全 文 ：第 15 卷 第 4 期
Vol. 15 No. 4
草 地 学 报
ACTA AGRESTIA SINICA
2007 年 7 月
Jul. 2007
文章编号: 1007-0435( 2007) 04-0386-08
Forage Cell Walls: Ruminal Degradation
Xue-zhao SU N
1, 2
, Simone O. Hoskin
2
, Keith N . Joblin
1
, Ian G. Andrew
3
and
Philip J. Harris
4
and Y-i bing Sheng
5
1. AgResearch Grass lan ds, Private Bag 11008, Palmerston North , New Zealand;
2. Inst itute of Veterinary, Animal and Biomedical S cien ces , Mass ey University, Private Bag 11222, Palmerston North, New Zealan d;
3. Ins titute of M olecu lar Biosciences, Massey U niver sity, Private Bag 11222, Palmers ton North, New Zealand;
4. S chool of Biological S ciences, The U niver sity of Au ckland, Private Bag 92019, Au ckland, New Zealand;
5. Shandong Academy of Agricu ltural S cien ces, J inan , S hand on g Province 250100, China;
curr ent ly visit ing scient ist at Mas sey University, New Zealand)
Abstract: Ruminal micr obes play a majo r ro le in the deg radation of for age cell w alls in the rumen. This review br ief-
ly discusses ruminal micr obes, mainly ruminal bacter ia, invo lved in forag e cel-l w all degr adation, enzymes secreted
from these microbes for wa ll po ly saccharide deg radation, outlines a model of cel-l w all deg radation, and constr aints
to cell wall degr adat ion.
Key words: Forage; Cell w alls; Rrumen; Degr adation; Po ly saccharides
牧草细胞壁:瘤胃内降解
孙学钊1, 2 , Simone O. Hoskin2 , Keith N. Joblin1, Ian G. A ndrew 3 ,
Philip J. Harris4 , 盛亦兵5
( 1. 新西兰农业研究院草地所, 信箱号 11008, 北帕莫斯顿,新西兰;
2. 兽医、动物和生物医学研究所,梅西大学, 信箱号 11222, 北帕莫斯顿, 新西兰;
3. 细胞生物学研究所,梅西大学, 信箱号 11222, 北帕莫斯顿,新西兰;
4. 生物学院, 奥克兰大学,信箱号 92019,奥克兰, 新西兰;
5. 山东农业科学院,山东 济南 250100; 目前为新西兰梅西大学访问学者)
摘要:牧草细胞壁在瘤胃内降解主要是瘤胃微生物起作用。本综述概略地介绍了牧草细胞壁降解所涉及的以细菌为主
的微生物,细胞壁多糖分解所需的酶, 细胞壁降解模型以及细胞壁降解的限制因素。
关键词: 牧草; 细胞壁; 瘤胃; 降解; 多糖
中图分类号: S812 文献标识码: A
Forage cell w alls prov ide the major energ y
source for r um inants[ 1] . H ow ever, this only hap-
pens af ter they ar e degraded by rumen micr obes to
produce vo lat ile fat ty acids for ruminants to absor b
and utilise as the primar y energ y source. Thus,
the ruminal degradat ion of fo rage cell w alls is cr it-i
cal to ruminant nutrit ion.
Ingested fo rages are f irst subjected to a reduc-
t ion in part icle size and then fur ther deg radation by
rumen microbes. T he reduct ion in part icle size re-
sults mainly f rom chew ing during r um inat ion and
eating[ 2] . For pect in-rich forag es such as chicory[ 3]
and white clover [ 4] , the rapid degr adation of pec-
t ins also appears to contribute to part icle break-
down in the rumen.
Af ter the reduct ion in part icle size, rumen m-i
cr obes play a major ro le in fo rag e cel-l w all deg rada-
t ion. Rumen micr obes include many dif ferent spe-
cies, predominant ly of bacteria, protozoa and fungi.
