CELL-CYCLE REGULATION OF HYDROXYPROLINE-RICH GLYCOPROTEIN HRGPNT3 GENE-EXPRESSION DURING THE INITIATION OF LATERAL ROOT-MERISTEMS

Analysis of transgenic tobacco plants containing a tobacco hydroxyprol ine-rich glycoprotein HRGPnt3 gene promoter-beta-glucuronidase (GUS) g ene fusion (HRGPnt3-uidA) showed that this promoter is active not only in the early stages of initiation of lateral roots as previously desc ribed, but also in the initiation of adventitious roots, with similar selective expression in a subset of pericycle cells. HRGPnt3 is also i nduced during initiation of hairy roots following transformation by Ag robacterium rhizogenes. The auxin indole acetic acid (IAA) induces an increase in the number of characteristic discrete sites of HRGPnt3 exp ression. It is shown that these sites are destined to form new root pr imordia from pericycle cells of both adventitious and main roots. Dose -dependent induction of root meristems by auxin overcomes the limitati ons of this naturally stochastic process and makes lateral root initia tion amenable to biochemical analysis. Quiescent pericycle cells, whic h are developmentally arrested in the G(2) phase of the cell cycle, ra pidly progress into M phase upon mitogenic stimulation. Colchicine and nocodazole, which block completion of mitosis, inhibited the activati on of the HRGPnt3 promoter but did not block auxin induction of parA, a marker for de-differentiation in leaf mesophyll cell-derived protopl asts. Hydroxyurea, which inhibits cell-cycle progression at the G(1)/S -phase transition and also blocks lateral root initiation, did not inh ibit HRGPnt3 induction. Thus, HRGPnt3 induction precedes completion of the first cell division during primordium formation, and is one of th e initial steps in a sequential program of gene expression activated u pon stimulation of cell division for the development of a new meristem during lateral root initiation.