JEM-1200EX, JEOL) .
1. 4　 Gas exchange analysis
Photosynthetic and fluorescent parameters were
detected in vivo 2 weeks after inoculation. Gas ex-
change was monitored with a LI-6400 portable pho-
tosynthesis system (LI-COR, Lincoln, USA) at a
constant airflow rate of 500 μmol·s -1 and at satura-
tion irradiance with incident photosynthetic photon
flux density (PPFD) of 800 μmol·m -2·s -1 . The
concentration of ambient CO2 was about 385 cm3·
m -3 and the temperature was about 25℃.
1. 5　 Chlorophyll fluorescence analysis
An Imaging-PAM Chlorophyll Fluorometer
(Walz, Effeltrich, Germany) was used to measure
chlorophyll ( Chl) fluorescence parameters. After
dark adaptation of the leaf for 1 h, the minimum fluo-
rescence yield (Fo) was obtained with a weak meas-
uring beam ( <0. 1 μmol·m -2·s -1) . A 600 - ms
saturating flash ( > 7 000 μmol·m -2 · s -1 ) was
applied to measure the maximum fluorescence yield
(Fm) . The leaf was then continuously irradiated
with an actinic light (1 000 μmol·m -2·s -1) and
equilibrated for 30 min to record the steady-state
fluorescence yield ( Fs) . The maximum fluores-
cence under irradiation (Fm′) was obtained by im-
posing another saturation flash. The maximal photo-
chemical efficiency (Fv / Fm) was defined as (Fm -
Fo) / Fm, the quantum efficiency of photosystem II
(ФPSII) was defined as ( Fm′ - Fs) / Fm′, photo-
chemical quenching (qP) was defined as (Fm′ -
Fs) / (Fm′ - Fo) and non-photochemical quenching
(qN) was defined as (Fm - Fm′) / (Fm - Fo) as
proposed by Genty et al [16] .
1. 6　 Statistical analysis
Analysis of variance (ANOVA) was done u-
sing Microsoft Excel © and other analyses were per-
formed using SYSTAT version 10. 0 ( SPSS, Chica-
go, USA) to generate the LSD of treatment means.
For all analyses, differences were considered to be
significant at the P <0. 05 level.
2　 Results
2. 1　 Phenotypic analysis
The resistance of O. meyeriana and O. sativa
cv. Dalixiang to Xoo was evaluated by inoculating
705
　
植物病理学报 42 卷
with 10 Xoo strains collected from the Philippines and
China. As shown in Table 1, the lesions in O. meye-
riana were all shorter than 2 cm. The longest lesion
(1. 78 cm) was from inoculation with PXO99, and
ZHE173 induced the shortest (0. 75 cm) . In the cul-
tivated rice Dalixiang, the lesions were all longer
than 13 cm, showing that Dalixiang was very suscep-
tible to Xoo. There was highly significant difference
between the two hosts (P <0． 001) . Two of the most
aggressive strains were PXO145 and PXO124 that
induced lesions 21. 3 cm and 20. 2 cm long, respec-
tively. These results are in agreement with the early
observations that O. meyeriana conferred a high level
of resistance to Xoo[10,11] .
2. 2　 Ultrastructural changes in chloroplasts
Electron microscopy images revealed that leaves
of healthy rice contained chloroplasts with normal
structure both in O. meyeriana and Dalixiang (Fig.
1-A, D) . Xoo invasion caused distinct alterations in
chloroplast structure of the cultivated rice. The chlo-
roplasts became deformed with swollen stroma and
loosened lamellae (Fig. 1-E, F) . In many chloro-
plasts, the membrane system was disrupted and the
contents were then released into the cytoplasm (Fig.
1-F) . In contrast, no prominent differences in chlo-
roplast structure were observed in O. meyeriana ino-
culated with Xoo.
2. 3 　 Changes in gas exchange and chloro-
phyll fluorescence
To further examine the effects of the pathogen
on the chloroplasts, the differences in gas exchange
and chlorophyll fluorescence between the wild and
cultivated rice with or without Xoo inoculation were
analyzed. In Dalixiang, net photosynthetic rate
(PN) was much lower in the treated plants (1. 41
μmol·m -2·s -1) than in the controls (15. 7 μmol
·m -2·s -1), a decrease of about 91% (Table 2) .
