Arsenic (As) is a toxic environmental pollutant. Growing Ricinus communis (castor) on As-contaminated land has the advantage that in addition to revegetation of contaminated land, it can produce bioenergy. To date, As tolerance mechanisms of this plant are not fully understood. In our previous study, we screened tolerant and sensitive genotypes of castor and reported higher total As concentration, enhanced reactive oxygen species (ROS) generation, and oxidative stress in sensitive genotypes of castor GCH 2 and GCH 4 in comparison to tolerant genotypes WM and DCH 177. In the present study, we compared the activity, isoenzyme profile, and gene expression of ROS-scavenging enzymes, proline content, and expression of nicotianamine synthase genes (RcNAS1, RcNAS2, and RcNAS3) in As-tolerant and As-sensitive genotypes of castor. SOD and GPX activity increased significantly in roots of tolerant genotype WM but remained the same or decreased in sensitive genotype GCH 2 and GCH 4 at 200 μM arsenate [As(V)] treatment indicating their important role in As tolerance in castor. CAT activity and proline content increased in sensitive genotypes but remained the same in tolerant genotypes due to As(V) treatment. APX activity showed no significant change in roots and leaves of both tolerant and sensitive genotypes. NAS genes (RcNAS1, RcNAS2, and RcNAS3) encode enzymes that catalyze trimerization of S-adenosylmethionine to form nicotianamine and are critical for metal chelation and heavy metal tolerance. Differential responses of RcNAS1, RcNAS2, and RcNAS3 genes in WM and GCH 2 due to As(V) treatment suggest their role in As(V) tolerance.
Keywords: Arsenic; Castor; Catalase; Metalloid tolerance; Phytoremediation; Proline; Superoxide dismutase; isoenzyme.