Auxin is Involved in Lateral Root Formation Induced by Drought Stress in Tobacco Seedlings

Drought stress is a common abiotic stress that affects plant root development. The goal of this study was to identify the role of auxin on aspects of root growth under water-deficient conditions in Nicotiana tabacum L. In this study, time-course development of lateral root (LR) growth in response to water deficiency (2.5% polyethylene glycol 6000, PEG) was analyzed in tobacco seedlings. LR formation in tobacco plants in response to drought stress depended on the experimental duration. First- and second-order LR formation was induced, respectively, by 43 and 28% on day 5 and by 57 and 40% on day 7 relative to control treatment. Thereafter, PEG-induced LR formation was not observed, and a decrease of 16% was recorded in first-order LR formation on day 11. A root pruning study suggested that PEG-induced LR formation plays a pivotal role in maintaining tobacco plant growth during drought stress. The change in the indole-3-acetic acid (IAA) concentration in leaves and roots was consistent with that of LR formation. This was corroborated by elevated DR5::GUS expression levels on day 5 and by decreased expression levels on day 9. Application of N-1-naphthylphthalamic acid (NPA) to PEG-treated seedlings reduced LR formation to levels similar to those of the control treatment; DR5::GUS expression was also similar between these treatments after NPA application. Results were similar following alpha-naphthylacetic acid application to control-treated seedlings. Measurement of [H-3]IAA transport indicated that polar auxin transport from shoots to roots was increased by 43% under PEG application. Quantitative reverse transcription PCR revealed higher expression of YUC and PIN genes in PEG-treated plants, indicating that they play key roles in auxin synthesis and transport regulation in drought stress. Overall, these suggest that drought stress regulates LR formation in tobacco seedlings via modification of auxin synthesis and polar transport depending on the experimental stage.