Release of Ca2 ' from internal stores and influx from the apoplastic space resulting in transient increases or oscillation of cytosolic Ca2+ levels are involved in many signaling networks in plants (15,47,48). The duration, intensity, and spatial distribution of Ca2+ transients appear to encode signal-response specificity. Direct modulation of cytosolic Ca2+ levels might therefore be a difficult task. However, downstream targets of Ca2+ regulation may represent future tools for engineering plants with improved stress tolerance.
Two calcium-dependent protein kinases (CDPKs) from tobacco (NtCDPK2 and 3) have been shown to be involved in defense and hypo-osmotic stress signaling (49,50). Both CDPKs are phosphorylated and thereby activated in a Ca2+-dependent manner and trigger an oxidative burst and programmed cell death (49). Ectopic expression of a related CDPK from A. thaliana in tomato protoplasts stimulated NADPH oxidase activity and an oxidative burst (51). Tightly regulated expression of constitutively active CDPK derivatives might offer a tool to engineer plants with enhanced flooding and disease resistance.
Calmodulin is a well-known target of calcium in multiple signal transduction pathways (47). Transgenic tobacco cells expressing a dominant-acting calmodulin derivative responded with a stronger oxidative burst and enhanced NADPH levels to treatments with various elicitors, infection with avirulent bacteria, and osmotic and mechanical stress compared with wildtype cells (52). Two specific calmodulin isoforms, SCaM4 and 5, and their transcripts were found to accumulate in cultured soybean cells upon elicitor treatment (53). These two calmodulin isoforms are most divergent from other isoforms described from plants and animals (54). Furthermore, their ability to activate calmodulin-dependent enzymes in vitro differed greatly from that of the other calmodulin isoforms of soybean. Transgenic tobacco plants constitutively expressing SCaM4 or SCaM5 exhibited spontaneous lesion formation, constitutive expression of defense-related genes, and increased resistance to virulent viral, bacterial, and oomycete pathogens (53). These findings suggest that specific calmodulin isoforms represent promising tools for engineering stress-tolerant plants.
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