Apoplastic permeability barriers: a central regulating factor enabling plant resistance to abiotic stresses and dormancy induction.

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Barriers have played a pivotal role enabling successful environmental adaptation throughout plant evolutionary history. Fundamental factors conserved in the progenitors of land plants provided essential tools for plant colonization on land. Development of barriers such as cuticular layers and complex polymers in the apoplast (middle lamella, cell wall) are among key factors enabling avoidance of water loss and preventing disease invasion. Thus, apoplastic complexes are often at the core of higher plant resistance mechanisms to both abiotic and biotic stresses. While the close relationship between multiple stresses and their importance to successful plant adaptation is well recognized, studies have not widely examined the role of specific structural apoplastic barriers as a common mediator across stresses and developmental processes. 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 and composed of cross-linking extensin glycoproteins and non-cellulosic polysaccharides including pectins. Pectins are dynamic, the major regulatory molecule dictating the porosity, permeability and therefore, barriers, of Type I cell walls but the precise mechanism is still not resolved. The functional ability to form pectin gels, regulate porosity and permeability is controlled through Pectin Methylesterase (PME), calcium, sucrose and modified by temperature. However, most of the evidence comes from the food processing literature. Remarkably, only a limited number of such studies in plants exist and we are not aware of any currently focussed specifically on sucrose, Ca2+ and PME as regulatory points controlling apoplast permeability barriers to enhance broad range plant adaptation to multiple stresses. Global warming has also placed more recent focus on temperature-driven environmental adaptation. Hypothesis: temperature-mediated plant stress and tree dormancy responses are induced through alterations in pectin apoplastic permeability barriers regulated by PME, calcium and sucrose. Objectives: 1) To demonstrate a reduction in apoplast permeability increases freezing stress resistance; 2) To determine if PME, sucrose and/or calcium levels are the key regulating factors to freezing, salt and drought stress resistance through apoplast permeability changes. 3) To determine if temperature-mediated dormancy induction is regulated by changes in apoplast permeability through PME, sucrose and/or calcium. Discovery apoplast permeability is at the centre of a range of abiotic stress resistances will have widespread impact, creating new tools for crop improvement and begins to provide insight into the role of barriers and regulation of water flow--fundamental principles to all plant growth, development and adaptation to the environment.