Elucidating the Mechanisms of Farnesylation-Mediated Heat Stress Response in Plants by Using Arabidopsis Hit5 Mutant(1/3)

Scientific consensus suggests that global warming induced heat stress, which results when the ambient temperature is elevated above the normal optimum, will impact agriculture and food production worldwide. Thus, elucidating how plants respond to heat stress is essential for developing heat-tolerant crops for meeting the future food needs. For this and to identify genetic determinants that induce such responses, we used a forward genetics approach to screen ethyl methane sulfonate-mutagenized Arabidopsis mutants that were more thermosensitive than the wild-type plants. One of these mutants, hit5 (for heat-intolerant 5), was isolated because prolonged incubation at 37°C for 4 days was determined to be lethal for the mutant, but not for the wild-type seedlings. However, sudden heat shock treatment at 44°C for 40 min was found to be lethal for the wild-type plants but not for the hit5 seedlings. The mutated locus was mapped to the ENHANCED RESPONS TO ABA 1 (ERA1) gene, which encodes the β-subunit of the protein farnesyl transferase (PFT) that is responsible for the covalent linkage of a 15-carbon farnesyl moiety to target proteins. Farnesylation is a type of post-translational modification. As the name implies, mutations in ERA1 gene are known to confer enhanced response to abscisic acid (ABA) phenotypes. This indicated that at least one of the PFT-targeting proteins is a negative regulator of ABA signaling. However, the effects of protein farnesylation on plant survival under high temperature conditions have not yet been investigated. Thus, the isolation and initial characterization of hit5/era1 mutants provided novel and direct evidences of farnesylation being an essential part of heat stress response in plants. Nevertheless, several questions need to be clarified. First, is the change in thermosensitivity in hit5/era1 involves ABA signaling? Second, depending on the form of heat stress, hit5/era1 exhibit either heat-sensitive or heat-tolerant phenotypes. Hence, how are PFT-mediated heat stress response divided? Can this division be further distinguished genetically? That is, can a revertant, in which only one of the hit5 temperature-dependent phenotype is reverted to the wild type, be isolated? Third, the accumulation of heat shock proteins (HSPs) is known to contribute to the ability of plants to survive heat shock. Does hit5 mutation affect HSP gene expression? If so, is this directly involved in the hit5-mediated heat shock tolerance phenotype? In this research proposal, we designed specific experiments to address these substantial and important questions. Results from this study might not only provide novel insights on plant HSRs, but also reveal the versatile roles of protein farnesylation in the regulation of plant cell functions.