Physiological, agronomic and molecular changes for early and late senescence maize inbred lines under abiotic stresses

Abstract

Senescence is the final stage of leaf development and leads to death. Senescence can be induced prematurely by abiotic stresses. Early senescence or induced senescence by abiotic stresses can be undesirable, affecting the growth and yield of plants. There is considerable genetic variation in the patterns of senescence in maize. The stay-green (SG) is a secondary trait that enables crop plants to maintain their green leaves and photosynthesis capacity for a longer time after silking. The objectives of this thesis were divided into two essential studies. Firstly, evaluate the effect of SG phenotype for maize (Zea mays L.) phenological, physiological, and agronomic characters; and assess how abiotic stresses affect these traits. The second objective was to identify genes differentially expressed (DEGs) during senescence for contrasting SG phenotype in inbred lines and to show how their expression changes under abiotic stresses. The first objective was made with eight inbred lines with contrasting SG phenotypes. The experiments were carried out for two successive years experiment 2018 and 2019. The evaluation was made in two locations, with two repetitions in each location for each year trial. The eight genotypes were evaluated under two water levels, with water stress and optimum water conditions; three nitrogen levels, N1 (0U), N2 (30U), and N3 (90U). The last factor studied was plant density, with two levels of high plant density (80000 plant ha-1) and low plant density (50000 plant ha-1). For the second objective, we used two representative genotypes from the complete set of genotypes used in the first objective, one with SG phenotype and the other with early senescence rate. RNA-seq analysis was made for different samples collected during different senescence stages, starting from silking to support the objective. For the first objective, the result shows that SG genotypes have better performance for most measured traits. Drought and nitrogen are the most critical stresses that negatively affect plant physiological activity and yield and promote leaf senescence. Plant density has a positive effect on maximal biomass and grain yield. However, it can reduce the individual plant yield and affect grain quality. For the second objective of genes’ expression, the results reveal that several genes are activated or repressed during the senescence period. Those genes were activated or repressed earlier for early senescence genotype, and these expressions were delayed for the stay-green line. We also identified the expression of some specific genes corresponding to each abiotic stress or combined stress. Down-regulated genes were mainly involved in photosynthesis, different processes of biosynthesis and metabolism. In contrast, the upregulated genes are involved in the degradation and catabolism process and different stimulus processes under abiotic stress. Furthermore, during the senescence process and under different abiotic stresses, we showed the expression of different transcription factors related to senescence and response to abiotic stress. From the previous result of these studies, we conclude that leaf senescence was under genetic control. It can be affected by different abiotic stresses, which can negatively affect plant physiology and yield. However, delaying leaf senescence can be helpful to maintain plant physiological activity for a long time then increase biomass and grain yield.