The Effects of Salinity on Nitrogen Fixation and Trehalose Metabolism in Mycorrhizal Cajanus cajan (L.) Millsp Plants

Arbuscular mycorrhizal (AM) fungi exist widely in natural ecosystems as well as in salt-affected soils and are considered suitable candidates for bio-amelioration of saline soils. Plants respond to salinity by accumulating sugars and other low-molecular-weight compatible solutes. One such compound is trehalose, which has been found to play an important role as a stress protectant. The aim of the present investigation was to study interactions between an AM fungus and salinity stress on growth, nitrogen fixation, and trehalose metabolism in Cajanus cajan (L.) Millsp. (pigeonpea). Two genotypes [Sel 85N (salt-tolerant) and ICP 13997 (salt-sensitive)] were subjected to saline treatments with and without mycorrhizal inoculations. Salinity reduced plant biomass (shoot and root) in both genotypes and resulted in a decline in shoot-to-root ratio (SRR); however, a smaller decline was observed in Sel 85N than in ICP 13997. AM colonization was reduced with increasing salinity levels but mycorrhizal responsiveness (MR) increased. Genotypic variability in nitrogen fixation and trehalose metabolism in response to salinity and mycorrhization was observed. An increment in nodule number was accompanied by a reduction in dry mass. Subsequently, nodular activity (leghemoglobin, acetylene-reduction activity [ARA], nitrogen content) was reduced under soil salinity, which was more profound in ICP 13997 than in Sel 85N. The symbiotic association with Glomus mosseae led to significant improvement in plant dry mass and nitrogen-fixing potential of nodules under salt stress. Salinity led to an increase in trehalose-6-P synthetase (TPS) and trehalose-6-P phosphatase (TPP) activities resulting in increased trehalose content in nodules, which was accompanied by inhibition of trehalose catabolism (trehalase activity). AM plants had lower trehalase activity under saline and non-saline conditions. Thus, a symbiotic relationship between plant roots and G. mosseae might have resulted in salinity tolerance in a genotype-dependent manner.