Similarly, pathogen invasion markedly decreased
stomatal conductance ( gs ) by about 85% ( Table
2) . In parallel with the inhibition of carbon assimila-
tion, intercellular CO2 concentration ( Ci ) was
prominently increased by about 29% ( Table 2 ) .
Maximal photochemical efficiency (Fv / Fm), quan-
tum efficiency of photosystem Ⅱ (ФPSⅡ) and pho-
tochemical quenching ( qP) all declined markedly
(42% , 82% and 81% , respectively) in Dalixiang
after inoculation with Xoo (Table 2) . These results
showed that the function of the photosynthetic sys-
tem was impaired by pathogen invasion. Non-photo-
chemical quenching (qN) often increase to dissipate
excess excitation energy and protect the photosyn-
thetic apparatus when carbon assimilation is rever-
sibly depressed[17,18] . However, in this study qN al-
so markedly declined in treated plants of Dalixiang
(Table 2), indicating that the photosynthetic system
was being irreversibly destroyed after pathogen inva-
sion. This result was in agreement with the informa-
tion obtained in electron microscopic observation
(Fig. 1-E, F) . In comparison with the susceptible
Dalixiang, O. meyeriana exhibited no significant
changes either in gas exchange or in chlorophyll fluo-
rescence parameters after Xoo inoculation (Table 2) .
Table 1　 Size of lesions (cm) 2 weeks after inoculation with different Xoo strains
in Oryzae meyeriana and O. sativa ssp. japonica cv. Dalixiang
Rice
Xoo strains
PXO86 PXO79 PXO71 PXO112 PXO99 PXO280 PXO145 PXO87 PXO124 ZHE173
O. meyeriana 1. 1 0. 9 1. 0 1. 1 1. 8 0. 9 1. 6 1. 1 1. 0 0. 8
O. sativa 15. 5 15. 3 16. 7 18. 8 20. 0 15. 8 21. 3 14. 8 20. 2 13. 6
Values are the means of 6 replicate measurements. SED for comparing treatments means = 0. 79 (108 d. f) .
805
　
　 5 期 YANG Yong,et al.:Xylem secondary cell-wall thickening involved in defense responses of O.meyeriana to Xoo
Fig. 1　 Effects of Xoo inoculation on chloroplast ultrastructure in
Oryzae meyeriana and O. sativa ssp. japonica cv. Dalixiang
Rice was inoculated with Xoo strain PXO124 and two weeks later leaf samples were collected for observation using
electron microscopy. A, B, C:O. meyeriana;D, E, F:Dalixiang; A, D:Mock-inoculation with water; B, C,
E, F: Inoculation with Xoo. White and black arrows indicate swollen chloroplast stroma and loosened lamellae,
respectively. Scale bars are indicated on each picture.
2. 4　 Ultrastructural changes in xylem
No bacteria were seen in the mock inoculated
rice (Fig. 2-A,B,H,I) . In contrast, after Xoo in-
oculation, xylem vessels at the site near the visible
lesion were filled with bacteria not only in Dalixiang
but also in the resistant O. meyeriana (Fig. 2-C to
F,J to M) . In rice xylem vessel cells, the secondary
cell wall developed discontinuously, and the vessel
pits separating xylem parenchyma and xylem ele-
ments became the prominent feature within the ves-
sels (Fig. 2) . In the present study, bacteria were
frequently discovered associated with the pit mem-
branes in both the wild and cultivated rice (Fig. 2-C
to F,J to M) . It was noticed that in O. meyeriana
Xoo infection induced thickening of the xylem se-
condary cell wall as compared with the control
(Fig. 2-C to F) and consequently the diameter of
the pit aperture was also prominently reduced (Fig.
2-C to F) . Bacteria were rarely seen within the
xylem parenchyma cells of the resistant wild rice
(Fig. 2-C,G) . In contrast, there was no significant
difference in the width of the xylem secondary cell
wall between Xoo and water inoculated Dalixiang,
and there was no decrease in the diameter of pit aper-
tures following inoculation by Xoo (Fig. 2-J to M).
Large numbers of bacteria were found in the xylem pa-
renchyma cells in Dalixiang after Xoo inoculation (Fig.
