A Dual Role for the Chloroplast Small Heat Shock Protein of Chenopodium album including Protection from Both Heat and Metal Stress
The chloroplast small heat shock proteins (Cp-sHSP) protect photosystem II and thylakoid membranes during heat and other types of stresses. The amount of Cp-sHSPs produced is correlated with plant thermotolerance. Cp-sHSPs of Chenopodium album have already been shown to be heat-regulated and have a novel promoter architecture containing some conserved regulatory motifs Shakeel et al. (Plant Physiol Biochem 49:898–908, 2011). To determine if similar or different Cp-sHSP isoforms protect plants against heat and metal stress, we isolated and characterized a novel Cp-sHSP isoform, CaHSP26.13p (GI # JX073659) from a Pakistani C. album ecotype and determined its expression under heat and metal stress by measuring transcript and protein levels. Presence of the same CaHSP26.13p transcript in heat- and metal-treated plants provided evidence of heat- and metal-regulated expression. Gene expression analysis indicated that Cp-sHSP transcript levels were maximum at 37 °C for 4 h (heat stress) and 20 mM cadmium (metal stress). Immunoblot analysis revealed that (1) maximum expression of precursor and processed protein was observed at 37 °C in the case of heat stress, (2) while only precursor protein of ~26 kDa was produced in the case of metal stress. The highest expression occurred when plants were treated with 15 mM cadmium. We speculate that post-translational regulation accounted for the lack of correlation between transcript and protein levels. This study demonstrates that differential regulation of the same C. album Cp-sHSP family member enables it to play a dual role in protecting the plant from heat and metal stress.
KeywordsHeat stress Metal stress HSP Chloroplast Gene expression
This research was fully supported by the Higher Education Commission of Pakistan (grant # 1212) to SNS. We thank Dr. Eric Schaller for giving us opportunity to complete some of the critical experiments in his lab. We also thank the reviewers for their excellent suggestions to improve the manuscript.
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