Mechanical wounding not only damages plant tissues, but also provides pathways for pathogen invasion. To understand plant responses to wounding at a genomic level, we have surveyed the transcriptional response of 8,200 genes in Arabidopsis plants. Approximately 8% of these genes were altered by wounding at steady-state mRNA levels. Studies of expression patterns of these genes provide new information on the interactions between wounding and other signals, including pathogen attack, abiotic stress factors, and plant hormones. For example, a number of wound-responsive genes encode proteins involved in pathogen response. These include signaling molecules for the pathogen resistance pathway and enzymes required for cell wall modification and secondary metabolism. Many osmotic stress-and heat shock-regulated genes were highly responsive to wounding. Although a number of genes involved in ethylene, jasmonic acid, and abscisic acid pathways were activated, many in auxin responses were suppressed by wounding. These results further dissected the nature of mechanical wounding as a stress signal and identified new genes that may play a role in wounding and other signal transduction pathways.

Mitochondrial stress releases mitochondrial DNA (mtDNA) into the cytosol, thereby triggering the type Ι interferon (IFN) response. Mitochondrial outer membrane permeabilization, which is required for mtDNA release, has been extensively studied in apoptotic cells, but little is known about its role in live cells. We found that oxidatively stressed mitochondria release short mtDNA fragments via pores formed by the voltage-dependent anion channel (VDAC) oligomers in the mitochondrial outer membrane. Furthermore, the positively charged residues in the N-terminal domain of VDAC1 interact with mtDNA, promoting VDAC1 oligomerization. The VDAC oligomerization inhibitor VBIT-4 decreases mtDNA release, IFN signaling, neutrophil extracellular traps, and disease severity in a mouse model of systemic lupus erythematosus. Thus, inhibiting VDAC oligomerization is a potential therapeutic approach for diseases associated with mtDNA release.

Although calcium is a critical component in the signal transduction pathways that lead to stress gene expression in higher plants, little is known about the molecular mechanism underlying calcium function. It is believed that cellular calcium changes are perceived by sensor molecules, including calcium binding proteins. The calcineurin B-like (CBL) protein family represents a unique group of calcium sensors in plants. A member of the family, CBL1, is highly inducible by multiple stress signals, implicating CBL1 in stress response pathways. When the CBL1 protein level was increased in transgenic Arabidopsis plants, it altered the stress response pathways in these plants. Although drought-induced gene expression was enhanced, gene induction by cold was inhibited. In addition, CBL1 -overexpressing plants showed enhanced tolerance to salt and drought but reduced tolerance to freezing. By contrast, cbl1 null mutant plants showed enhanced cold induction and reduced drought induction of stress genes. The mutant plants displayed less tolerance to salt and drought but enhanced tolerance to freezing. These studies suggest that CBL1 functions as a positive regulator of salt and drought responses and a negative regulator of cold response in plants.

Members of the Arabidopsis calcineurin B-like Ca 2 ؉ binding protein (AtCBL) family are differentially regulated by stress conditions. One AtCBL plays a role in salt stress; another is implicated in response to other stress signals, including drought, cold, and wounding. In this study, we identified a group of novel protein kinases specifically associated with AtCBL-type Ca 2 ؉ sensors. In addition to a typical protein kinase domain, they all contain a unique C-terminal region that is both required and sufficient for interaction with the AtCBL-type but not calmodulin-type Ca 2 ؉ binding proteins from plants. Interactions between the kinases and AtCBLs require micromolar concentrations of Ca 2 ؉ , suggesting that increases in cellular Ca 2 ؉ concentrations may trigger the formation of AtCBL-kinase complexes in vivo. Unlike most serine/threonine kinases, the AtCBL-interacting kinase efficiently uses Mn 2 ؉ to Mg 2 ؉ as a cofactor and may function as a Mn 2 ؉ binding protein in the cell. These findings link a new type of Ca 2 ؉ sensors to a group of novel protein kinases, providing the molecular basis for a unique Ca 2 ؉ signaling machinery in plant cells. INTRODUCTIONAmong the extracellular signals eliciting changes in Ca 2 ϩ concentration in the cytoplasm of plant cells are plant hormones, light, stress factors, and pathogenic or symbiotic elicitors (Knight et al., 1991(Knight et al., , 1996(Knight et al., , 1997Neuhaus et al., 1993; Trewavas and Knight, 1994; Ehrhardt et al., 1996;McAinsh et al., 1997; Wu et al., 1997). In addition, many intrinsic growth and developmental processes, such as elongation of the root hair and pollen tube, are accompanied by Ca 2 ϩ transients (Franklin-Tong et al., 1996; Felle and Hepler, 1997; Holdaway-Clarke et al., 1997; Wymer et al., 1997). Because different signals often elicit distinct and specific cellular responses, an interesting question is how do cells distinguish between the Ca 2 ϩ signals produced by different stimuli?Studies with both animal and plant cells suggest that a Ca 2 ϩ signal is represented not only by Ca 2 ϩ concentration but also by spatial and temporal information, including Ca 2 ϩ localization and oscillation (Franklin-Tong et al., 1996; Holdaway-Clarke et al., 1997; Dolmetsch et al., 1998;Li et al., 1998). Although such complexity in Ca 2 ϩ parameters may partially explain the specificity of cellular responses triggered by a particular stimulus, the signaling components that "sense" and "interpret" the Ca 2 ϩ signals hold the key to linking the changes in these parameters to specific cellular responses.If Ca 2 ϩ signaling pathways constitute "molecular relays," the first "runner" after Ca 2 ϩ should be a component that serves as the Ca 2 ϩ "sensor" to monitor changes in Ca 2 ϩ parameters. Such sensors often are proteins that bind Ca 2 ϩ and, in so doing, change conformation in a Ca 2 ϩ -dependent manner. Several families of Ca 2 ϩ sensors have been identified in higher plants. Perhaps the best known is the family of calmodulin (CaM) and CaM-related prot...

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