2-J,N). The bacteria might have invaded the parenchy-
ma cells through the pits from xylem vessels, a sugges-
tion supported by Fig. 2-K which showed the primary
wall of a xylem vessel cell twisted by bacterial inva-
sion. In addition, Horino[19] and Hilaire et al. [20] had
905
Table 2　 Effects of Xoo inoculation on gas exchange and chlorophyll fluorescence
parameters in Oryzae meyeriana and O. sativa ssp. japonica cv. Dalixiang
Rice Inoculation
PN
(μmol·m -2·s -1)
gs
(μmol·m -2·s -1)
Ci
(μmol·mol -1)
Fv / Fm ФPSⅡ qP qN
O. meyeriana
H2O 12. 70 ±0. 40 a 0. 33 ±0. 02 a 274 ±6. 93 a 0. 79 ±0. 01 a 0. 36 ±0. 03 a 0. 75 ±0. 05 a 0. 77 ±0. 01 a
Xoo 12. 60 ±0. 20 a 0. 32 ±0. 01 a 272 ±1. 53 a 0. 79 ±0. 02 a 0. 35 ±0. 04 a 0. 72 ±0. 07 a 0. 77 ±0. 02 a
O. sativa
H2O 15. 70 ±0. 56 a 0. 34 ±0. 03 a 239 ±6. 24b 0. 79 ±0. 01 a 0. 34 ±0. 01 a 0. 72 ±0. 05 a 0. 79 ±0. 01 a
Xoo 1. 41 ±0. 16 b 0. 05 ±0. 02 b 309 ±16. 5 a 0. 46 ±0. 03 b 0. 06 ±0. 01 b 0. 14 ±0. 03 b 0. 36 ±0. 01 b
Rice was inoculated with Xoo strain PXO124 by leaf-clipping method. Gas exchange and chlorophyll fluorescence parameters were detected in vivo two weeks after inocula-
tion. Values are the mean ± s. e. of six infected leaves. Values followed by different letters between the control and Xoo inoculation are significantly different at P <0. 05.
PN:Net photosynthetic rate; gs:Stomatal conductance; Ci:Intercellular CO2 concentration; Fv / Fm:Maximal photochemical efficiency; ФPSⅡ:Quantum efficiency of photo-
system Ⅱ; qP:Photochemical quenching; qN:Non-photochemical quenching.
　
　 5 期 YANG Yong,et al.:Xylem secondary cell-wall thickening involved in defense responses of O.meyeriana to Xoo
Fig. 2　 Effects of Xoo inoculation on xylem ultrastructure in Oryzae meyeriana
and O. sativa ssp. japonica cv. Dalixiang
Rice was inoculated with Xoo strain PXO124 and two weeks later leaf samples were collected for observation using
electron microscopy. A to G:O. meyeriana; H to N:Dalixiang;A, B, H, I:Mock-inoculation with water;
C to G, J to N:Inoculation with Xoo. XV:Xylem vessel;P: Pit aperture; PW:Xylem primary cell-wall; SW:
Xylem secondary cell-wall;XPC:Xylem parenchyma cell; F:Fibrillar material. White, black and gray triangles indicate
pit aperture, xylem primary cell-wall and secondary cell-wall, respectively, and black arrow indicates a bacterium. G and
N are enlargements from the corresponding parts of C and J, respectively. Scale bars are indicated on each picture.
observed fibrillar material abundant in rice vessels
and suggested that this material was one of the re-
sponses to bacteria. Here, similar material was also
found in certain xylem vessels in O. meyeriana but
not in Dalixiang (Fig. 2-F) .
3　 Discussion
In agreement with previous research[10,11], the
present study showed that O. meyeriana was highly
resistant to Xoo, and the leaf lesions were much
shorter than that in susceptible cv. Dalixiang after in-
oculation with the pathogen (Table 1) . Electron mi-
croscopic observations also showed that chloroplast
structure was severely destroyed in inoculated Dalix-
iang (Fig. 1) and this was associated with a marked
decline in gas exchange and chlorophyll fluorescence
parameters indicating that the performance of the
photosynthetic apparatus was badly damaged by
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植物病理学报 42 卷
pathogen invasion (Table 2) . Alterations in chloro-
plast structure provided a reasonable explanation for
the phenotypic changes in rice leaves that included
loss of green pigment, yellowing and lesions. No
such changes occurred in the resistant O. meyeriana
(Fig. 1, Table 2) . All of the results supported the
value of O. meyeriana as an important genetic source
that could be used in breeding programs for impro-
ving Xoo resistance of commercial rice cultivars.
