Molecular Mechanism behind Stay Green Trait in Bread Wheat (Triticum aestivum L.)

Abstract

Stay-green refers to ―heritable delayed foliar senescence during grain filling‖, is associated with retention of chlorophyll and improved photosynthesis together with the maintenance of assimilated carbon supply during grain filling stage. Consequently, confirming maximum mass per grain. Stay-green trait has received substantial attention from crop breeders owing to crop improvement under abiotic and biotic stresses. The stay-green trait needs to be further explored at morpho-physiological, biochemical, and, molecular levels for introgression in the future breeding programs. Our first study aimed to unraveled the genetic composition of the stay-green trait in a diverse germplasm consisting of landraces, green revolution, post green revolution, elite cultivars, and CIMMYT advance cultivars using 90K SNP array by employing general linear model, mixed linear model, and fixed and random model circulating probability unification based genome-wide association mapping. Forty eight loci were detected for chlorophyll content, chlorophyll fluorescence, NDVI, stay-green indices, plant height, and tiller number. Annotation of thirty six putative genes extracted from identified loci revealed their role in plant development, defense responses under stress, flowering time control, chloroplast development, and damage tissues regeneration. The second study aimed to demonstrate the impact of the stay-green trait in bread wheat under terminal heat stress. Field experiments (2014-2015 & 2015-2016) were conducted to investigate the influence of terminal heat stress on the morpho physiological traits in different stay-green types. In addition, the greenhouse experiment was performed to dissect the stay-green trait in functional stay-green, non-functional stay-green, and non-stay-green genotypes. Field experiments confirmed that genotypes exhibiting the stay-green trait have significantly high chlorophyll content, normalized difference vegetative index, grain yield, biological yield, kernel weight, and low canopy temperature under control and heat stress conditions. In the greenhouse experiment, functional stay-green and non-functional stay-green genotypes showed a high chlorophyll xiv content and photochemical efficiency, whereas biological yield and grain yield showed a significant relation with the functional stay-green genotype. The sequencing and expression analysis of chlorophyllide a oxygenase (CaO), light-harvesting complex (Cab), stay-green (SGR), and red chlorophyll catabolite reductase (RCCR) in functional stay-green, non-functional stay-green, and non-stay-green genotypes revealed variations in the exons of CaO and RCCR; and significant difference in the regulation of CaO and Cab at 7 days after anthesis under terminal heat stress. The third study aimed to demonstrate the metabolic regulation of the secondary stay-green trait using stay-green and non-stay-green genotypes under control and heat stress treatments by employing Ultra-High‐Performance Liquid Chromatography High‐ Resolution Mass Spectrometry (UHPLC-HRMS). Analysis of Variance, Partial Least Squares Discriminant Analysis, and Significant Analysis of Metabolites identified 166 significant known metabolites. The predominant metabolites that showed significant high accumulation in non-stay-green genotype were deoxyuridine, deoxycytidine, 5'- deoxyadenosine, glycyl-L-leucine, leucyl-proline, cytidine, uridine, isocytosine and adenine, whereas the dominant metabolites that showed high accumulation in stay-green genotype were ADP, phosphocholine, glutathione, 2-phosphoglyceric acid, cADPR, allantoin, trigonelline, syringic acid, spermine, and hexanesulfonic acid sulfate under control and heat stress treatments. The variation in the levels of metabolites in non-stay green and stay-green genotypes highlighted variable metabolic adjustment in stay-green genotype that reduces heat impacts.