Physiological and Molecular Mechanisms of Iron-Toxicity Tolerance in Rice and Implication for Breeding

Abstract

Iron toxicity is a widespread nutrient disorder which affects lowland rice mostly in poorly drained fields where reduced iron becomes available at levels exceeding plant‟s requirements. Different strategies are employed by plants to avoid iron toxicity. However, lack of a clear understanding of adaptation mechanisms used by donor parents and the physiological and molecular factors governing iron-toxicity tolerance mechanisms seriously limits breeding efficiency. The aim of this study was to identify physiological and molecular mechanisms underlying iron toxicity tolerance in rice as well as related QTLs and design breeding strategies that would lead to the development of superior germplasm. Hydroponic and geoponic screening of a diverse set of 17 rice cultivars/varieties obtained from AfricaRice germplasm was done based on split plot randomized complete block design and data analysed using statistical tool for agricultural research (STAR). Based on the preliminary results, four rice varieties contrasting for iron toxicity tolerance were selected for in-depth characterization. Traits associated with Fetoxicity tolerance were identified through the analysis of morphological (leaf bronzing, plant height, leaf width and length, shoot and root biomass), anatomical (root traits, aerenchyma), physiological (stomatal conductance, photosynthesis, chlorophyll content, fluorescence, cell membrane stability, relative water content, anti-oxidative activities) and metabolic parameters (carbohydrates, proteins, sugars, lignin, suberin). Gene expression changes were assessed by Real Time quantitative PCR (RT-qPCR) using RNA extracted from root and shoot tissues of both control and stressed plants. QTL mapping was conducted in a bi-parental population genotyped with 1,090 SNPs. Among the 22 varieties tested, CK801, CK90, Suakoko8 and IR841 showed the highest tolerance to iron toxicity with limited growth reduction, high number of new lateral roots, low leaf injury and balanced photosynthesis. Detailed analysis of CK801 and Suakoko8 revealed that these two tolerant varieties used different strategies to overcome iron toxicity. Inclusion followed by strong dilution of iron concentration in the cells and efficient reactive oxygen species (ROS) scavenging by anti-oxidants and anti-oxidative enzymes were identified, as some of the additional tolerance mechanisms used by CK801. Suakoko 8 mainly used strong mobilization of carbohydrates at the early stage of the stress period to anticipate metabolite shortage and powerful iron exclusion at the root surface. Iron toxicity tolerance was also expressed through the early down regulation of genes involved in iron uptake and translocation and gradual increments in the upregulation of other genes. In the preliminary QTL analysis, QTLs for root biomass and number of green leaves were detected on chromosomes 2, 4, 9 and 10. Tolerant varieties identified in this study can be used as donors in breeding programs as well as the target traits associated, for the development of superior lowland rice. The advanced Supa x CK801 population (F6) obtained in this thesis will be a valuable tool for multi-location testing and validation of the QTLs reported herein. Selection and characterization of the best performing lines within the populations obtained through the three way crosses with CK801, Suakoko8 and Supa will confirm the efficiency of guided trait combinations as a relevant breeding strategy to achieve stable tolerance to iron toxicity.