Auxin/gibberellin interactions in pea leaf morphogenesis

Abstract

Pea (Pisum sativum) has been an important model plant for several generations of plant physiologists, geneticists and developmental biologists. There exists an extensive body of knowledge on the genetics of many developmental processes including gibberellic acid (GA) biosynthesis and GA and auxin interactions during stem elongation. Auxin is a morphogen and is transported from cell to cell via vesicle-mediated secretion from sites of synthesis. This creates auxin concentration gradients, which can regulate genes controlling morphogenesis. Among the genes regulated are those of GA biosynthesis and this subsequently sets up secondary, parallel gradients of GA concentration. Both these hormones have been proposed to play roles during leaf development in other plant species. The question posed is whether auxin/GA interactions control leaf morphogenesis in pea. It was addressed by (1) looking at effects of auxin and GA inhibitors on leaf development, (2) attempting to rescue leaf form mutants by hormone application and (3) looking for genes expressed during leaf development that might be regulated by auxin and GA. Auxin and GA inhibitors produce common abnormalities during pea leaf development of wildtype (WT) cultured plantlets, which include: inhibition of leaf initiation, reductions in the number of pinna pairs produced and conversion of terminal tendrils into leaflets. Both GA and auxin rescued the tendrilled acacia (uni-tac) mutant by (1) increasing the number of pinna pairs produced and (2) converting terminal leaflets into tendrils. The Uni gene was transcriptionally up-regulated by auxin in shoot tips of cultured WT plantlets. Stable mRNA levels of three genes (PsPIN1, LE and IAA 4/5) known to be auxin-regulated also increased. These results suggest that both auxin and GA are involved in leaf initiation, promote leaf tip growth, and function in gradients and at multiple levels during pea leaf morphogenesis. © 2006 The Linnean Society of London.