AMF modulated rhizospheric microbial enzyme activities and their impact on sulphur assimilation along with thiol metabolism in pigeonpea under Cd stress

Presence of cadmium (Cd) in the soil has a negative impact on soil enzymatic activities, which, thus, reduce growth of the inhabiting plant species. Symbiotic association between plants and arbuscular mycorrhizal fungi-AMF have potential to overcome Cd stress. Therefore, the present study investigated the role of different AMF species [Claroideoglomus claroideum-Cc, C. etunicatum-Ce, Funneliformis mosseae-Fm and Rhizoglomus intraradices-Ri] in imparting Cd tolerance to pigeonpea-Cajanus cajan (L.) Millsp. Plants. The experiment was a complete factorial combination of two factors comprising of Cd concentrations (0, 25, 50 mg kg⁻¹) and AMF treatments (Control/Non-AM, Cc, Ce, Fm, Ri) with twelve replicates for each treatment. A negative correlation between Cd concentration and plant growth, along with reactive oxygen species (ROS) build up were recorded. All mycorrhizal species reduced Cd uptake and lowered oxidative burden with maximum benefits provided by + Ri inoculation, followed by + Fm and + Ce, with + Cc the least. These effects could be associated with their relative root colonizing (RC) abilities and glomalin production in the soil. Greater efficiency of Ri than other three species could be related to maximum soil enzymatic activities [urease (URE), phosphatases (PHAs), dehydrogenase (DHase), arylsulfatase (AS)], which, enhanced the bioavailability of nutrients (P, S, N, K) to the plants. Moreover, higher activities of S assimilating enzymes [ATP sulfurylase (ATPS), γ-Glutamylcysteine synthetase (γ-ECS)] led to greater synthesis of thiol compounds [glutathiones (GSH), phytochelatins (PCs), non-protein thiol (NP-SH)] which, further reduced Cd translocation from roots to shoots. The study indicated that pigeonpea was highly responsive to different AMF species, with host-Ri combination displaying the strongest potential to alleviate Cd stress.