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

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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.