DIFFERENTIAL GENE-EXPRESSION IN RESPONSE TO AUXIN TREATMENT IN THE WILD-TYPE AND RAC, AN ADVENTITIOUS ROOTING-INCOMPETENT MUTANT OF TOBACCO

Histological analyses of auxin-treated cuttings from the wild type and the rac mutant of tobacco (Nicotiana tabacum cv Xanthii) previously r evealed that some rac phloem parenchyma or inner cortical parenchyma c ells form callus in response to exogenous auxin treatment but these ce lls never undergo the organized divisions associated with adventitious root initiation in the wild type. Here we report the effect of the ra c mutation on the temporal and spatial expression patterns of three ge nes previously shown to be associated with adventitious root meristems , HRGPnt3, iaa4/5, and gh3. Using histochemical staining analyses of H RGPnt3-GUS transformant cuttings, we determined that the rac mutation blocks auxin activation of the HRGPnt3 promoter. Thus, activation of t he HRGPnt3 promoter occurs specifically during adventitious root initi ation in tobacco cuttings. Histochemical staining analyses of iaa4/5-G US and gh3-GUS transformant cuttings revealed that the rac mutation do es not repress the auxin activation of the iaa4/5 and gh3 promoters. B ased on our histochemical staining analyses, we conclude that differen tial gene expression occurs in response to auxin treatment during adve ntitious root initiation in the wild type compared with callus formati on in rac cuttings. We also determined that HRCPnt3 mRNA accumulation occurs in response to components of our root-induction protocol other than auxin, indicating that HRCPnt3 expression is regulated both devel opmentally and environmentally.