Cell‐cycle regulation of hydroxyproline‐rich glycoprotein HRGPnt3 gene expression during the initiation of lateral root meristems

Abstract

SummaryAnalysis of transgenic tobacco plants containing a tobacco hydroxyproline‐rich glycoprotein HRGPnt3 gene promoter‐β‐glucuronidase (GUS) gene fusion (HRGPnt3‐uidA) showed that this promoter is active not only in the early stages of initiation of lateral roots as previously described, but also in the initiation of adventitious roots, with similar selective expression in a subset of pericycle cells. HRGPnt3 is also induced during initiation of hairy roots following transformation by Agrobacterium rhizogenes. The auxin indole acetic acid (IAA) induces an increase in the number of characteristic discrete sites of HRGP‐nt3 expression. It is shown that these sites are destined to form new root primordia from pericycle cells of both adventitious and main roots. Dose‐dependent induction of root meristems by auxin overcomes the limitations of this naturally stochastic process and makes lateral root initiation amenable to biochemical analysis. Quiescent pericycle cells, which are developmentally arrested in the G2 phase of the cell cycle, rapidly progress into M phase upon mitogenic stimulation. Colchicine and nocodazole, which block completion of mitosis, inhibited the activation of the HRGPnt3 promoter but did not block auxin induction of parA, a marker for de‐differentiation in leaf mesophyll cell‐derived protoplasts. Hydroxyurea, which inhibits cell‐cycle progression at the G1/S‐phase transition and also blocks lateral root initiation, did not inhibit HRGPnt3 induction. Thus, HRGPnt3 induction precedes completion of the first cell division during primordium formation, and is one of the initial steps in a sequential program of gene expression activated upon stimulation of cell division for the development of a new meristem during lateral root initiation.