These can be divided into several groups according
收稿日期: 2006-09-18; 修回日期: 2007-06-16
基金项目: 新西兰科学技术研究基金委优等生博士奖学金
作者简介: 孙学钊( 1964- ) ,江苏海安人,博士,博士后研究员,主要从事牧草消化及草地管理研究, E-mail: xuezhao. sun@ agresearch. co. nz
第 4期 SUN Xue- zhao et al: Forage cel l w alls : Ruminal degradat ion
to their functions[ 5] . F ibro lyt ic microbes refer to
the g roup invo lved in the br eakdown of forage cell
w al ls. Pr oteo lyt ic microbes refer to those w hich
breakdown dietary pr oteins. Others ar e respons-i
ble fo r the breakdown o f lipids. In the follow ing
sect ions, fibroly tic microbes w ill be discussed be-
cause they are responsible fo r for ag e cel-l w all deg-
r adation.
Increasing the dig est ion of forage cell w al ls by
rum inal microbes is a key factor in improving an-i
mal performance. Scient ists from many disciplines
such as plant anatomy, plant bio chem ist ry, forag e
breeding, ruminant nutrit ion and ruminal microb-i
o logy have been involved in this area. T his topic
has been exhaustedly review ed [ 6~ 8] and this review
only focuses on rumen f ibro lyt ic m icrobes and their
act ivities in cell w all degr adat ion.
1 Forage cell walls
The major cult ivated forag e plants are either
monocotyledons, mainly g rasses, or dicoty ledons,
mainly legumes. Some new novel herb forages
from other families, e. g. asteraceae, are also d-i
cotyledonous pla-nts. T her e are no g reat dif fer-
ences in cell w alls betw een forag e and non- forag e
herbaceous plants, but forages tend to be utilised be-
fore the mature stage when there is a low degree of lig-
nificat ion. The basic structure of cellulose in all plants
is the same, although the relative abundance may dif-
fer. The non-cellulosic polysaccharide compositions of
forage grasses are quite different from those of forage
legumes[ 9] . In grass primary cell walls, the main non-
cellulosic polysaccharides are arabinoxylan and glucu-
ronoarabinoxylan, w ith smaller amounts of xylog lucan
and ( 1→3) , ( 1→4)-B-glucans[ 10] ; the predom inant
non-cellulosic polysaccharides in the secondary cell
w al ls have similar str uctures, but low er degrees o f
subst itut ion of the backbone xylan [ 11] . In contrast ,
in legumes, the predom inant non-cellulosic po ly-
saccharides in the primary w al ls are pect ic polysac-
char ides, xy log lucans, and small proport ions o f
heterox ylans and ( g alacto- ) glucomannans; in the
secondar y w alls, they are predominant ly 4-O-
methy-l g lucur onoxy lans with smal ler proport ions
of g lucomannans[ 9] .
Lignin is present in the lignif ied walls. T he
lignif ied legume w alls appear indigestible. In con-
trast , the lignif ied grass w alls are readily dig ested
although lignin itself is indig estible
[ 12]
. In add-i
t ion, cel l w alls contain minor amounts of suberin,
cut in, tannins, w axes and minerals [ 13] .
T he proport ions of the various poly saccharides
dif fer betw een g rasses and dicoty ledonous for ag es.
T he most significant difference is that pectic po-
lysaccharides are much more abundant in forage leg-
ume w alls than forage grass walls. For example, the
content of pectic polysaccharides is 10% ~ 30% in the
cell walls of alfalfa (Medicago sat iva)
[ 14]
and 3% ~
4% or less in the cell walls of grasses
[ 15]
.
2 Rumen microbes
2. 1 Bacteria
T he main rumen microbes invo lved in degrad-
ing plant cell w alls are listed in T able 1[ 16] . Bacte-
ria and fungi account for about 80% of plant cel-l
w all degradat ion in the rumen; protozo a account
for approx imately 20% [ 17] .
