Improving salt stress resistance in cereals

Abstract

Salinity is a complex environmental stress that affects growth and production of agriculturally important crops world-wide. Tolerance to salt stress is variable for different plants and involves integration of various physiological and biochemical mechanisms. This thesis investigates the role in salt tolerance of various transporter genes in important agricultural cereal crops, i.e. rice and barley. Transgenic lines overexpressing OsTPKa, OsTPKb and OsAKT1 were generated using Agrobacterium mediated rice transformation, while for OsAKT1 a loss of function mutant was also isolated. Transgenic and mutant plants were characterized to study the role of these genes in rice. For barley, transgenic lines overexpressing HvHKT2;1 were obtained and studied for its role in barley salt tolerance. Transgenic rice plants overexpressing TPKa did not show any growth phenotype under salt stress conditions, however, the performance of TPKa overexpressors was better at different K+ stress conditions compared to control lines. In contrast, transgenic lines overexpressing TPKb showed improved growth under all K+ and Na+ stress conditions, suggesting that TPK channels plays a crucial role in K+ nutrition and in maintaining a higher K+/Na+ ratio under different K+ and Na+ stress conditions. Characterization of rice AKT1 mutants and overexpressors showed the involvement of the AKT1 channel in Na+ uptake at low [K+]ext or high [Na+]ext concentrations and both overexpression and loss of function resulted in reduced growth under these conditions. On the other hand, data from experiments with barley HKT2;1 overexpressing lines showed improved growth under salt stress conditions possibly via Na+ exclusion or accumulation of excessive Na+ in the shoots. Overall, the findings point to two important aspects of salt tolerance: firstly, the contribution of TPKa and TPKb to K+ homeostasis, particularly that of TPKb in maintaining ion homeostasis during different K+ and Na+ stress conditions. Secondly, a role of AKT1 and HKT2;1 in Na+ uptake at the root soil boundary is inferred. These findings reconfirm the idea that maintaining a high K+/Na+ ratio is crucial for salt tolerance in both rice and barley. In barley, HvHKT2;1 overexpressors showed improved salt tolerance via Na+ redistribution from shoot to root and accumulation of Na+ in older leaves. The transgenic lines overexpressing TPKs and HKT2;1 and the information gained from this study could be used in future breeding programs or to generate multiple overexpressors to study the additive or synergistic effects of traits that will add to the present knowledge of ion transport in rice and barley