Jie Yang, Hang He, Yuming He, Qiaozhen Zheng, Qingzhong Li, Xin Feng, Pengcheng Wang, Guocheng Qin, Yangtao Gu, Ping Wu, Chao Peng, Shilei Sun, Yi Zhang, Mingzhang Wen, Rong Chen, Yang Zhao, and Tongda Xu

PNAS June 15, 2021 118 (24) e2102544118

Significance

The complex and diverse functions of the phytohormone auxin rely on its concentration. A high concentration of auxin has distinct effects on plant development and environmental adaptation in comparison with a low level of auxin. In this study, we show that a high concentration of auxin is capable of stimulating ABA responses partially through a TMK1-based mechanism. This involves the phosphorylation of ABI1/2, two negative regulators of ABA signaling, and following the activation of downstream ABA responses. Our work reveals the interplay between the phytohormone auxin and ABA, and provides a basis for interpreting differential phytohormone responses during plant development.

Abstract

Differential concentrations of phytohormone trigger distinct outputs, which provides a mechanism for the plasticity of plant development and an adaptation strategy among plants to changing environments. However, the underlying mechanisms of the differential responses remain unclear. Here we report that a high concentration of auxin, distinct from the effect of low auxin concentration, enhances abscisic acid (ABA) responses in Arabidopsis thaliana, which partially relies on TRANS-MEMBERANE KINASE 1 (TMK1), a key regulator in auxin signaling. We show that high auxin and TMK1 play essential and positive roles in ABA signaling through regulating ABA INSENSITIVE 1 and 2 (ABI1/2), two negative regulators of the ABA pathway. TMK1 inhibits the phosphatase activity of ABI2 by direct phosphorylation of threonine 321 (T321), a conserved phosphorylation site in ABI2 proteins, whose phosphorylation status is important for both auxin and ABA responses. This TMK1-dependent auxin signaling in the regulation of ABA responses provides a possible mechanism underlying the high auxin responses in plants and an alternative mechanism involved in the coordination between auxin and ABA signaling.

See: https://www.pnas.org/content/118/24/e2102544118

Figure 1: TMK1 is involved in the high auxin-stimulated ABA responses. (A) Quantification of the cotyledon greening ratio of 6-d-old wild-type seedlings treated with different concentrations of yucasin together with or without 0.5 μM ABA. The data are means ± SD (n = 3). (B) Quantification of the cotyledon greening ratio of 7-d-old wild-type seedlings treated with different concentrations of IAA together with both 10 μM yucasin and 0.5 μM ABA. The data are means ± SD (n = 3). (C and D) Expression of auxin-responsive genes in germinating seedlings treated with 10 nM IAA or 100 nM IAA together with 10 μM yucasin and/or 20 μM ABA (C). Expression of ABA-responsive genes in germinating seedlings treated with 10 nM IAA or 100 nM IAA together with 10 μM yucasin and/or 20 μM ABA (D). Three biological replicates were performed with consistent results. The figures show results from one representative biological repeat. (n = 3, two-tailed Student’s t test). (E–G) Cotyledon greening phenotype of Col-0, tmk1-1, and the complemented line in response to 2 μM IAA and 0.5 μM ABA. Representative figures at day 7 are shown in E. The germination greening ratios under 0.5 μM ABA treatment during the continuous 11 d were quantified in F. Data show the means ± SD (Col-0 vs. tmk1-1: P < 0.0001, Col-0 vs. gTMK1- flag;tmk1-1: P = 0.4634; n = 3 two-way ANOVA). The cotyledon greening ratios on day 7 are quantified in G. The change ratio of ABA + IAA vs. ABA indicate the auxin sensitivity of ABA responses. Data show the means ± SD (n = 3, two-way ANOVA). Approximately 80 seedlings were analyzed in each replicate. (Scale bars, 2 mm.)