Apoplastic alterations as a common mechanism to freezing avoidance and growth cessation/dormancy induction through temperature-mediated pectin gelation

植物与环境缺乏同步性是造成非生物胁迫伤害的主要原因，造成巨大的经济损失。植物是固定的有机体，必须适应环境才能生存。在整个植物进化史上，温度一直是适应的关键驱动力。然而，这些热适应的机制仍不清楚。质外体空间是调控冰繁殖和细胞生长的关键区域。质外体区域是渗透性屏障，以避免冰通过阿拉伯木聚糖聚合物或抗冻蛋白直接进入植物细胞。细胞壁部分是由纤维素微纤维通过凝胶状基质结合在一起的两相结构。这种凝胶状基质占细胞壁干重的三分之二(占杨树初生细胞壁的47%)，由交联性伸展素糖蛋白和非纤维素多糖组成，包括果胶(多聚半乳糖醛酸(PGAs)和鼠李半乳糖醛酸(RG)I和II)和半纤维素。果胶是决定细胞壁孔隙度的主要调节分子。质外体的胶凝性、硬度和随后的渗透性是由果胶基质通过果胶甲基酯酶(PME)决定的，PME可以去除果胶甲基，允许Ca~(2+)与羧酸盐离子结合并产生钙交联键。这种刚性和果胶凝胶的形成还通过改变水的渗透性来影响水流动和膨胀压力的关键细胞膨胀参数。PME和果胶凝胶是动态的和温度敏感的，但大多数证据来自食品加工文献。值得注意的是，在植物中进行的此类研究数量有限。本项目将通过1名博士生和1名硕士研究生解决以下主要假设。学生和4名本科生夏季学生：通过果胶凝胶化的质外体变化导致：a)通过减少冰向细胞内空间的传播来避免冰冻，以及b)通过减少水流来诱导依赖温度的休眠。

Lack of plant synchrony with the environment is the primary cause of abiotic stress injury resulting in significant economic loss. Plants are stationary organisms and must adapt to their environment in order to survive. Temperature has been a key driver to adaptation throughout plant evolutionary history. However, the mechanism of these thermal adaptations is still not understood. The apoplastic space represents a key region regulating both ice propagation and cell growth. Apoplastic regions are permeability barriers to avoid ice propagation directly into the plant cell through arabinoxylan polymers or Anti-Freeze Proteins. The cell wall fraction is a biphasic structure consisting of cellulose microfibrils held together by a gel-like matrix. This gel-like matrix constitutes up to two-thirds of the cell wall's dry weight (47% of primary cell walls of Populus), and is composed of cross-linking extensin glycoproteins and non-cellulosic polysaccharides including pectins (polygalacturonic acids (PGAs), and rhamnogalacturonan (RG) I and II) and hemicelluloses. Pectins are the major regulatory molecule dictating the porosity of cell walls. The gelation, rigidity and subsequent permeability of the apoplast is dictated by the pectin matrix through Pectin Methyl Esterase (PME) which removes pectin methyl groups, allowing Ca2+ binding to the carboxylate ions and creating calcium cross-linkages. This rigidity and pectic gel formation also influences key cell expansion parameters of water flow and turgor pressure through changes in water permeability. PME's and pectin gelation are dynamic and temperature-sensitive but most of the evidence comes from the food processing literature. Remarkably, there are only a limited number of such studies in plants. This project will address the following overarching hypothesis through 1 Ph.D. student, 1 M.Sc. student and 4 undergraduate summer students: Apoplastic alterations through pectin gelation result in: a) freezing avoidance by reduced ice propagation into the intracellular space, and b) temperature-dependent dormancy induction through reduction of water flow.