In the rumen, there are a few dominant cellu-
loly tic bacteria: Fibrobacter succinogenes , Rumi-
nococcus albus , R . f lavef aciens and Buty r iv ibrio
f ibri solv ens
[ 16, 18] . How ever, the cellulolyt ic bacte-
ria in the rumen var y w ith the subst rate. In New
Zealand, perennial r ye-gr ass/ w hite clover pasture
is the main feed for g razing ruminants and the
dominant cellulo lyt ic bacter ia in the rumen of these
ruminants are Ruminococcus albus , R. f lav ef a-
ciens, F ibrobacter succinogenes , Buty r iv ibrio
f ibrisolvens( Keith Joblin, 2003, unpublished data) .
2. 2 Fungi
Rumen fung i have been less studied than ru-
men bacteria. T ypical fungi asso ciated w ith cell
w all degradat ion ar e N eocal l imastix f r ontal is , N .
p atr i ciar um, Piromyces communis and Or p inomy-
ces bov is
[ 19, 20]
. They have enzymat ic and physical
ro les in the degr adation of cell w alls in the rumen.
T he cellulases and xy lanases f rom fung i are highly
act ive, and rumen fung i produce a wider range of
polysaccharidases than bacteria
[ 20]
. The fungal removal
of wall components that lim it access by bacter ia to
the substrate facilitates further degradat ion. Forsberg
and Cheng[ 21] reported that fungal growth is apparently
387
草 地 学 报 第 15卷
Table 1 Identit y and enzyme act ivities of r uminal
microbes invo lved in the deg radation
o f plant cell w alls in the rumen
Organism
Degrada tive act ivity
Celluloly tic Hemicellulolyt ic Pect inoly tic
Bacter ia
Fi brobacter succinogenes + + +
Rumi nococcus albus + + +
Rumi nococcus f lav ef aciens + + +
Buty riv ibr io f i br iso lvens + +
Eubact er i um cell ul osol vens + +
Cl ostr i dium long isp orum +
Cl ostr i dium locheadii + +
Prev otell a rum inantium + +
Eubact er i um xy lanop hi lum +
Rumi nobacter amy lophil us +
Succinimonas amy l oly ti ca +
Succiniv ibr io dex tr inosolvens +
Selenomonas ruminantium +
Selenomonas lactil y tica +
Lachnosp i ra multi parus +
Strep tococcus bovi s + +
Megasphaera elsd enii + +
F ungi
Neocall imasti x f rontali s + + +
Neocall imasti x p atr i ci arum + + +
Neocall imasti x j oy onii + +
Caecomyces communis + + +
Piromyces communi s + + +
Orp inomyces bov is + +
Rumi nomyces elegans + +
P ro to zoa
Eudip lodinium maggi i + + +
Ostracodi ni um dil obum + + +
Ep idinium caudatum + +
Metadi nium af f ine + + +
Eudip lodinium bovis + + +
Orphry oscol ex caudatus + + +
Poly p last ron mul tiv esiculatum + + +
Dip l odiniump entacanthum +
Endop l op last ron tr il or icatum +
Orphy roscol ex t r icoronatus +
Ostracodi ni um graci le +
Entodi nium caudatum + +
Iso tr icha int es ti nalis + + +
Iso tr icha p rost oma + + +
Adapted f rom Deho rity [16] . + indicates activit y. T he term hemicellulo-
ly tic refers t o mainly xy lano lyt ic activity .
limited to the recalcit rant lignif ied w alls of scleren-
chyma f ibres. Fung i can penetr ate the cut icle at the
plant surface as w ell as the cell w alls o f lignified
tissue[ 22] . Fung i can also play a different ro le fr om
bacteria in the degr adation of cell w alls, e. g. me-
chanically rupture forage plant st ructure[ 23, 24] .
Fung i appear to play an important ro le in the deg-
r adation of the r esistant components of fo rag es,
but a lesser role in the digest ion of fo rages w ith a
low lignocellulosic component[ 23] . This may be due
to the rapid clear ance of the fo rage digesta fr om the
rumen of for ag es w ith a low lignocellulo sic compo-
nent .
