Evidence for nitrate reductase expression during initiation of lateral roots by NAA in chicory

The supply of -naphthaleneacetic acid(NAA), to excised chicory roots induced the formation of lateral rootmeristems mainly localized proximal to the pre-existing apical rootmeristem, in a region which does not initiate any lateral roots in controlconditions. Inhibition of root elongation and concomitant enlargement ofthe apices were also observed. Quantification of NAA and cytokinin levelsshowed that the most reproducible and significant changes occurring afterthe NAA treatment consisted of a decrease in the level ofzeatin-O-glucoside conjugates. Hydrolysis of theseconjugates might deliver free zeatin-type compounds which were consumedduring the lateral root growth. After 5 d, control excised roots containeda high level of amino acids, mainly as asparagine and arginine, probablyissued from proteolysis associated to a senescent-like process. Conversely,in the presence of NAA, neither accumulation of amino acids nor a decreaseof the total protein content of the tissue could be detected. Newlyinitiated meristems expressed the nia gene whichencodes nitrate reductase, the first enzyme of the nitrate assimilatorypathway. Thus the increased expression of nitrate reductase which wasobserved in excised roots of chicory supplied with NAA (Vuylstekeret al., 1997b) may be ascribed to lateral rootformation and development. The reinduction of nitrate reduction activitywas driven by the increased demand for reduced nitrogen. Thus, thenia gene is one of the genes expressed during theearly stages of root meristem formation.Keywords:Auxins, chicory, in situ hybridization,lateral root, nitrate reductase.