Genetic and Environmental Modulation of the Chickpea - Mesorhizobium Symbiosis

Abstract

Chickpea (Cicer arietinum) is a valuable crop grown worldwide. It fixes atmospheric nitrogen by establishing a symbiotic relationship with rhizobia and secretes organic acids from its roots and green tissues that play a role in interactions with beneficial microbes, insect herbivores, and pathogens. As chickpea is nodulated with diverse Mesorhizobium species, genetic factors related to both rhizobia and host plant, and environmental conditions are likely to influence plant growth and defence, and chickpea-Mesorhizobium symbiosis. The primary objective of this thesis is to investigate the effects of genetic and environmental factors, and their interactions, on plant traits related to growth, nitrogen fixation and responses to herbivory. A quantitative review was conducted using published data to investigate the effect of environmental factors, particularly drought, on legume growth, nitrogen fixation and its related traits in the light of a hierarchy of phenotypic plasticity. Drought reduced total nitrogen fixation and average nodule mass more severely than plant shoot mass and elicited a hierarchy of plasticities whereby number of nodules per plant varied substantially, and average nodule mass and nitrogen fixation per unit nodule mass were relatively conserved. Four experiments were carried out in a glasshouse, with specific objectives converging to the primary objective. The objective of experiment 1 was to investigate the interaction between plant variety, Mesorhizobium strain, and environment, and their effects on plant growth and nitrogen fixation and its related traits, from the perspective of phenotypic plasticity. Experiment 2 investigated the effects of rhizobia on the growth dynamic of chickpea varieties, nodulation and bacteroid morphology under different water regimes. Experiment 3 and 4 investigated the effects of drought and herbivory on chickpea growth and defence and explored the trade-offs between exudation of organic acids and growth. Experiment 1. The phenotypic plasticity of chickpea varieties and mesorhizobia strains was quantified in an experiment combining factorially five chickpea varieties, seven Mesorhizobium strains and three photothermal regimes. Phenotypic plasticity was quantified for shoot dry weight, nodule dry weight, nodules per plant, nodule colour, symbiotic efficiency, and nitrogen cost. The working hypotheses were that there is a hierarchy of plasticities between plant growth and nitrogen fixation traits, and the plasticity of these traits depends on the genetic variation in both partners. Phenotypic plasticity of nodules per plant was higher than the plasticity of shoot dry weight, verifying a hierarchy of plasticities between these traits. The variation in phenotypic plasticity for shoot dry weight and nitrogen fixing traits was larger among strains than among varieties. Experiment 2. The effects of rhizobia on the growth of chickpea varieties, nodulation and bacteroid morphology were investigated in a factorial combining four varieties, four nitrogen sources including two Mesorhizobium strains, and two un-inoculated controls (nitrogen fertilised and un-fertilised), and two water regimes, well-watered and droughted. Shoot growth rate showed temporal variations in response to strain CC1192 under well-watered and drought conditions. Across sources of variation, leaf exudates varied 3.4-fold, total plant biomass 3.0-fold, nodules per plant 3.9-fold, nodule dry weight 3.0-fold, symbiotic efficiency 1.5-fold, bacteroid size 1.4-fold and amount of polyhydroxybutyrate 1.4-fold. Plant biomass was negatively correlated with bacteroid size and the amount of polyhydroxybutyrate under well-watered conditions. Symbiotic efficiency was negatively correlated with both bacteroid size and the amount of polyhydroxybutyrate under drought. Experiment 3 and 4. Experiment 3 was a factorial including twelve chickpea varieties and three water regimes to investigate the effects of variety and drought, and their interaction, on plant growth and exudation of organic acids. In experiment 4, six chickpea varieties, two water regimes, and two herbivory treatments were used, with plants challenged with Helicoverpa armigera larvae or as untreated controls. Drought decreased, and herbivory increased, the amount of leaf exudates. Water regime modulated the response of leaf exudates to herbivory, potentially leading to change in chickpea-herbivore interaction. Leaf damage, and survival and size of H. armigera larvae were larger in water-stressed plants and correlated negatively with the leaf exudates in both water regimes. There was no trade-off between exudates and growth traits in most cases; a weak trade-off was apparent under water stress, suggesting a trade-off between growth and defence contingent on water availability. All experiments were conducted meticulously under controlled conditions using small pots. Therefore, the results cannot be extrapolated to field environments but may provide some fruitful avenues of research to pursue. This study, therefore, lays the groundwork for future research, which is imperative in the improvement of chickpea productivity for sustainable agriculture.