2. 3 Protozoa
Because cell w al l dig est ion in the rumen w as
found to decr ease when protozoa w ere absent
[ 25]
and fibrolyt ic enzyme act ivit ies have been detected
in protozo a[ 26] , protozoa are believed to contribute
signif icantly to the digest ion of plant cell w alls.
Studies to further def ine their cont ribut ion have
been hindered by dif ficult ies in culturing protozoa.
3 Cell wall degradation by rumen mi-
crobes
3. 1 Fibrolytic enzymes
T he major f ibro lyt ic enzymes secreted by ru-
men m icrobes w hich hydro lyse forag e cell w al ls are
listed in T able 2[ 26, 27] . The polysaccharide hydro-
lases and their encoding genes in micr obes have
been review ed in detail by Warren
[ 28]
.
Plant cell w alls contain cellulo se, pectic poly-
saccharides and a range o f non-cellulosic po lysac-
charides. Rumen m icrobes usually produce mult-i
ple enzymes to deg rade these materials. Cellulo lyt-
ic enzymes ar e of ten act ive against xy log lucans,
and specif ic x ylo glucan-deg rading enzymes are
commonly co-produced by celluloly tic r umen m-i
cr obes
[ 29]
. Due to the insolubility of cellulose and
xylo glucans, these f ibro lyt ic enzymes must be e-i
ther ex t racellular and free in the solution or on the
cel l surface. As a consequence, f ibro lyt ic enzyme
systems can be div ided into cell associated o r ex t ra-
cellular classes [ 7] .
Ex tr acellular enzymes, non-complexed cellu-
lase systems [ 29] , are synthesized and secr eted sepa-
rately
[ 7]
, but appear less impor tant in the r umen
than cellulase enzymes present in cellulosomes.
T hr ee major types o f enzymat ic act iv it ies o ccur: a)
endog lucanases including ( 1→4)-B-D-g lucan-4-g lu-
canohydr olases, b) ex oglucanases, including ( 1→
4)-B-D-g lucan g lucohydrolases ( cellodext rinases)
and ( 1→4)-B-D-g lucan cellobiohydrolases ( cello-
biohydro lases ) , c ) b-g lucosidases or b- glucoside
388
第 4期 SUN Xue- zhao et al: Forage cel l w alls : Ruminal degradat ion
glucohydro lases [ 29] . The endoglucanases hydro lyse
internal amorphous reg ions of cellulose chains,
generat ing reducing and non-reducing ends o f ol-i
g osaccharides o f various lengths. Exoglucanases
at tack the reducing or non-reducing ends in a pro-
gr essive manner , liber at ing g lucose ( glucohydro-
lases) o r cel lobio se ( cellobiohydr olases) . Exog lu-
canases hydro lyse cry stalline cellulose, w hereas b-
g lucosidases hydro lyse so luble cellodex t rins and
cel lobiose to glucose[ 29] .
