Molecular Basis of Hormone Antagonism in Cereal Aleurone Cells

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The antagonism between two phytohormones, gibberellins (GA) and abscisic acid (ABA), regulates the transition from seed formation to seed germination as well as several other physiological and developmental processes in plants. This proposed project is designed to investigate the molecular basis of this hormone antagonism in the aleurone cells of barley seeds, where the action of these two hormones can be conveniently monitored by the activity of several marker genes encoding enzymes such as a-amylases, proteases, or a group of unique proteins called late embryogenesis abundant (LEA) proteins. The GA/ABA antagonism has long been recognized as non-competitive in nature, and it has been observed that the action of ABA in this antagonism is sensitive to inhibitors of the gene transcription process. Recent results suggest that an ABA induced regulatory molecule, the protein kinase PKABA1, is a key intermediate in suppressing the GA-regulated gene expression. The objective of this proposed project is to further elucidate the role of PKABA1 in GA/ABA antagonism and to investigate the mechanisms underlying its interactions with other signal transduction components mediating the action of GA and ABA. The specific aims in this project include: 1) to investigate how ABA and GA regulate the synthesis of PKABA1, 2) to determine whether the expression of PKABA1 is mediated by the activity of a lipid-degrading enzyme, phospholipase D (PLD), an early ABA signal transduction component, 3) to "map" the site of action of PKABA1 in relation to other known signaling molecules, 4) to determine the functional domains of the PKABA1 molecule, which are important for its regulatory roles, and 5) to study how the levels and activities of regulatory molecules important for the GA induction of gene expression can be affected by ABA treatment or PKABA1. A multi-discipline approach employing biochemical, genetic, cell and molecular biology techniques will be followed in carrying out this project. It is hoped that this proposed project would not only reveal new insights into the molecular basis of interactions between two important groups of phytohormones, but also the role of a unique protein kinase in plants. Since regulation of seed germination has been an important problem in agriculture, it is conceivable that the information obtained in this project could also lead to potential applications. In addition to the training of one postdoctoral fellow and one graduate student, the participation of several undergraduate students throughout this project is also anticipated.