Control of adventitious root formation in Arabidopsis

Abstract

Adventitious or de novo root organogenesis is a process that occurs from wounded or detached plant tissues or organs. In tissue culture experiments, the available hormone concentrations in the medium play significant roles in inducing adventitious roots. However, regeneration from detached organs in natural conditions depends on endogenous hormones. To imitate natural conditions, Arabidopsis thaliana Col-0 leaf explants were cultured on B5 medium without any added hormones, in order to investigate the endogenous hormonal signalling and molecular mechanisms that lead to de novo root organogenesis. Use was made of a series of hormone signalling reporter lines in transgenic Arabidopsis, to understand better the roles of auxin, cytokinin, ethylene and gibberellin signalling. Cell proliferation was monitored over a developmental time course, and the expression of a number of genes, and their functional roles through mutant analysis, was also investigated during the regeneration process. It was demonstrated that auxin, gibberellin and cytokinin signalling becomes focused at the wound site in the petiole, associated with the induction of adventitious roots. Auxin signalling-defective mutants such as axr1, axr3 and pls were unable to form adventitious roots as well as wild type, reflected in defective expression of auxin pathway genes such as YUC family genes and WOX5. pls and axr1 were also found to be defective in the expression of the transcription factor gene NAC1. Mutants and transgenic overexpression lines for transcriptional regulators RAP2.7, MDF1 and NAC1 showed that the three genes are required for adventitious root formation, and function in an auxin-independent manner to mediate root regeneration. Adventitious root formation from the Arabidopsis leaf therefore requires coordinated expression of a number of transcription factors that work in both an auxin-dependent and -independent manner, and cross talk between auxin and other hormones is important for correct organogenesis.