T able 2 Major enzyme activ ities present in the rumen and required for the hydrolysis of plant cell w all po lymers
Wall polymer T arget bond for hyd rolysis E nzyme effect ing hydrolysis
Cel lulose
Cel lulose ( 1→4)-b-glu cose lin kage Endo- ( 1→4)-b-glucanase
Cel lulose ( n on-reducin g end) ( 1→4)-b-glu cose lin kage Exo- ( 1→4)-b-glucanase
Cel lobiose ( 1→4)-b-glu cose lin kage ( 1→4)-b-glu cosidase
Soluble cellooligomers ( 1→4)-b-glu cose lin kage Cel lodext rinase
Cel lulose or xylan ( 1→4)-b-glu cose lin kage or xylose link age Xylocellulase
Pect ic polys accharides
H om ogalacturonan ( 1→4)-a- galactur onide linkages Pectate lyase
Methy-l esterified HG ( 1→4)-a- galactur onide linkages Pect in lyase
Oth er non- cel lulosic polysaccharides
Xylan ( 1→4)-b-xylose link age Endo- ( 1→4)-b-xylanase
Xylob iose ( 1→4)-b-xylose link age ( 1→4)-b-xylos idas e
Arab inoxylan a-Araf-( 1→3)- xy-l linkage Arab inofu ranos idas e
Glucur on ox ylan 4- O-methy-l a- D-glucu ronic acid-( 1→2)-a-D-xylos e linkage Glucur on idas e
Acetylxylan Acetyles ter bond O-Acetyl xylan esteras e
Ferul ic acid Feru loyles ter bond Ferul ic acid esterase
p-C oumaric acid p- Coumaryl ester b on d or linkage p-C oumaric acid esterase
Adapted f rom White et al[27] . , an d Wang and M cAllis ter [ 26]
In addit ion to ex t racellular enzymes, comple-
xes o f cel lulases occur as cellulosomes. T hese com-
plexes include both catalyt ic and carbohydr ate-
binding modules ( CBM s) , bringing the catalyt ic
domain close to the insoluble cellulose, so facilita-
t ing cellulo se hydro lysis and sho rtening the dis-
tance for the ho st bacter ia to take up hydroly sis
products. Carbohydr ate-binding modules are par-
t icularly important for the init iation and processing
of exoglucanases [ 29] .
The deg radat ion of pect ic poly sacchar ides in-
volves three major enzymes, polygalacturonate lyase,
po lygalacturonase and pect in methyester ase[ 27] .
The degradat ion o f other non-cellulosic po ly-
saccharides requires a range of enzymes. Fo r exam-
ple, enzymes fo r the deg radation of heterox ylans
include endo- ( 1→4)-B-xy lanase, exo- ( 1→4)-B-xy-
lanase, ( 1→4)-B-xy losidase, A- L-arabinofuranos-i
dase, A-g lucuronidase, O-acetyl x ylan esterase and
ferulic acid esterase[ 27] .
3. 2 Cel-l wal l polysaccharide degradation
Var ious models of cel-l w all poly saccharide
deg radat ion in the r umen have been proposed
[ 30]
.
L ignificat ion has been taken into considerat ion in
almost all of these models. Chesson[ 31] proposed a
mechanist ic model ( F ig. 1) of w all po lysaccharide
deg radat ion for for ag es w ith lignified cell w alls
based on know ledge that the residue composition of
barley st raw af ter degradat ion w as similar to the o-
riginal substrates before deg radat ion
[ 32]
. T he deg-
radat ion of cell w alls is a process of er osion of the
surface of cell w alls and all wall components are re-
leased at the same rate.
A model for the degr adation of non- lignif ied
primary w alls has not been proposed. Chesson et
al[ 33] found that the degradat ion r ates of different
polysaccharides in the mesophy ll and epidermis iso-
lated f rom ryegr ass did not differ. A study on the
macer at ion of w hite clover during incubation w ith
the pectinolyt ic bacterium L achnosp i r a mul tiparus
suggested that pect in hydrolysis was the init ial
step in degradat ion[ 4] . Sun[ 3] found that pectic po-l
ysaccharides of cel l w alls f rom forage chico ry ( Ci-
chorium inty bus L. ) w ere deg raded faster than
other w all poly sacchar ides.
389
草 地 学 报 第 15卷
F ig. 1 A model of for age cell wall deg radation ( from Chesson and Fo rberg [7] ) . A ) depicts the initial stag es o f co loniza-
t ion and deg radation, C) the f ina l stages when deg radation has effectiv ely ceased. Open boxes represent str uctur al polysaccha-
r ides not associated w ith lignin and the tw o forms o f closed boxes, lignin-carbohydrate com plexes with differ ent proper ties and
distr ibution. Dots represent the cell w all deg rading enzymes produced by the bacterium.
