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Plant Physiology 134:1293-1294 (2004) © 2004 American Society of Plant Biologists Immunophilins in PlantsImmunophilins are receptors for several types of immunosuppressive drugs, including cyclosporin A (CsA), FK506, and rapamycin. The CsA receptors are referred to as cyclophilins (CYPs) and FK506- and rapamycin-binding proteins are abbreviated as FKBPs. These two group of proteins (collectively called immunophilins) share little sequence homology, but both have peptidyl prolyl cis/trans isomerase activity that is involved in protein folding. Immunophilins have been identified in all organisms examined including bacteria, fungi, animals, and plants. Nevertheless, the physiological functions of immunophilins are poorly understood in all organisms. Based on a survey of the Arabidopsis genome, He et al. (pp. 1248–1267) have discovered 52 genes that encode for putative immunophilins, including 23 putative FKBPs and 29 putative CYPs. This is by far the largest immunophilin family identified in any organism. A striking feature of immunophilins in Arabidopsis is that a large fraction of FKBPs and CYPs are localized in the chloroplast, and this may be the reason why plants have a larger immunophilin family than animals. Immunophilin genes are generally expressed throughout the plant except for those encoding chloroplast members that are often detected only in the green tissues. The large number of genes and diversity of structure domains suggest that immunophilins are a superfamily of proteins with diverse functions. A protein (AtFIP37) that interacts with a type of immunophilin in plants, notably FKBP12, a regulator of the cell cycle in Arabidopsis, is the topic of a report from Vespa et al. (pp. 1283–1292). The authors show that AtFIP37 is expressed in the nuclei of undifferentiated cells throughout development, and that knockout mutants of AtFIP37 show an embryo-lethal phenotype that is caused by a delay in endosperm development and embryo arrest. The overexpression of AtFIP37 induces the formation of large trichome cells with up to six branches. These large trichomes have a DNA content of up to 256C, implying that these cells have undergone extra rounds of endoreduplication. Interactions between Auxin and Brassinosteroids Brassinosteroids (BRs) and auxin regulate many of the same aspects of plant growth and development, including cell division and expansion, vascular differentiation, root growth, and senescence. Many auxin-induced responses are synergistically enhanced by BR treatments, suggesting a possible interaction between these two hormones. In this issue, Bao et al. (pp. 1624–1631) show that the BR induction of both lateral root formation and the expression of an auxin-inducible promoter are suppressed by the auxin transport inhibitor N-(1-naphthyl) phthalamic acid (NPA). These observations provide evidence that BR and auxin functionally interact at least in part through BR regulation of auxin transport. Goda et al. (pp. 1555–1573) provide further information concerning the interactions between BR and auxin. These authors employed GeneChip technology to study transcript profiles over 24 h in response to auxin and BR. They identified 409 genes as BR-inducible, 276 genes as IAA-inducible, and 637 genes in total. These two hormones regulated only 48 genes in common, suggesting that most of the actions of each hormone are mediated by gene expression that is unique to each. Many IAA-up-regulated genes were induced quickly by IAA, and more slowly by BR, suggesting divergent physiological roles. IAA- and BR-specific genes were also identified, which should help to elucidate the specific actions of each hormone. The TGTCTC element, a core element of the previously reported auxin response element (AuxRE), was not enriched in genes specifically regulated by IAA, but was enriched in the 5'-flanking region of genes up-regulated by both IAA and BR. Such gene classification should be useful for predicting the functions of unknown genes and in understanding the interactions of these two hormones. Calcium and Herbivore Attack When attacked by herbivores, several plant species, including Lima bean (Phaseolus lunatus), emit volatiles that attract natural predators of the damaging insects. Volicitin, N-(17-hydroxylinolenoyl)-glutamine, a major component from the regurgitant of Mediterranean climbing cutworm (Spodoptera littoralis Boisd.) larvae, has been shown to induce a systemic release of volatiles from some plants. Little is known, however, about the early effects of herbivore attack at the membrane level. In this issue, Maffei et al. (pp. 1752–1762) present data on the early effects that feeding of the Mediterranean climbing cutworm has on the membrane properties of Lima bean leaves. Their results show that herbivore attack induces a strong depolarization at the bite zone and a wave of depolarization that spreads throughout the entire leaf. This depolarization was induced by the regurgitant but not by volicitin alone. An enhanced influx of Ca at the very edge of the bite was also observed, and this influx was halved by the Ca channel blocker verapamil. In transgenic aequorin-expressing soybean (Glycine max) cells, the influx of Ca by several N-acyl glutamines (volicitin, N-palmitoyl-glutamine, and N-linolenoyl-glutamine) from the larval oral secretion was concentration–dependent. The herbivore wounding causes a response in the plant cells that cannot be mimicked by mechanical wounding. These results suggest an involvement of Ca in the wound signaling that follows herbivore attack. Mastoporan: Not a G-Protein Agonist in Plants
Mastoparan, an amphiphilic tetradecapeptide isolated from wasp venom, is widely used in animal studies as a G-protein agonist, capable of directly stimulating the guanine nucleotide exchange reaction of the Functional Differentiation of Cytokinin Receptors The cytokinin cis-zeatin (cZ) was long ignored by researchers under the assumption that it showed only weak activity in many assays. Many recent studies, however, suggest that cZ may be important in some types of plants and tissues. For example, cZ-type cytokinins are abundant in some plants, such as maize (Zea mays). Moreover, the maize cytokinin O-glucosyltransferases, cisZOG1 and cisZOG2, predominantly catalyze (and inactivate) cZ. The apparent regulation of cZ by O-glucosylation suggests that cZ may be functional as a cytokinin in maize. If cZ has physiological activity in maize, it should be recognized by cytokinin receptors. In this issue, Yonekura-Sakakibara et al. (pp. 1654–1661) report that all three of the identified maize cytokinin-responsive histidine kinases (ZmHKs) are activated by cZ. These cytokinin-activated histidine kinases are parts of two-component receptors believed to be involved in cytokinin signal transduction. These receptor kinases exhibited differential sensitivity to other cytokinins. For example, isopentenyladenine was most affective for ZmHK1, whereas ZmHK2 tended to be most sensitive to trans-zeatin and the riboside. In addition to providing further evidence for a role for cZ as an active cytokinin in maize, this study also suggests that there is a strong functional differentiation of cytokinin receptors in regard to ligand preference.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.103.900107. Related articles in Plant Physiol.:
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-subunit of animal heterotrimeric G-proteins. In plants, mastoporan has been reported to increase cytoplasmic Ca, induce an oxidative burst, and stimulate phosphoinoisitide turnover as well as the activities of certain phospholipases and protein kinases. Usually, these effects of mastoporan on plant function have been interpreted as evidence for a role for heterotrimeric G-proteins in the processes studied. In this issue, Miles et al. (pp. 
(agb-1) proteins were disrupted. They report that the mastoporan-stimulated phosphorylation of myelin basic protein, a known substrate of eukaryotic MAPKs, was unaffected by the loss-of-function of either G-protein subunit. If mastoporan doesn't act through G-proteins in plants, then what is its mode of action? Using a variety of pharmacological agents, the authors provide evidence that the effects of mastoporan on cultured tobacco (Nicotiana tabacum) cells involve an influx of extracellular Ca, the production of reactive oxygen species, and the activity of a MAPK kinase.