DIFFERENTIAL ACTIVATION OF THE PRIMARY AUXIN RESPONSE GENES, PS-IAA4 5 AND PS-IAA6, DURING EARLY PLANT DEVELOPMENT/

The plant growth hormone auxin typified by indoleacetic acid (IAA) tra nscriptionally activates early genes in pea, PS-IAA4/5 and PS-IAA6, th at are members of a multigene family encoding short-lived nuclear prot eins. To gain first insight into the biological role of PS-IAA4/5 and PS-IAA6, promoter-beta-glucuronidase (GUS) gene fusions were construct ed and their expression during early development of transgenic tobacco seedlings was examined. The comparative analysis reveals spatial and temporal expression patterns of both genes that correlate with cells, tissues, and developmental processes known to be affected by auxin. GU S activity in seedlings of both transgenic lines is located in the roo t meristem, sites of lateral root initiation and in hypocotyls undergo ing rapid elongation. In addition, mutually exclusive cell-specific ex pression is evident. For instance, PS-IAA4/5-GUS but not PS-IAA6-GUS i s expressed in root vascular tissue and in guard cells, whereas only P S-IAA6-GUS activity is detectable in glandular trichomes and redistrib utes to the elongating side of the hypocotyl upon gravitropic stimulat ion. Expression of PS-IAA4/5 and PS-IAA6 in elongating, dividing, and differentiating cell types indicates multiple functions during develop ment. The common and yet distinct activity patterns of both genes sugg est a combinatorial code of spatio-temporal co-expression of the vario us PS-IAA4/5-like gene family members in plant development that may me diate cell-specific responses to auxin.