Cross-talk between calcium-calmodulin and nitric oxide in abscisic acid signaling in leaves of maize plants

Using pharmacological and biochemical approaches, the signaling pathways between hydrogen peroxide (H2O2), calcium (Ca2+)-calmodulin (CaM), and nitric oxide (NO) in abscisic acid (ABA)-induced antioxidant defense were investigated in leaves of maize (Zea mays L.) plants. Treatments with ABA, H2O2, and CaCl2 induced increases in the generation of NO in maize mesophyll cells and the activity of nitric oxide synthase (NOS) in the cytosolic and microsomal fractions of maize leaves. However, such increases were blocked by the pretreatments with Ca2+ inhibitors and CaM antagonists. Meanwhile, pretreatments with two NOS inhibitors also suppressed the Ca2+-induced increase in the production of NO. On the other hand, treatments with ABA and the NO donor sodium nitroprusside (SNP) also led to increases in the concentration of cytosolic Ca2+ in protoplasts of mesophyll cells and in the expression of calmodulin 1 (CaM1) gene and the contents of CaM in leaves of maize plants, and the increases induced by ABA were reduced by the pretreatments with a NO scavenger and a NOS inhibitor. Moreover, SNP-induced increases in the expression of the antioxidant genes superoxide dismutase 4 (SOW), cytosolic ascorbate peroxidase (cAPX), and glutathione reductase I (GR1) and the activities of the chloroplastic and cytosolic antioxidant enzymes were arrested by the pretreatments with Ca2+ inhibitors and CaM antagonists. Our results suggest that Ca2+-CaM functions both upstream and downstream of NO production, which is mainly from NOS, in ABA- and H2O2-induced antioxidant defense in leaves of maize plants.