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

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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.