Glutamic Acid and Poly-γ-glutamic Acid Enhanced the Heat Resistance of Chinese Cabbage (Brassica rapa L. ssp. pekinensis) by Improving Carotenoid Biosynthesis, Photosynthesis, and ROS Signaling

Heat stress is one of the most common agrometeorological risks in crop production in the middle and lower reaches of the Yangtze River in China. This study aimed to investigate whether glutamic acid (Glu) or poly-gamma-glutamic acid (gamma-PGA) biostimulants can improve the thermotolerance of a cool-season Chinese cabbage (Brassica rapa L. ssp. pekinensis) crop. Priming with Glu (2.0 mM) or gamma-PGA (20 mg.L-1) was conducted at the third leaf stage by applying as daily foliar sprays for 5 days before 5 days of heat stress (45 degrees C in 16-h light/35 degrees C in 8-h dark). Coupled with morphophysiological and biochemical analyses, transcriptomes of Glu or gamma-PGA-primed Chinese cabbage under heat stress were examined by RNA-seq analysis. The results showed that the thermotolerance conferred by Glu and gamma-PGA priming was associated with the increased parameters of vegetative growth, gas exchange, and chlorophyll fluorescence. Compared with the control, the dry weights of plants treated with Glu and gamma-PGA increased by 51.52% and 39.39%, respectively. Glu and gamma-PGA application also significantly increased the contents of total chlorophyll by 42.21% and 23.12%, and carotenoid by 32.00% and 24.00%, respectively. In addition, Glu- and gamma-PGA-primed plants markedly inhibited the levels of malondialdehyde, electrolyte leakage, and super-oxide anion radical, which was accompanied by enhanced activity levels of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD). Enrichment analysis of Kyoto Encyclopedia of Genes and Genomes (KEGG) categories within the differentially expressed genes (DEGs) functional clusters of RNA-seq data indicated that the expression levels of the genes for DNA replication, DNA repair system, linoleic acid metabolism, cysteine and methionine metabolism, glutathione metabolism, purine and pyrimidine metabolism, carotenoid biosynthesis, and plantpathogen interaction were commonly up-regulated by both Glu and gamma-PGA priming. Glu treatment enhanced the expression levels of the genes involved in aliphatic glucosinolate and 2-oxocarboxylic acid, while gamma-PGA treatment activated carotenoid cleavage reaction to synthesize abscisic acid. Taken together, both Glu and gamma-PGA have great potential for the preadaptation of Chinese cabbage seedlings to heat stress, with Glu being more effective than gamma-PGA.

Automated summary

This study investigated the potential of glutamic acid (Glu) and poly-gamma-glutamic acid (gamma-PGA) biostimulants in enhancing the thermotolerance of Chinese cabbage under heat stress. The results showed that both Glu and gamma-PGA treatments improved the parameters of vegetative growth, gas exchange, and chlorophyll fluorescence. Moreover, these treatments increased the contents of total chlorophyll and carotenoid and reduced oxidative damage by enhancing the activity levels of antioxidant enzymes. Transcriptome analysis revealed that the expression levels of genes involved in various metabolic pathways were up-regulated by both Glu and gamma-PGA priming. Overall, Glu was found to be more effective than gamma-PGA in enhancing the thermotolerance of Chinese cabbage.