O. meyeriana showed strong resistance to all of
the 10 Xoo strains (Table 1) . This multiple resis-
tance specificity is highly desirable in breeding pro-
grams for durable resistance, but its molecular basis
is still not better understood. It is possible that multi-
ple resistance genes in O. meyeriana recognize each
unique pathogen elicitors produced by different
pathogen avirulence genes. Another possibility is
that this wild rice contains a single resistance gene
capable of recognizing diverse pathogen elicitors,
which can be explained by the “ guard” hypothesis
that resistance proteins might guard a limited set of
key cellular targets of pathogen virulence factors[21] .
Three Xoo-resistance genes that have been identified
from wild rice also confer resistance to multiple Xoo
strains. The cloned Xa21 gene conferred resistance
to 29 strains[22] . The cloned Xa27 gene conferred re-
sistance to 27 strains[2] . The fine-mapped Xa23 gene
showed resistance to 20 strains[23] . However, from
current data, we are still unclear whether O. meyeri-
ana contains multiple functional genes with each
possessing different resistance specificity or whether
it harbors a single gene that confers broad resistance.
It will be necessary to identify and clone the resis-
tance gene( s) from this wild rice if this problem is
to be better understood. We have therefore intro-
duced the Xoo resistance trait from O. meyeriana into
the cultivar Dalixiang using asymmetric somatic
hybridization[11,12], and following this we are isola-
ting and cloning the related gene(s) through a map-
based cloning strategy.
Uncertainty about the resistance mechanism in
O. meyeriana has delayed the isolation of resistance
gene(s) and the use of this material in breeding pro-
grammes. From current data, it is possible that the
high resistance of O. meyeriana may have multiple
causes. The leaf waxiness is unfavorable for patho-
gen adherence to leaf surface, which can decrease
pathogen invasion into leaf cells. In this study, large
numbers of bacteria were observed in the xylem ves-
sels of both Dalixiang and O. meyeriana even two
weeks after leaf-clipping inoculation ( Fig. 2 ),
which suggests that the bacteria could invade and re-
produce in the leaves of the wild rice. This means
that O. meyeriana also possesses certain inherent cel-
lular and physiological mechanisms accounting for its
high Xoo resistance besides of particular leaf physical
characteristics such as waxiness. Horino[19] and Hi-
laire et al. [20] reported that Xoo cells were enveloped
by abundant fibrillar material within rice vessels and
the bacterial cells became irregular in shape in in-
compatible interactions. We also found similar fibril-
lar material in certain vessels of O. meyeriana (Fig.
2F) . Additionally, we have observed that Xoo ino-
culation induced the re-location of Rubisco activase
from the stroma to the chloroplast thylakoid mem-
brane and this may help to protect the thylakoid
membrane against damage from Xoo infection[24] .
Cheng et al. reported that there was no signifi-
cant difference in mesophyll structure, top cells,
thin-wall cells, vescular and thick-wall cells between
O. meyeriana and cultivated rice without Xoo inocu-
lation, and suggested that the strong resistance of
this wild rice was not caused by leaf physical struc-
ture[14] . We also discovered the similar results in O.
meyeriana under mock-inoculation conditions, and
hardly found significant alteration induced by Xoo
infection in the number and shape of leaf vescular
and other cells (Fig. 1, Fig. 2) . However, in con-
trast to susceptible rice, Xoo induced the thickening
of the xylem secondary cell wall in the resistant O.
meyeriana. As a consequence, the diameter of the
pit aperture was decreased, effectively reducing the
area of pit membrane exposed for access by bacteria,
and there were hardly bacteria found within the
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　 5 期 YANG Yong,et al.:Xylem secondary cell-wall thickening involved in defense responses of O.meyeriana to Xoo
xylem parenchyma cells (Fig. 2) . Cheng et al. did
not concern the change in xylem secondary cell
wall[14] . However, Hilaire et al. reported that xy-
lem secondary cell wall thickened in incompatible,
but not compatible, interactions between Xoo and
rice cultivar IRBB10 that contains the Xa10 gene
conferring bacterial blight resistance and suggested
that this response might be involved in the defense of
Xa10 to Xoo[20] . We suggest that xylem cell wall
thickening may similarly be one of the reasons for
the high resistance of O. meyeriana to Xoo.
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[2] 　 Gu K, Tian D, Yang F, et al. High-resolution genetic
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