Assessing Stress Tolerance of Organelle Small Heat Shock Protein Mutants in Arabidopsis Thaliana

Molecular chaperones are proteins found in virtually every organism and are essential to cell survival. When plants are heat stressed, they upregulate and downregulate multiple genes, many of which are associated with the heat shock response. Small heat shock proteins (sHSPs) are one class of molecular chaperones that are upregulated during heat shock. They are proposed to act as the first line of defense by binding to heat sensitive proteins and preventing their irreversible aggregation. However, many details of sHSP function remain to be discovered and exactly what proteins they protect is unresolved. In addition to cytosolic sHSPs found in other organisms, plants also produce sHSPs that are targeted to organelles. In this study, I focus on the mitochondria and chloroplast localizing sHSPs: HSP23.5-MTI/CP, HSP23.6-MTI/CP, HSP25.3-CP, and HSP26.5-MTII in Arabidopsis thaliana. The heat tolerance of knockout mutants of these different organelle-localized sHSPs, including single, double, triple, and quadruple knockouts was assessed through various stress assays. A hypocotyl elongation assay indicated a mild heat sensitive phenotype for many of the sHSP knockout mutants and plants lacking all four sHSPs showed the greatest reduction in hypocotyl elongation following heat stress. In an vi assay with light grown seedlings, I observed plants that lacked the chloroplast-localizing HSP25.3-CP were sensitive to acute heat stress. In stress assays involving arsenic, plants that did not express mitochondrial sHSPs were the most sensitive to excess arsenic. Interestingly, plants lacking the four sHSPs were more resistant to salt and cadmium stress. The phenotypes of these sHSPs will bring us closer to defining their mechanism of action during heat or heavy metal stress and the mutants will provide a platform for further studies of sHSP structure and function