The primary cell wall of dicotyledenous plants is comprised of approximately equal parts cellulose, cross-linking glycans, and pectin, with a relatively minor fraction of protein. The role that pectin may play in regulating cell elongation was studied in two systems in Arabidopsis seedlings; the primary root with its well-defined zone of elongating cells, and the hypocotyl that grows almost exclusively by cell elongation. A collection of mutants affected in root (cob-1, cob-3, lit-1 (rsw2), qui-1 (prcl-9), sab-1, shr-2) and hypocotyl (gal-3, gai) elongation was used to identify altered pectin phenotypes that correlated with the growth phenotypes. FT-IR microspectroscopy was used to show that roots and hypocotyls affected in cell elongation have pectin with lower degree of esterification (DE) values compared to wild-type. This was quantified and confirmed by chemical assays. DE is unchanged where cell elongation is increased by GA- and dark-induced growth, suggesting that a threshold of DE may exist that is permissive for growth, but below which reduced values of DE correlate with reduced elongation. The cob-1 mutant was studied in more detail, and found to have very low DE values in the region of greatest cellulose deficiency, together with thickened walls and other compositional changes. Other cellulose-deficient mutants (qui-1, lit-1) were similarly affected, suggesting that enrichment of the wall with pectin of low DE is used to conserve wall strength. Analysis of GA-deficient mutants gave some evidence that galactan side-chains, and to a lesser extent arabinan sidechains of RG-I may be required throughout the wall for elongation, and that the outer wall of roots and hypocotyls may be growth-limiting. Thinning of the cell wall correlates with hypocotyl length, suggesting that biosynthesis does not keep pace with elongation. Finally, root and hypocotyl cell walls have different DE values, suggesting that the properties of pectin in these systems may vary with different wall composition. Various mechanisms by which pectin DE may become limiting to the rate of cell elongation are proposed. DE is regulated by pectin esterases, which in Arabidopsis comprises a large gene family. To test the hypothesis that pectin esterification is limiting to cell elongation, four transposon insertion mutants in putative pectin esterases were screened for cell elongation and wall phenotypes, with the prediction that higher DE values in the mutant walls might result in increased growth. Under various conditions in the different mutants, DE was both raised and lowered and cell elongation promoted and inhibited. To avoid problems of genetic redundancya nd the absence of functional characterisation in this family, a PME with demonstrated biochemical activity from Aspergillus aculeatus was over-expressed in Arabidopsis and 30 transgenic lines generated. However, the phenotypes of these plants have not been analysed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:251441 |
| Date | January 2002 |
| Creators | Derbyshire, Paul |
| Publisher | University of East Anglia |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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