Molecular Genetic Analysis of lls1, An Inhibitor of Cell Death in Plants

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Johal 9729608 The focus of this research is to elucidate the function of a novel gene from maize that appears to act as a suppressor of cell death in plants. This gene, designated llsl (lethal leaf spot) is identified by a recessive mutation and characterized by the initiation of necrotic lesions that expand to kill leaves. Since llsl spots visually resemble lesions incited by race 1 of Cochliobolus carbonum on susceptible maize, this mutation has been placed in the general class of mutations called disease lesion mimics. Plants with these genetic defects appear to undergo aberrant cell death, resulting in the formation of spontaneous lesions that sometimes resemble known diseases of maize. The genetic and phenotypic behavior of llsl predicts that the normal function of this gene is to prevent the spread of cell death, which may be triggered by any stimulus, including a pathogen infection, physical wounding or a genetic lesion. Once a cell dies on an llsl plant, it initiates a chain reaction of cell death, leading to the formation of a continually expanding (in a radial fashion) zone of dying cells that eventually consumes the entire leaf tissue. Interestingly, the expression of llsl lesions is developmentally programmed, cell autonomous, and light-dependent. It appears that llsl lesions only initiate and propagate in photosynthetically active tissues. In order to understand the nature of the llsl gene, it has been cloned from maize following tagging with Mutator. Sequence analysis of Llsl reveals that it encodes a novel protein highly conserved between monocots and dicots. It is present in a single copy in both maize and Arabidopsis. Phenotypic comparison along with mapping data of the Arabidopsis homolog predict it to be the acdl (accelerated cell death) gene. This hypothesis is strengthened by the demonstration that at least two EMS-induced acdl alleles have undergone single base-pair mutations in conserved residues of the llsl homolog. In this proposal, a multi-faceted approach employing genetic, histological and molecular biology tools, will be used to unveil the function of Llsl in planta. The experimental plan will take advantage of two excellent plant model systems - maize and Arabidopsis. Specific goals are to determine: * how llsl is regulated during both normal and pathological situations; * where its site of action is, both at the cellular and intracellular levels; * what its target/substrate is in the cell; and * what its relationship is to the acdl mutant of Arabidopsis7 It is highly likely that the results obtained from this study will provide a unique and important insight into the mechanisms of regulation of cell death in plants. In addition to contributing to basic research, these studies may eventually permit the conception of rational new approaches for the biotechnological manipulation of cell death in agriculture. Some obvious areas are plant protection, transformation, stress resistance, propagation and male sterility.