Mungbean tonoplastic intrinsic protein gene (VrTIP1;1) regulates drought and salinity responses in Arabidopsis

Abstract Aquaporins are major players controlling water status across membranes in plants. Through this work we report the functional characterization of a group 1 tonoplast intrinsic protein (TIP) gene from mungbean (Vigna radiata (L.) R. Wilczek), VrTIP1;1, in Arabidopsis to evaluate its role in drought and salinity associated osmotic stress. The Arabidopsis plants overexpressing VrTIP1;1 depicted increased germination rate than the wild type (WT) under stimulated dehydration and salinity induced by mannitol and NaCl respectively. Though the roots of both WT and the transgenic lines showed root growth suppression under stress condition the root lengths were markedly higher with respect to the transgenic lines. The VrTIP1;1 overexpressing lines showed higher retention of water and chlorophyll, along with lower lipid peroxidation and proline accumulation under both drought and salinity. The antioxidant enzyme activities of glutathione reductase (GR) and catalase (CAT) indicated an enhanced oxidative defense capacity in the transgenic lines, whereas the transcript abundance of the antioxidant genes CAT and SOD was lower compared to WT. This divergence between transcript levels and enzyme activity may arise from post transcriptional regulation or differences in protein turnover. Similarly, the transcript levels of key stress responsive transcription factors (DREB1A, DREB2A, and myb60) were reduced in the transgenic lines under stress, while their basal (non-stress) levels were comparable to WT. We interpret this pattern as reflecting reduced perception of stress in VrTIP1;1 overexpressing plants: because TIP1;1 improves cellular water balance (higher RWC, maintained chlorophyll, and lower oxidative damage), the plants experience a smaller osmotic disturbance and therefore do not require strong induction of emergency stress responsive transcriptional programs. To have a better visualization into the survivability of the transgenics in stress conditions, soil stress assay was conducted. Following 10 days of water withdrawal, the transgenic lines retained better physiological integrity than WT plants, as confirmed by visible and infrared imaging. The WT plants, being severely wilted and near death, exhibited cooler canopies reflecting background soil temperature, whereas transgenic plants retained active tissue capable of emitting detectable infrared radiation. Similar responses were observed under salinity stress.