Physiological and molecular responses of two Chinese cabbage genotypes to heat stress

A comparative investigation of heat stress-mediated physiological and biochemical parameters in conjunction with the expression analysis of heat shock transcription factors (BrHSF) from two different Chinese cabbage genotypes was done to understand the mechanism of heat tolerance. Our results show that the heat-tolerant (2013-33) genotype had a smaller relative electric conductivity, a less malondialdehyde content and a higher maximal efficiency of photosystem II photochemistry than the heat-sensitive (AM160) genotype, and was able to develop the leaf head under heat stress, whereas 'AM160' flailed to develop it. The results also indicate that '2013-33' accumulated a higher amount of soluble sugars and protein under heat stress condition than 'AM160'. However, it warrants to mention that proline content and antioxidant enzymes, such as the peroxidase, catalase, and superoxide dismutase activities, in the 2013-33 genotype under HS were recorded, being significantly lower than in 'AM160'. Additionally, the expression profile of BrHSF genes was checked and classified to three main groups, (i) HS-induced HSFs expressed in both genotypes (group I), (ii) suppressed by HS in both genotypes (groupII), and (iii) genotype-specific expression of HSFs (repressed in the AM160 heat-sensitive genotype whereas induced in the '2013-33' heat tolerance genotype; group III). Furthermore, the result of promoter analysis shows that group III BrHSFs, i.e., 23, 30, and 33 gene promoter regions possessed a difference between '2013-33' and 'AM160'. In conclusion, the results of our study identify that '2013-33' had more heat tolerance than 'AM160' because of a higher accumulation of sugar and protein and an enhanced expression of group III HSFs, and the differential response of group III HSFs to HS in these two genotypes may be because of a promoter sequence difference. The study provides us a clue towards understanding the mechanism of heat tolerance in Chinese cabbage and offers a valuable source for further improvement of heat tolerance in Chinese cabbage.