What do transgenic plants tell us about the regulation and function of cell-wall structural proteins like extensins?

Plant primary cell walls appear as complex structures composed of two inter connected networks embedded in a pectin matrix. The first network is that o f cellulose microfibrils wrapped in and cross-linked with xyloglucans. The second network constitutes structural proteins connected by intermolecular cross-links. Extensins constitute a major class of structural proteins and have been classified according to the content of characteristic amino acid motifs: Ser-(Pro)(4) repeats, Tyr-X-Tyr-Lys motifs involved in the formatio n of intramolecular isodityrosine cross-links, and X-Pro-Val-Tyr-Lys motifs potentially forming intermolecular cross-links. Several extensin genes wer e cloned, and transgenic plants were used as tools to characterize their pa tterns of expression and to define tissue-specific or stress-responsive reg ulatory elements. Major contributions of such studies were to confirm that all extensin genes are regulated in different ways. Moreover, extensin gene s are not expressed in all plant cells: they may be expressed in root or st em phloem cells, in cells under mechanical constraints, in response to woun ding, and in cells proliferating under hormone control. Promoter deletion s tudies allowed us to define regulatory domains using both histochemical det ection of GUS (beta-glucuronidase) activity driven by extensin promoter fra gments and fluorometric analyses. Common regulatory elements remain to be d etermined. Finally, functional approaches were tentatively performed. Howev er, transgenic plants underexpressing two different extensins did not show any clear phenotype confirming that extensins are not required in all plant cell walls and suggesting that other structural proteins may replace exten sins where cell-wall reinforcement is essential.