Cadmium toxicity-induced proline accumulation is coupled to iron depletion

Investigations were conducted to elucidate the key factor behind Cd²⁺-toxicity-induced proline accumulation in Indian mustard (Brassica juncea) by raising seedlings, independently in distilled water (DW) and mineral growth medium (MGM) in the presence of 0-500 μM CdCl₂. Invariably, Cd²⁺-induced toxicity, measured in terms of growth, was significantly more prominent in seedlings raised in DW than those raised in MGM. Cd²⁺ brought about a significant reduction in growth and photosystem II activity with a concomitant increase in proline levels, in a concentration-dependent manner. Interestingly, the level of iron in shoots of seedlings decreased proportionately with increase in Cd²⁺ toxicity. Cd²⁺-promoted proline accumulation was significantly higher in seedlings raised in DW than those raised in MGM. Depletion of essential cations (viz. Ca²⁺, Mg²⁺, K⁺, and Fe²⁺) from MGM one at a time revealed that depletion of Fe²⁺ leads to maximal proline accumulation under Cd²⁺ toxicity. Interestingly, proline level in seedlings raised under Cd²⁺ toxicity in DW supplemented with Fe²⁺ was similar to that recorded in seedlings raised in MGM. Our results convincingly demonstrated that Cd²⁺-induced iron deficiency promotes proline accumulation.