Pivotal role of heterotrimeric G protein in the crosstalk between sugar signaling and abiotic stress response in plants.

Heterotrimeric G-proteins are key modulators of multiple signaling and developmental pathways in plants, in which they act as molecular switches to engage in transmitting various stimuli signals from outside into the cells. Substantial studies have identified G proteins as essential components of the organismal response to abiotic stress, leading to adaptation and survival in plants. Meanwhile, sugars are also well acknowledged key players in stress perception, signaling, and gene expression regulation. Connections between the two significant signaling pathways in stress response are of interest to a general audience in plant biology. In this article, advances unraveling a pivotal role of G proteins in the process of sugar signals outside the cells being translated into the operation of autophagy in cells during stress are reviewed. In addition, we have presented recent findings on G proteins regulating the response to drought, salt, alkali, cold, heat and other abiotic stresses. Perspectives on G-protein research are also provided in the end. Since G protein signaling regulates many agronomic traits, elucidation of detailed mechanism of the related pathways would provide useful insights for the breeding of abiotic stress resistant and high-yield crops.

Lipid transport protein ORP2A promotes glucose signaling by facilitating RGS1 degradation.

Heterotrimeric GTP-binding proteins (G proteins) are a group of regulators essential for signal transmission into cells. Regulator of G protein signaling 1 (AtRGS1) possesses intrinsic GTPase-accelerating protein (GAP) activity and could suppress G protein and glucose signal transduction in Arabidopsis (Arabidopsis thaliana). However, how AtRGS1 activity is regulated is poorly understood. Here, we identified a knockout mutant of oxysterol binding protein-related protein 2A, orp2a-1, which exhibits similar phenotypes to the arabidopsis g-protein beta 1-2 (agb1-2) mutant. Transgenic lines overexpressing ORP2A displayed short hypocotyls, a hypersensitive response to sugar, and lower intracellular AtRGS1 levels than the control. Consistently, ORP2A interacted with AtRGS1 in vitro and in vivo. Tissue-specific expression of 2 ORP2A alternative splicing isoforms implied functions in controlling organ size and shape. Bioinformatic data and phenotypes of orp2a-1, agb1-2, and the orp2a-1 agb1-2 double mutant revealed the genetic interactions between ORP2A and Gß in the regulation of G protein signaling and sugar response. Both alternative protein isoforms of ORP2A localized in the endoplasmic reticulum (ER), plasma membrane (PM), and ER-PM contact sites and interacted with vesicle-associated membrane protein-associated protein 27-1 (VAP27-1) in vivo and in vitro through their two phenylalanines in an acidic track-like motif. ORP2A also displayed differential phosphatidyl phosphoinositide binding activity mediated by the pleckstrin homology domain in vitro. Taken together, the Arabidopsis membrane protein ORP2A interacts with AtRGS1 and VAP27-1 to positively regulate G protein and sugar signaling by facilitating AtRGS1 degradation.

Diurnal variation of transitory starch metabolism is regulated by plastid proteins WXR1/WXR3 in Arabidopsis young seedlings.

Transitory starch is the portion of starch that is synthesized during the day in the chloroplast and usually used for plant growth overnight. Here, we report altered metabolism of transitory starch in the wxr1/wxr3 (weak auxin response 1/3) mutants of Arabidopsis. WXR1/WXR3 were previously reported to regulate root growth of young seedlings and affect the auxin response mediated by auxin polar transport in Arabidopsis. In this study the wxr1/wxr3 mutants accumulated transitory starch in cotyledon, young leaf, and hypocotyl at the end of night. WXR1/WXR3 expression showed diurnal variation. Grafting experiments indicated that the WXRs in root were necessary for proper starch metabolism and plant growth. We also found that photosynthesis was inhibited and the transcription level of DIN1/DIN6 (Dark-Inducible 1/6) was reduced in wxr1/wxr3. The mutants also showed a defect in the ionic equilibrium of Na+ and K+, consistent with our bioinformatics data that genes related to ionic equilibrium were misregulated in wxr1. Loss of function of WXR1 also resulted in abnormal trafficking of membrane lipids and proteins. This study reveals that the plastid proteins WXR1/WXR3 play important roles in promoting transitory starch degradation for plant growth over night, possibly through regulating ionic equilibrium in the root.

Young seedlings adapt to stress by retaining starch and retarding growth through ABA-Dependent and -independent pathways in Arabidopsis.

The metabolism of starch, the major resource of carbohydrates and an essential determinant of plant fitness in plants, often exhibits plasticity in response to challenging environmental conditions. Previous study of leaf starch turnover under abiotic stresses documented conflicting patterns. To understand this discrepancy and examine the process in more detail, we grew Arabidopsis seedlings under a series of typical stress conditions, observed and quantified leaf starch content at different time. By electron microscope, iodine staining and starch quantification, we confirm that short-term (<8 h) stress treatments, such as osmotic stress, high salinity, caused rapid starch decrease during the night while long-term (>56 h) stresses increased starch content significantly. Exogenous abscisic acid (ABA) induced similar effects to the abiotic stresses. Comparison of the starch content in wild type (WT) and the ABA receptor quadruple mutant pyr1;pyl1;pyl2;pyl4 suggests that the stress-induced starch turnover change is also mediated by ABA-independent pathways. In addition, more energy supply caused severer growth defect under stress conditions. And the transcription levels of both starch biosynthesis enzymes (APL1 and APL3) and starch degradation enzymes (SEX1, SEX4, BAM1 and BAM3) exhibited differential increase under long-term stresses. Taken together, our data strongly suggest that young seedlings adapt to stress by retaining starch and retarding growth through both ABA-dependent and -independent pathways.