STUDIES ON ROLES OF GENES INVOLVED IN CELL-WALL CONSTRUCTION DURING PLANT MORPHOGENESIS

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To gain insight into the functional diversity of the XTH (xyloglucan endotransglucosylase/hydrolase) gene family, we analyzed their expression profiles and obtained the results as followes :1) The two azuki bean VaXTH genes exhibit essentially similar but different tissuespecific expression profiles. Whereas VaXTH1 is expressed in xylem cells in the basal part of the internode, little or no expression of VaXTH2 is found in the xylem. Furthermore, they exhibit spatially divergent expression profiles along the internode, VaXTH1 being expressed nearer to the top of the internode than VaXTH2Auxin up-regulates both of the mRNA levels, however, this effect of auxin on the VaXTH1 gene is nullified in 0.25 M mannitol, which prevents cell expansion without affecting auxin action per se. On the other hand, the auxin-induced up-regulation of the VaXTH2 gene is not affected by mannitol.2) We examined expression profiles of the 33 Arabidopsis AtXTH genes, using quantitative real-time RT-PCR, and revealed that most members exhibit distinct expression profiles in terms of tissue specificity and responses to hormonal signals, with some members exhibiting similar expression patterns. By comparing the flanking sequences of individual genes, we identified four sets of large-segment duplicatoins and two sets of solitary gene duplications.3) We have conducted promoter analysis of the AtXTH17, 18, 19 and 20 and identified cis-acting regions responsible for root specific expression profiles.The results obtained, thus far, support the idea that each member of the XTH gene family is regulated specifically by different sets of plant hormones, and is committed to a certain specific process in a specific tissue, at specific stages of development. Based on these considerations, we advance a hypothesis that the cell wall in a certain cell-type is constructed, maintained and restructured by a series of collaborative actions of a set of enzymes characteristic to the cell-wall type.