Transcription factor WRKY46 regulates osmotic stress responses and stomatal movement independently in Arabidopsis

Abstract

SUMMARYDrought and salt stress severely inhibit plant growth and development; however, the regulatory mechanisms of plants in response to these stresses are not fully understood. Here we report that the expression of a WRKY transcription factor WRKY46 is rapidly induced by drought, salt and oxidative stresses. T-DNA insertion of WRKY46 leads to more sensitivity to drought and salt stress, whereas overexpression of WRKY46 (OV46) results in hypersensitivity in soil-grown plants, with a higher water loss rate, but with increased tolerance on the sealed agar plates. Stomatal closing in the OV46 line is insensitive to ABA because of a reduced accumulation of reactive oxygen species (ROS) in the guard cells. We further find that WRKY46 is expressed in guard cells, where its expression is not affected by dehydration, and is involved in light-dependent stomatal opening. Microarray analysis reveals that WRKY46 regulates a set of genes involved in cellular osmoprotection and redox homeostasis under dehydration stress, which is confirmed by ROS and malondialdehyde (MDA) levels in stressed seedlings. Moreover, WRKY46 modulates light-dependent starch metabolism in guard cells via regulating QUA-QUINE STARCH (QQS) gene expression. Taken together, we demonstrate that WRKY46 plays dual roles in regulating plant responses to drought and salt stress and light-dependent stomatal opening in guard cells.

Rethinking Guard Cell Metabolism

“…In most cases, starch content in guard cells has been estimated on a relative value scale by comparing the intensity of iodine-stained guard cell chloroplasts (Tallman and Zeiger, 1988;Kang et al, 2007;Valerio et al, 2011;Prasch et al, 2015). In a few studies, starch content was determined quantitatively using the oil-well technique on freeze-dried leaflets Ding et al, 2014) or spectrophotometrically using guard cell-enriched epidermal fragments (Daloso et al, 2015), but, in these cases, no temporal dynamics were reported. Thus, our knowledge of starch metabolism in guard cells has remained fragmentary, mostly correlative in nature, and even contradictory.…”

A WRKY Transcription Factor Regulates Fe Translocation under Fe Deficiency

“…Second, both WRKY46 and VITL1 are expressed in root stele as well as in vasculature tissues in shoots ( Fig. 2; Gollhofer et al, 2011;Ding et al, 2014), suggesting they have similar expression patterns. Third, down-regulation of VITL1 significantly increases Fe concentrations in xylem sap of wrky46 mutants and thus restores Fe homeostasis and Fe deficiency response of wrky46 mutants to that of the wild type (Fig.…”

“…Previously, our microarray data revealed that the expression of two Fe translocation-related genes, NIC-OTIANAMINE SYNTHASE2 (NAS2) and VACUOLAR IRON TRANSPORTER1-LIKE1 (VITL1), was affected by WRKY46 overexpression (Ding et al, 2014). NAS2 encodes an enzyme that synthesizes nicotianamine and is responsible for Fe transport inside the body of plants (Schuler et al, 2012).…”

“…Plants were grown on modified half-strength MS agar medium or solution (pH 5.6) as described previously (Ding et al, 2014). For hydroponic Fe deficiency treatment, 6-week-old various genotypes grown in half-strength MS nutrient solution were transferred into the same fresh solution with or without Fe supply for different periods (3-6 d).…”

“…WRKY46 has been reported to function in various processes, e.g. biotic and abiotic stress responses (Hu et al, 2012;Ding et al, 2013Ding et al, , 2014) and lateral root development (Ding et al, 2015), indicating WRKY46 as an important regulator in Arabidopsis. In this study, we discovered a previously unknown role for WRKY46 and demonstrated that WRKY46 promotes Fe translocation from root to shoot by suppressing the expression of a nodulin-like gene (VIT1-like1) under Fe deficiency.…”