4 Effects of cel-l wall characteristics
on ruminal cel-l wall degradation
The rumen m icrobial ecosystem is pow erful
and sophist icated for the digest ion of plant cell
w al ls. How ever , the poly sacchar ides of cell w alls
ing ested by ruminants into the r umen are almost
never completely deg raded. T his results from sev-
eral lim itat ions 1) plant structure and composition,
w hich regulate bacterial access to nutrients, 2 )
bacteria that cont rol the development o f dig est iv e
microbial consort ia, and adhesion and hydroly sis
by complexes o f hydroly tic enzymes secreted fr om
adherent bacter ia, and 3) the animal, w hich in-
creases the availability o f nut rients through mast-i
cation, salivat ion and dig esta kinetics[ 34, 35] . How-
ever, only plant st ructur e and composit ion ar e
w ithin the scope o f this review .
4. 1 Plant anatomy and cell types
T he parts of forag e plants that ruminants con-
sume, such as internode, sheath and leaf in g rasses
and leaf lamina, midrib, pet iole and stem in leg-
umes, ar e composed of a w ide r ange of cell types.
T he w alls o f each cell type have their unique char-
acterist ics, for ex ample, the thickness o f w all, lig-
nif ied o r non- lignif ied. T hese lead to dif ferences in
the cell w all degradat ion of each cell type ( T able
3) . Half the variat ion in overall cell wall degrad-
ability among cult iv ar s w as found to be der iv ed
f rom differences in plant mo rpholo gy
[ 31]
. T he in-
cr eased deg radability of leaves in comparison w ith
stems may result from higher proport ions o f cell
types w ith thin and non- lignif ied w alls, such as
mesophy ll, and less propor tions of cell types w ith
thick or lignified walls, such as scler enchyma,
bundle sheath and xy lem cells, in leaves than in
stems
[ 36]
.
Table 3 The relative dig est ibility o f for ag e cell types/ tissues
Cell type/ ti ssue Digest ibilit y Characterist ics Dis trib ut ion
Mesophyll H igh T hin wall, no lignin loos ely ar ranged in legumes an d C3 gras ses
Parenchym a Moderate to high
H ighly diges tible wh en imma-
tu re
In midrib of grass an d main vein of legume leaves, leaf
s heath, and s tem of g ras ses , and petiole and stem of
legumes
Collenchym a Moderate to high T hick w al l, n ot lignif ied In legume leaves and t issu e
Parenchym a bundle
sheath
Moderate to high
Wall m oderately thick and
w eakly lignif ied
Su rrounds vascular t issue in C4 leaf b lades
Phloem f ibre Moderate Often is n ot lignif ied In legume petioles and s tems
Epidermis Low to high
Outer wall thicken ed, some-
tim es lign ified
Xylem None
Both prim ary and secondary x y-
lem lign ified and thick ened
Major cont ributor to indigest ible f ract ion.
Sclerenchym a None to low T hick, lignif ied w all U p to 1200 Lm lon g and 5-20 Lm in diam eter
Cut icle non e
Adapted f rom Bux ton & Redfearn[ 37] ; Ch eng et al. [ 34] ; J ung & Engels[ 38] ; and Jung & Engels[ 39]
390
第 4期 SUN Xue- zhao et al: Forage cel l w alls : Ruminal degradat ion
Physical and st ructural barriers may limit f ibr e
degradat ion also. T he cutin layer is not completely
degradable by rumen m icrobes, part icular ly bacte-
r ia, except some r umen fung i w hich may penetrate
it, and silica ex ist ing in the st ructur e enhances the
resistance of the cut icle to r upture
[ 34]
. Waxes and
the cut icle on the surface of epidermis prevent ru-
men bacteria from at tacking forag e t issues[ 40] . A
ring of thick lignif ied w alls in both g rass and leg-
ume stems is resistant to deg radation
[ 22]
.
4. 2 Wall surface area
The degradat ion of forag e cell w alls r equires
rumen bacterial at tachment [ 41] . T her efore, the cell
w al l surface area available for adhesion is impor-
tant. T he w all sur face area is inf luenced by cell
w al l thickness and part icle size.
