Towards a mechanistic understanding of global regulators of plant lifecycle transitions and seedling responses to environmental stresses

种子休眠的特点是在通常有利于发芽的条件下，分散的种子暂时不能发芽。某些针叶树（如黄杉）的种子在成熟时处于明显的休眠状态，在传播过程中表现出较低的发芽能力，需要数月的潮湿冷却才能打破休眠。农业和林业工业所依赖的种子在萌发后表现出高的发芽能力和旺盛的同步生长。然而，这些行业遇到了与种子质量差和持续休眠有关的问题。对于一些栽培物种来说，休眠通常被认为是一种不受欢迎的特性。黄杉是一种对加拿大非常重要的高海拔树种，其种子在秋季传播，种子休眠是一种关键的适应特征，可以防止在寒冷的冬季发芽。我们已经确定了影响针叶树种子休眠和质量的一些机制因素，并开发了改善针叶树种子萌发和幼苗生长的工具。然而，我们最引人注目的发现是关于发芽抑制剂脱落酸（ABA）和一种叫做ABI3的蛋白质，它有助于传递ABA信号。ABI3蛋白控制着许多对种子存活至关重要的事件。这包括食物储备的沉积、休眠的施加和种子组织对干燥的耐受性的获得。最近，ABI3蛋白在其他植物组织的休眠中，以及在试图“抵御”环境压力（如极端高温和低温、高盐度和干旱）的幼苗中，发挥了更广泛的作用。我们从黄杉中分离出了ABI3基因（CnABI3），并确定了该蛋白的一些功能。此外，我们已经确定了三种与CnABI3相互作用的黄杉蛋白，并进一步发现CnABI3可能在开花调控中发挥作用。在树木和其他植物物种中，ABI3蛋白可能作为一个全局调节剂，调节植物生命周期的所有关键转变。然而，该蛋白多种功能背后的机制尚不完全清楚。这种理解的一部分将来自于对cnai3的一个相互作用蛋白的详细描述，我们称之为“CnAIP2”。我们知道这种蛋白在植物生命周期的关键转变中也起着重要作用，但它是ABI3蛋白的“对抗”或拮抗剂。本提案的总体目标是：(1)了解ABI3-CnAIP2蛋白作为关键植物生命周期转变的“看门人”的机制，并定义这两个全球调节剂在种子和幼苗存活过程以及生命周期决策中的作用。(2)探讨长时间寒冷作为调节生命周期决策（特别是种子发芽和开花能力）的关键环境信号之间的有趣相似之处。(3)了解ABI3蛋白及其反蛋白如何帮助幼苗应对环境胁迫。提出的研究有很大的潜力产生对种子和幼苗生存至关重要的过程的新的机制见解，这将对林业和农业产生实际影响。我们将从进化的角度来看种子和幼苗生存的过程，以理解树木（裸子植物）和其他植物（被子植物）之间的联系。长期的研究将产生关于调节蛋白如何参与对气候和海拔适应重要的环境信号的整合的信息。

Seed dormancy is characterized by a transient inability of the dispersed seed to germinate under conditions that are normally conducive to germination. The seeds of certain conifer species (e.g. yellow-cedar) develop pronounced dormancy at maturity, exhibiting a low capacity for germination upon dispersal, and requiring several months of moist chilling to break dormancy. The agricultural and forestry industries rely upon seeds that exhibit high germinability and vigorous, synchronous growth after germination. However, these industries encounter problems related to poor seed quality and persistent dormancy. For some cultivated species then, dormancy is often considered an undesirable trait. For yellow-cedar, a high elevation species of great importance to Canada, whose seeds are dispersed in the fall, seed dormancy represents a critical adaptive trait that prevents germination during the cold winter months. We have defined some of the mechanistic factors that underlie the dormancy and quality of conifer seeds, and have developed tools for improving conifer seed germination and seedling growth. However, our most striking discoveries are those concerning the germination inhibitor abscisic acid (ABA), and a protein called ABI3, which helps to relay ABA signals. ABI3 proteins control many events that are critical to seed survival. This includes the deposition of food reserves, dormancy imposition and the acquisition of a tolerance of seed tissues to desiccation. Recently more widespread roles for ABI3 proteins have become apparent – in the dormancy of other plant tissues – and in young seedlings attempting to “fend off” environmental stresses, like extreme heat and cold temperatures, high salinity and drought. We have isolated the ABI3 gene from yellow-cedar (CnABI3) and have defined some of the functions of this protein. In addition, we have identified three yellow-cedar proteins that interact with CnABI3, and further have made the exciting discovery that CnABI3 may play a role in the regulation of flowering. In trees and other plant species, the ABI3 protein may function as a global regulator, acting to regulate all key transitions of the plant lifecycle. However, the mechanisms underlying the multiple functions of this protein are not fully understood. Part of this understanding will come from detailed characterization of one of the interacting proteins of CnABI3 that we have called “CnAIP2”. We know that this protein also plays important roles during key transitions of the plant lifecycle, but it acts as a “counter” or antagonist to the ABI3 proteins. The overall objectives of this proposal are: (1) To understand the mechanisms by which ABI3-CnAIP2 proteins function as “gatekeepers” of key plant lifecycle transitions and define how the two global regulators act in seed and seedling survival processes, and in lifecycle decisions. (2) To address aspects of the interesting parallels between prolonged cold as a key environmental signal that regulates lifecycle decisions (especially seed germination and flowering competence). (3) To understand how the ABI3 proteins and their counter-proteins help young seedlings cope with environmental stresses. The proposed studies have great potential to yield novel mechanistic insights into processes critical to seed and seedling survival that will have practical implications for forestry and agriculture. We will look at the processes involved in seed and seedling survival from an evolutionary point of view to understand connections between trees (gymnosperms) and other plants (angiosperms). Long-term research will generate information about how the regulatory proteins may participate in the integration of environmental signals of importance to climatic and altitudinal adaptation.