Abstract
Symbiotic nitrogen fixation (SNF) in legume root nodules injects millions of tons of nitrogen into agricultural lands and provides ammonia to non-legume crops under N-deficient conditions. During plant growth and development, environmental stresses, such as drought, salt, cold, and heat stress are unavoidable. This raises an interesting question as to how the legumes cope with the environmental stress along with SNF. Under drought stress, dehydrin proteins are accumulated, which function as protein protector and osmotic substances. In this study, we found that the dehydrin MtCAS31 (cold-acclimation-specific 31) functions in SNF in Medicago truncatula during drought stress. We found that MtCAS31 is expressed in nodules and interacts with leghemoglobin MtLb120-1. The interaction between the two proteins protects MtLb120-1 from denaturation under thermal stress in vivo. Compared to wild type, cas31 mutants display a lower nitrogenase activity, a lower ATP/ADP ratio, higher expression of nodule senescence genes and higher accumulation of amyloplasts under dehydration conditions. The results suggested that MtCAS31 protects MtLb120-1 from the damage of drought stress. We identified a new function for dehydrins in SNF under drought stress, which enriches the understanding of the molecular mechanism of dehydrins.
Highlights
The most significant characteristic of legumes is symbiotic nitrogen fixation (SNF)
The plant materials used in this study were Medicago truncatula R108 wild type, MtCAS31 knockout mutants generated by transcription activator-like effector nuclease (TALEN) and MtCAS31 re-transposon Tnt1-insertion mutant, NF5714
The longitudinal section of nodules showed that MtCAS31 expression was detected in meristematic zone (I), infection zone (II), and nitrogen-fixation zone (III) (Figure 1B)
Summary
The most significant characteristic of legumes is symbiotic nitrogen fixation (SNF). 68% of all ammonia fixation is done by legumes in nodules (Peoples et al, 2009; Biswas and Gresshoff, 2014). Nod factor (NF) released from rhizobium triggers the dedifferentiation and division of root cortical cells. Rhizobium invades the roots, which is facilitated by infection threads (IT), which are tubular structures formed by invagination of the cell wall and plasma membrane. ITs ramify and release rhizobium in cortical cells by endocytosis. The released rhizobia are surrounded by a plant membrane called the symbiosome membrane. The symbiosome membrane together with the enclosed bacteria is called a symbiosome in which SNF occurs. During SNF, atmospheric N2 is converted into ammonia, which is catalyzed
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