Calcium/calmodulin up-regulates a cytoplasmic receptor-like kinase in plants

Calcium-calmodulin is required for abscisic acid-induced antioxidant defense and functions both upstream and downstream of H2O2 production in leaves of maize (Zea mays) plants

* Using pharmacological and biochemical approaches, the role of calmodulin (CaM) and the relationship between CaM and hydrogen peroxide (H(2)O(2)) in abscisic acid (ABA)-induced antioxidant defense in leaves of maize (Zea mays) plants were investigated. * Treatment with ABA or H(2)O(2) led to significant increases in the concentration of cytosolic Ca(2+) in the protoplasts of mesophyll cells and in the expression of the calmodulin 1 (CaM1) gene and the content of CaM in leaves of maize plants, and enhanced the expression of the antioxidant genes superoxide dismutase 4 (SOD4), cytosolic ascorbate peroxidase (cAPX), and glutathione reductase 1 (GR1) and the activities of the chloroplastic and cytosolic antioxidant enzymes. The up-regulation of the antioxidant enzymes was almost completely blocked by pretreatments with two CaM antagonists. * Pretreatments with CaM antagonists almost completely inhibited ABA-induced H(2)O(2) production throughout ABA treatment, but pretreatment with an inhibitor or scavenger of reactive oxygen species (ROS) did not affect the initial increase in the contents of CaM induced by ABA. * Our results suggest that Ca(2+)-CaM is involved in ABA-induced antioxidant defense, and that cross-talk between Ca(2+)-CaM and H(2)O(2) plays a pivotal role in ABA signaling.

Abscisic acid stimulates the expression of two isoforms of plasma membrane Ca2+-ATPase in Arabidopsis thaliana seedlings

AT-ACA8 and AT-ACA9 are two plasma membrane (PM) Ca (2+)-ATPases of ARABIDOPSIS THALIANA. In this article the expression of AT-ACA8, AT-ACA9, and of AT-ACA10, a third isoform of Ca (2+)-ATPase closely related to PM Ca (2+)-ATPases, was analysed and the effect of the hormone abscisic acid (ABA) on the expression level of PM Ca (2+)-ATPase specific transcripts was investigated. In adult plants of A. THALIANA, AT-ACA8 and AT-ACA10 are expressed in all organs considered whereas AT-ACA9 is expressed only in flowers. All isoforms of PM Ca (2+)-ATPases can be detected in young seedlings but the amount of AT-ACA9 mRNA is much lower than those of AT-ACA8 and AT-ACA10. ABA markedly and rapidly stimulates the expression of both AT-ACA8 and AT-ACA9 genes in young seedlings but not that of AT-ACA10. ABA also increases the level of AT-ACA8 protein at the PM, suggesting a role for PM Ca (2+)-ATPases in ABA signalling.

Nodulation independent of rhizobia induced by a calcium-activated kinase lacking autoinhibition

Legumes, such as Medicago truncatula, form mutualistic symbiotic relationships with nitrogen-fixing rhizobial bacteria. This occurs within specialized root organs--nodules--that provide the conditions required for nitrogen fixation. A rhizobium-derived signalling molecule, Nod factor, is required to establish the symbiosis. Perception of Nod factor in the plant leads to the induction of Ca2+ oscillations, and the transduction of this Ca2+ signal requires DMI3 (refs 2, 3), which encodes the protein kinase Ca2+/calmodulin-dependent protein kinase (CCaMK). Central to the regulation of CCaMK is an autoinhibitory domain that negatively regulates kinase activity. Here we show that the specific removal of the autoinhibition domain leads to the autoactivation of the nodulation signalling pathway in the plant, with the resultant induction of nodules and nodulation gene expression in the absence of bacterial elicitation. This autoactivation requires nodulation-specific transcriptional regulators in the GRAS family. This work demonstrates that the release of autoinhibition from CCaMK after calmodulin binding is a central switch that is sufficient to activate nodule morphogenesis. The fact that a single regulation event is sufficient to induce nodulation highlights the possibility of transferring this process to non-legumes.

Early responses to Nod factors and mycorrhizal colonization in a non-nodulating Phaseolus vulgaris mutant

Legumes can acquire nitrogen through a symbiotic interaction with rhizobial bacteria. The initiation of this process is determined by a molecular dialogue between the two partners. Legume roots exude flavonoids that induce the expression of the bacterial nodulation genes, which encode proteins involved in the synthesis and secretion of signals called Nod factors (NFs). NFs signal back to the plant root and trigger several responses, leading to bacterial invasion and nodule formation. Here, we describe the molecular and cellular characterization of a Phaseolus vulgaris non-nodulating mutant (NN-mutant). Root hair cells of the NN-mutant plant respond with swelling and branching when inoculated with Rhizobium etli, albeit without curling induction. Furthermore, neither initiation of cell division in the outer cortex, nor entrapment of bacteria nor infection thread formation was observed. Both the bean wild-type and the NN-mutant responded with elevated intracellular calcium changes in the root hairs. Although the NN-mutant is deficient in early nodulin gene expression when inoculated with R. etli, it can be effectively colonized by arbuscular mycorrhizal fungi (Glomus intraradices). Our data indicate that the P. vulgaris NN-mutant is not blocked at the NFs early perception stage, but at later downstream stages between Ca(2+) signaling and early nodulin induction. This supports the idea that both microsymbionts are perceived and trigger different downstream pathways in the host plant.

Sending the right signals: regulating receptor kinase activity

Knowledge of the functions of plant receptor-like-kinases (RLKs) is increasing rapidly, but how their cytoplasmic signalling activity is regulated and how signals are transduced to cytoplasmic or nuclear proteins remain important questions. Recent studies, particularly of the BRASSINOSTEROID INSENSITIVE1 RLK, have begun to shed light on the mechanistic details of RLK activation, including the possible role of ligand binding. Studies of this and other RLKs have also highlighted the potential importance of hetero-oligomerisation and receptor internalisation in RLK signalling. Finally, a range of potential regulatory proteins and putative downstream signalling substrates have been identified for various RLKs. Despite some similarities with animal receptor kinase signalling systems, mechanisms that affect the intracellular behaviour, regulation and interactions of RLKs appear to be very diverse, potentially explaining how signalling specificity is maintained at the cytoplasmic level.