Cadmium toxicity-induced proline accumulation is coupled to iron depletion.

Investigations were conducted to elucidate the key factor behind Cd-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 CdCl. Invariably, Cd-induced toxicity, measured in terms of growth, was significantly more prominent in seedlings raised in DW than those raised in MGM. Cd 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 Cd toxicity. Cd-promoted proline accumulation was significantly higher in seedlings raised in DW than those raised in MGM. Depletion of essential cations (viz. Ca, Mg, K, and Fe) from MGM one at a time revealed that depletion of Fe leads to maximal proline accumulation under Cd toxicity. Interestingly, proline level in seedlings raised under Cd toxicity in DW supplemented with Fe was similar to that recorded in seedlings raised in MGM. Our results convincingly demonstrated that Cd-induced iron deficiency promotes proline accumulation.