Cadmium Alters the Metabolism and Perception of Abscisic Acid in Pisum sativum Leaves in a Developmentally Specific Manner

Abstract

Abscisic acid (ABA) plays a crucial role in plant defense mechanisms under adverse environmental conditions, but its metabolism and perception in response to heavy metals are largely unknown. In Pisum sativum exposed to CdCl2, an accumulation of free ABA was detected in leaves at different developmental stages (A, youngest, unexpanded; B1, youngest, fully expanded; B2, mature; C, old), with the highest content found in A and B1 leaves. In turn, the content of ABA conjugates, which was highest in B2 and C leaves under control conditions, increased only in A leaves and decreased in leaves of later developmental stages after Cd treatment. Based on the expression of PsNCED2, PsNCED3 (9-cis-epoxycarotenoid dioxygenase), PsAO3 (aldehyde oxidase) and PsABAUGT1 (ABA-UDP-glucosyltransferase), and the activity of PsAOγ, B2 and C leaves were found to be the main sites of Cd-induced de novo synthesis of ABA from carotenoids and ABA conjugation with glucose. In turn, β-glucosidase activity and the expression of genes encoding ABA receptors (PsPYL2, PsPYL4, PsPYL8, PsPYL9) suggest that in A and B1 leaves, Cd-induced release of ABA from inactive ABA-glucosyl esters and enhanced ABA perception comes to the forefront when dealing with Cd toxicity. The distinct role of leaves at different developmental stages in defense against the harmful effects of Cd is discussed.