Recent Insights into Auxin-Mediated Molecular Cross Talk Events Associated with Regulation of Root Growth and Architecture During Abiotic Stress in Plants

Abstract

Plant roots exhibit a plethora of signalling mechanisms operative during abiotic stress (drought, salinity, heavy metal and temperature). Among various primary growth regulators or phytohormones, auxins exert primary influence in the polar and lateral growth of primary roots. Auxin-induced regulation of growth and morphogenesis in plants mostly operates through the mechanisms of altered gene expression. Extensive investigations in the recent past have revealed the modulatory role of auxin in regulating plant growth during abiotic stress. To summarize the current understandings of auxin action on root growth it is necessary to understand the functioning of auxin transporter proteins and auxin responsive elements (AUXRE, PGP, and PIN, GH3, Aux/IAA and SAUR) operative under stressful situations. Implications of microarray and RT-PCR approaches have revealed the spatio-temporal differentiation of auxin responsive elements in root tissues of major crop plants. Regulation of ZIFL1 (superfamily transporter) gene expression is mediated by alternate splicing which further regulates stress-induced auxin transport functions. Furthermore, auxin transporter gene transcripts undergo modulation by hormonal crosstalk during drought and salinity stress in rice. Remodelling of root architecture is caused by altered distribution of PIN family auxin efflux proteins during abiotic stress. Various external cues like nitrogen, potassium and soil pH associate with auxin-mediated control of abiotic stress-induced root growth. Plant tolerance to heavy metal, temperature and salinity stress seems to be operative through resuming of polar auxin gradient (AUX1) in shoot and root tissues. During abiotic stress conditions nitric oxide (NO) and reactive oxygen species (ROS) function as major regulators of AUX/IAA elements in roots. Elevated auxin levels have been associated with regulation of various components of cell signalling and metabolism. Auxin, brassinosteroids and ABA seem to exert synergistic actions towards the regulation of auxin-mediated root growth during abiotic stress.