Joon-Yung Cha, Jeongsik Kim, Song Yi Jeong, Gyeong-Im Shin, Myung Geun Ji , Ji-Won Hwang , Laila Khaleda, Xueji Liao, Gyeongik Ahn , Hee-Jin Park , Dong Young Kim , Jose M. Pardo , Sang Yeol Lee , Dae-Jin Yun , David E. Somers, and Woe-Yeon Kim

PNAS August 8, 2022; 119 (33) e2207275119

Significance

The circadian clock is an endogenous molecular timekeeper that coordinates biological rhythms of physiological and developmental processes in living organisms. Compensation of the circadian clock plays a crucial role in maintaining constant periodicity in fluctuating diurnal environments. Although ion contents in plants fluctuate daily or seasonally due to changes in transpiration rates and soil moisture, the precise molecular mechanisms of salt compensation are not well established. We report that SOS1 is essential for maintaining the circadian clock period under saline conditions and mediates salt compensation by directly protecting GI. These results suggest that plants may have recruited the GI protein to coordinate homeostatic functions against various levels of salt stress via the salt-dependent association between GI and individual SOS proteins.

Abstract

The circadian clock is a timekeeping, homeostatic system that temporally coordinates all major cellular processes. The function of the circadian clock is compensated in the face of variable environmental conditions ranging from normal to stress-inducing conditions. Salinity is a critical environmental factor affecting plant growth, and plants have evolved the SALT OVERLY SENSITIVE (SOS) pathway to acquire halotolerance. However, the regulatory systems for clock compensation under salinity are unclear. Here, we show that the plasma membrane Na⁺/H⁺ antiporter SOS1 specifically functions as a salt-specific circadian clock regulator via GIGANTEA (GI) in Arabidopsis thaliana. SOS1 directly interacts with GI in a salt-dependent manner and stabilizes this protein to sustain a proper clock period under salinity conditions. SOS1 function in circadian clock regulation requires the salt-mediated secondary messengers cytosolic free calcium and reactive oxygen species, pointing to a distinct regulatory role for SOS1 in addition to its function as a transporter to maintain Na⁺ homeostasis. Our results demonstrate that SOS1 maintains homeostasis of the salt response under high or daily fluctuating salt levels. These findings highlight the genetic capacity of the circadian clock to maintain timekeeping activity over a broad range of salinity levels.

See https://www.pnas.org/doi/10.1073/pnas.2207275119

Figure 1: SOS1 is essential for maintaining circadian clock in response to increasing salinity. (A–D) CAB2-LUC activity monitored in WT (Col-gl, n = 22 for A and B; n = 26 for C and D), sos1-1 (n = 19; n = 24), sos2-2 (n = 19; n = 26), and sos3-1 (n = 20; n = 24) seedlings in the presence (A and B) or absence (C and D) of 25 mM NaCl under constant blue light (31 μmol m⁻² s⁻¹). (E and F) NaCl concentration–dependent CAB2-LUC activity examined in WT seedlings treated with 0 mM (n = 13), 10 mM (n = 15), or 25 mM NaCl (n = 15) and sos1-1 seedlings treated with 0 mM (n = 13), 10 mM (n = 13), or 25 mM NaCl (n = 12). (A, C, and E) Bioluminescence traces (normalized to the mean expression level over 12- to 144-h sampling schedule) and (B, D, and F) period estimates (means ± SEM) of LUC reporter activity. Different characters indicate that the means are significantly different between genotypes or treatments (P < 0.05, one-way ANOVA with Tukey’s multiple comparisons test). n refers to the number of monitored plants.