Salicylic Acid And Reduced Glutathione Ameliorate Chromium-induced Physio Chemical And Genomic Changes In Brassica Napus Cultivars Differing In Tolerance Response

Heavy metals toxicity in soils is one of the major constraints for oilseed rape (Brassica napus L.) production. One of the best ways to overcome this constraint is to develop tolerant genotypes against heavy metals, and use of different plant growth regulators to improve the growth under abiotic stress also has significant importance. The current comprehensive study comprised of four different experiments and following useful findings were achieved:(1) Toxic roles of chromium (Cr) were studied in6day-old seedlings of four different cultivars of B. napus L. viz. ZS758, Zheda619, ZY50and Zheda622. The results revealed that elevated Cr levels inhibited the seedling growth of all cultivars, decreased content of photosynthetic pigments, activities of antioxidant enzymes, and increased the content of malondialdehyde (MDA) and reactive oxygen species (ROS). Cr contents in different parts of plant were higher in cultivar Zheda622as compared to other cultivars. The electron microscopic study showed that ultrastructures of mesophyll and root tip cells were totally damaged at400μM Cr stress and these changes were more prominent in Zheda622. On the basis of these findings, it was suggested that cultivar Zheda622was found to be more sensitive as compared to other three cultivars.(2) In the second study, toxic effects of Cr were studied in hydroponically grown seedlings of four different cultivars of Brassica napus L. The study revealed that elevated Cr levels reduced the plant growth and biomass as compared to respective controls in all cultivars and this decline was more obvious in cultivar Zheda622. It was observed that reduction of photosynthetic attributes was more pronounced in Zheda622as compared to other cultivars, and cultivar ZS758performed better under Cr-toxicity. Accumulation of ROS and MDA were induced under different Cr concentrations. However, to scavenge the negative effect of ROS and MDA, some of anti-and n on-antioxidant enzyme activities in leaves and roots were increased under the Cr-toxicity. The electron microscopic study showed that ultrastructural damages in leaf mesophyll and root tip cells were more prominent in Zheda622as compared to other cultivars under400μM Cr stress level. Under higher Cr concentration (400μM), changes like broken cell wall, immature nucleus, damaged cell membrane, more number of mitochondria, ruptured thylakoid membranes and large size of vacuole and starch grains were observed in leaf ultrastructures. The damages in root cell were observed in the form of disruption of Golgi-bodies and diffused cell wall under the higher concentration of Cr. On the basis of these observations, it was hypothesized that cultivar Zheda622was found to be more sensitive as followed by ZY50, Zheda619and ZS758under Cr-toxicity.(3) It is considered that salicylic acid (SA) induces tolerance mechanisms in plants against a wide spectrum of biotic and abiotic stresses. An experiment was planned to investigate how SA regulates plant defense in two cultivars of B. napus under Cr stress. Results showed that exogenously applied SA enhanced plant growth, dry biomass and strengthen the reactive oxygen scavenging system by improving the cell organelles that were severely damaged by Cr-toxicity. Chromium and SA contents were found significantly higher in both plant parts viz. leaves and roots of cultivar Zheda622(yellow color) as compared to ZS758(black color). The dose dependent increase was observed in ROS generation under Cr stress. To alleviate the adverse effects of ROS, plants defense apparatus was activated, and SA further enhanced the capability of protection. Gene expression analysis showed that combined treatments of Cr and SA increased the antioxidants related genes expression. In this study, it was demonstrated that SA induced the enzymatic antioxidants activities and their related genes expression, secondary metabolism and transcript level of specific stress related proteins in the leaves and roots of two B. napus cultivars under Cr-toxicity.(4) In4th study, besides the physiological parameters, we employed the RNA sequence analyses on the two differentially Cr tolerant B. napus cultivars to insight the stress mechanism against the Cr-toxicity. Results showed that Cr significantly reduced the number of DEGs as compared to control plants in both cultivars. Besides. GO analysis explored the GO terms that were related to Cr-toxicity and respond positively. Among all GO terms, promising ones were i.e. biological regulation, response to stress and stimulus. Furthermore, KEGG pathways analyses narrated the hazardous effects of Cr in the form of inhibiting zeatin, nitrogen and photosynthesis antenna protein pathways. Moreover, data suggested that Cr-toxicity reduced the tryptophan related pathway; however, it also increased the ribosome metabolic pathway on the other hand. The present study delineated that exogenously applied reduced glutathione (GSH) recovered the above mentioned attributes as compared to Cr-stressed conditions. Moreover, there were noticed that exogenous GSH induced the transducer activity in treated plants which directly related to prepare the cell for foreign stress.