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

在整个植物进化史中，屏障在成功适应环境方面发挥了关键作用。陆地植物祖先中保存的基本因子为植物在陆地上定殖提供了必要的工具。外质体（中间薄片、细胞壁）中角质层和复杂聚合物等屏障的形成是避免水分流失和防止疾病侵袭的关键因素之一。因此，外胞体复合物通常是高等植物抵抗非生物和生物胁迫机制的核心。虽然多种胁迫之间的密切关系及其对植物成功适应的重要性已得到充分认识，但研究尚未广泛探讨特异性结构外胞质屏障作为胁迫和发育过程之间的共同中介的作用。细胞壁部分是由凝胶状基质结合在一起的纤维素微原纤维组成的双相结构。这种凝胶状基质构成了细胞壁干重的三分之二，由交联延伸蛋白糖蛋白和包括果胶在内的非纤维素多糖组成。果胶是动态的，主要的调控分子决定了I型细胞壁的孔隙度、渗透性和屏障，但精确的机制仍未解决。形成果胶凝胶、调节孔隙度和渗透率的功能能力是通过果胶甲基酯酶（PME）、钙、蔗糖和温度修饰来控制的。然而，大多数证据来自食品加工文献。值得注意的是，只有有限数量的此类研究存在于植物中，并且我们不知道目前有任何专门关注蔗糖，Ca2+和PME作为控制外质体通透性障碍的调控点，以增强植物对多种胁迫的广泛适应。全球变暖也使人们更加关注温度驱动的环境适应。假设：温度介导的植物胁迫和树木休眠反应是通过PME、钙和蔗糖调节的果胶胞外通透性屏障的改变而诱导的。目的：1)证明外质体渗透性的降低增加了抗冻应力；2)通过外质体渗透性的变化，确定PME、蔗糖和/或钙水平是否为抗冻、抗盐和抗旱的关键调节因子。3)确定温度介导的休眠诱导是否受PME、蔗糖和/或钙介导的外质体通透性变化的调节。外质体渗透性是一系列非生物抗逆性的核心，这一发现将产生广泛的影响，为作物改良创造新的工具，并开始提供对水流障碍和调节作用的见解——所有植物生长、发育和适应环境的基本原则。＊＊＊

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. ***