A) Cell wal l thickness. Wilson and M ertens
( 1995) [ 4 2] not iced that less than 20% o f thick-
w al led cells can be dig ested in the r umen, since the
rate at w hich part icles leave the rumen is faster
than the dig estion rate. Cell w all thickness influ-
ences the rat io o f cell w all surface ar ea to total cell
w al l volume ( SA/ CWV) . T he SA/ CWV decreases
as cell w all thickens. By calculation, the m inimum
time to degr ade the half thickness o f w alls expo-
nent ially increases w ith orig inal w all thickness
[ 12]
.
B) Particle size. Cell wall dig est ibility is im-
proved af ter the physical or ganizat ion o f cells is
disr upted[ 39] , since the w all surface area w ill in-
crease as part icle size decreases. Reducing the par-
t icle size o f lucerne and maize stems to the level o f
indiv idual cells by bal-l m illing increased the de-
g radability of x ylan
[ 43]
.
4. 3 Lignification
Lignin is well know n fo r it s interfer ence in
fo rag e cell w all deg radat ion by rumen micr obes. A
decrease in the extent o f cell w all degradat ion re-
sulting f rom lignin is due to the fo rmat ion of a
physical barrier prevent ing bacterial access and
crosslinking of lignin with polysaccharides. Reducing
the proportion of lignified cells or selecting cult ivars
containing part ially degradable lignin could improve
w all digestibility[ 37] .
Phenolic-carbohydrate complex ( PC-LCC) is a u-
nit of the lignified secondary cell walls. Phenolic-car-
bohydrate compounds may be toxic to fibre-degrading
bacteria
[ 42]
. Even free phenolic acids and soluble phe-
nolic-carbohydrate complexes prevent microbial adhe-
sion and inhibit microbial activit ies[ 26] . Therefore, the
ruminal degradation of underlying cell w alls has been
found to be physically and biochemically prevented by
PC-LCC complexes.
L ignificat ion can cause a st ructural bar rier a-
round highly lignified middle lamel la and primary
w alls in legumes. T his middle lamella-pr imary w all
str ucture ( M L-PW ) may limit cell w all deg rada-
t ion of thick-w alled cells such as lignified scleren-
chyma
[ 12]
. L ignif ied middle lamella and primary
w alls ar e indigest ible[ 12] . This means that these
cel ls have to be disr upted for rumen microbes to
access cell w al ls fr om w ithin the cell cavity[ 42] .
How ever, due to lar ge par ticle sizes, access to the
cel l cavity is lim ited. Disrupt ion of these cells can
be achieved by mast icat ion[ 42] o r fungal penet ra-
t ion
[ 44]
.
5 Conclusion
A range of celluloly tic rumen bacteria are in-
volv ed in the deg radat ion of forage cell w alls. Fun-
gi are also involved, but are considered less impo r-
tant . Rumen bacteria secr ete mult iple enzymes to
hydroly se various po lysaccharide linkages in forage
cel l w alls. Fibroly t ic enzyme systems can be divid-
ed into cell associated or ext racellular classes. Ac-
co rding to Chesson. s mechanist ic model, the deg-
radat ion of lignif ied g rass cel l w alls is a process of
erosion of the surface of cell w alls and all po lysac-
charides are released at the same rate. How ever,
for the cell w alls of legumes and other dicoty ledon-
ous fo rages, polysaccharides are differ ent ially de-
gr aded.
Wall surface area, as inf luenced by cell w all
thickness and par ticle size, and lignif icat ion have
effects on w all deg radation. Since w all char acteris-
t ics, such as w all thickness and lignificat ion, va-
391
草 地 学 报 第 15卷
r ies among cell types, w all digest ibility greatly va-
r ies f rom one cell type to ano ther. Because o f
dif ferences in the proport ions of cell types betw een
stems and leaves, leaves tend to be more readily
digested than stems.
Understanding the interact ion betw een rumen
bacteria and forage cell w alls is key to the improve-
ment o f rum inant dig estion.
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