Differential regulation of Ku gene expression in etiolated mung bean hypocotyls by auxins

Plant Ku genes were identified very recently in Arabidopsis thaliana, and their roles in repair of double-stranded break DNA and maintenance of telomere integrity were scrutinized. In this study, the cDNAs encoding Ku70 (VrKu70) and Ku80 (VrKu80) were isolated from mung bean (Vigna radiata L.) hypocotyls. Both genes were expressed widely among different tissues of mung bean with the highest levels in hypocotyls and leaves. The VrKu gene expression was stimulated by exogenous auxins in a concentration- and time-dependent manner. The stimulation could be abolished by auxin transport inhibitors, N-(1-naphthyl) phthalamic acid and 2,3,5-triiodobenzoic acid implicating that exogenous auxins triggered the effects following their uptake by the cells. Further analysis using specific inhibitors of auxin signaling showed that the stimulation of VrKu expression by 2,4-dichlorophenoxyacetic acid (2,4-D) was suppressed by intracellular Ca²⁺ chelators, calmodulin antagonists, and calcium/calmodulin dependent protein kinase inhibitors, suggesting the involvement of calmodulin in the signaling pathway. On the other hand, exogenous indole-3-acetic acid (IAA) and α-naphthalene acetic acid (NAA) stimulated VrKu expression through the mitogen-activated protein kinase/extracellular signal-regulated kinase pathway. Altogether, it is thus proposed that 2,4-D and IAA (or NAA) regulate the expression of VrKu through two distinct pathways.