Physio-Biochemical Characteristics of Root Cell Wall in Salinity Tolerance Mechanisms in Wheat

概要

（Format No. 13）

SUMMARY OF DOCTORAL THESIS
Name: Yang Shao
Title: Physio-Biochemical Characteristics of Root Cell Wall in Salinity Tolerance
Mechanisms in Wheat
（コムギの耐塩性機構としての根細胞壁の生理生化学的特性）
Wheat (Triticum aestivum L.) production has been severely affected by soil
salinization. There is need to improve wheat production in salinized soils. The
importance of studying salinity tolerance mechanisms in wheat cannot be
overemphasized. So far, wheat salinity tolerance has been studied from the aspects of
osmotic adjustment, membrane transportation, hormone regulation, signal
transduction, etc. However, studies on root cell walls were very limited. This study was
conducted to investigate the salinity tolerance mechanisms of wheat, focusing on the
function of root cell wall. The objectives of this study were to elucidate the interactions
of cell wall composition, extensibility, expansin expression, root extension, and root
growth under salinity stress and characteristics of root cell wall that contribute to root
growth under salinity. Two salt tolerant (JS-7, Xinchun-31 (XC-31)) and two salt
sensitive (YL-15, GS-6058) spring wheat cultivars were selected as experimental
materials. These cultivars were cultivated at 0 (control), 40, 80 and 120 mM NaCl
concentrations. When root length showed significant difference among the cultivars,
chemical compositions (pectin, hemicellulose I and II, cellulose, and uronic acid in each
composition), extensibility, expansin expression and apoplastic pH in apical root (0-10
mm) cell walls, as well as cation exchange capacity of the whole root were investigated.
The main results are described as follows:
1. Chemical compositions and properties of root cell wall in relation with root growth
under salinity stress
Cultivars of JS-7 and XC-31 had higher root growth under salinity stress
compared with YL-15 and GS-6058. This confirmed that the former is more tolerant to
salinity stress than the latter. Salinity stress significantly decreased the pectin content
in the elongation zone in all cultivars except JS-7. Hemicellulose I and II were
significantly increased in the elongation and adjacent zones in sensitive cultivars
under salinity stress. Similarly, the cellulose content increased significantly across the
cultivars in both root zones. This increment was more pronounced in the sensitive
cultivars than in the tolerant cultivars. The uronic acid content in pectin in the
elongation zone was decreased significantly in the sensitive cultivars relative to the
tolerant cultivars, conversely, the uronic acid content in hemicellulose showed a
reversed tendency. The cation exchange capacity of the root cell wall was significantly
lower in sensitive cultivars than the tolerant cultivars. A positive correlation existed
between root growth and relative content of pectin in elongation zone, and cation
exchange capacity of the whole roots. However, root growth and relative content of
cellulose were negatively correlated. These results indicate that a high pectin content
and cation exchange capacity, as well as low hemicellulose and cellulose contents in the
cell wall benefit root growth and thus, tolerance under salinity stress conditions.
2. Extensibility of root cell wall in relation with root growth under salinity stress

The extensibility of root cell wall was significantly decreased in sensitive cultivars,
whereas, that in tolerant cultivars was maintained at the same level as that in the
control. Root extension and the differences between cultivars were largely dependent
on elastic extension, which accounted for one-half to two-thirds of the total extension.
Viscosity and the plastic extension of the root cell walls had no difference across the
treatments and cultivars. The significant decrease in cell wall elasticity in the root
elongation region was one of the factors that depressed root growth in sensitive
cultivars under salt stress. The well-maintained elasticity of tolerant cultivars
alleviated the depression of root growth by NaCl. Cell wall elasticity was positively
correlated with the relative pectin and hemicellulose I contents and negatively
correlated with the relative cellulose content. Under saline conditions, the relative
hemicellulose II content was not altered in the sensitive cultivars; however, it
decreased significantly in the tolerant cultivars. Therefore, changes in chemical
composition of cell wall corresponded with the cell wall extensibility and root growth in
wheat cultivars at different levels of salinity tolerance.
Salinity decreased the root cell wall extension significantly, especially in sensitive
cultivars through an increased extension resistance, but, there were no significant
effects on the tolerant cultivars. The elastic properties of root cell wall of wheat under
salinity were more pronounced in root elongation as compared with the plastic
properties. The increment in pectin and hemicellulose I better improved the elastic
extension in the root cell wall, relative to the deposition of cellulose.
3. Specific expression of expansins in response to apoplastic pH under salinity stress
Salinity treatment significantly reduced apoplastic pH in apical root in both
tolerant and sensitive cultivars. The apoplastic pH in elongation zone was about 6.28
under non-saline condition, while it decreased to about 5.3 under salinity in both
cultivars. For the roots grown under the non-saline condition, the optimal pH for cell
wall extension was 6.0 and 4.6 in tolerant and sensitive cultivars, respectively. In
contrast, roots grown under salinity showed that the optimal pH for cell wall extension
was 5.0 in the tolerant and 6.0 in the sensitive cultivars. Therefore, the apoplastic pH
(5.3) under salinity was favorable to root extension in tolerant cultivars, but not in the
sensitive ones. Expansin gene expressions in root cell wall were generally suppressed
by salinity. Gene expressions of TaEXPA3, TaEXPA6, TaEXPB1 and TaEXPB10 were
reduced in both cultivars under salinity stress. However, those of TaEXPA5 and
TaEXPA8 in tolerant cultivars were increased under salinity. This increment may
improve root extension under salinity. The expansin activity of the tolerant cultivar
was significantly higher than that of the sensitive one. TaEXPA8 mediated cell wall
loosening especially at pH 5.0, whereas, TaEXPA5 activated especially at pH 6.0.
Under salinity stress with lowered apoplastic pH, expansins in the tolerant cultivar
resulted in the maintenance of cell wall extensibility, whereas those in the sensitive
cultivars had no such activity.
This study investigated the root cell wall from the aspects of chemical composition,
physical property and expansin expression. Each component of the root cell wall has its
own effect on root extension. The extension of root cell wall under saline conditions,
with reduced turgor pressure, was adversely depressed in the sensitive cultivars, but
maintained to some extent in the tolerant ones. The wall loosening corresponded to the
elastic extension, which involved wall expansins and all other components. When the
cell wall loosens, new wall materials fill in the space or bind to the old wall. These
materials correspond with the plastic nature, i.e. the final elongation of the root. The
present study revealed the regulation role of cell wall in root growth. Cultivar
differences in salinity tolerance can be related to the property of root cell wall.
Characteristics of root cell wall such as higher amount of uronic acids and pectin, lower
amount of cellulose, and specific expansin expression were of importance for root
extension and growth under saline stress.

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