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Plant Physiology 140:1137-1138 (2006) © 2006 American Society of Plant Biologists On the InsideThe Scoop on Night SoilPlants protect themselves from pathogens by various defense responses, including the accumulation of antimicrobial compounds (phytoalexins), the induction of pathogenesis-related (PR) proteins, and hypersensitive cell death. Molecules called elicitors induce these defense responses. Composts fermented by animal feces have recently been demonstrated to suppress plant disease. Koga et al. (pp. 1475–1483) hypothesize that compounds from animal feces may function as elicitors of plant defense responses. They have isolated an elicitor of rice (Oryza sativa) defense responses from human feces and have identified it as cholic acid (CA), a primary bile acid. Treatment of rice leaves with CA induced the accumulation of phytoalexins and PR proteins, heightened hypersensitive cell death, and increased the resistance of plants to subsequent infection by virulent pathogens. The phytoalexins induced by CA were mainly phytocassanes. The authors' results indicate that CA is specifically recognized by rice and that this bile acid induces a wide array of defense responses. Extracellular ATP Generates Reactive Oxygen Species
Extracellular ATP (eATP) is a well-characterized signaling agent in mammals. It induces the respiratory burst in phagocytes via a signaling chain that typically includes P2 receptors and increases in cytosolic Ca2+ concentration ([Ca2+]cyt). Recent reports indicate that eATP signaling also may occur in plants. Low micromolar concentrations of eATP induce increases in [Ca2+]cyt and membrane depolarization in Arabidopsis (Arabidopsis thaliana) roots. Among the signaling changes induced by eATP in animal cells is enhanced production of reactive oxygen species (ROS) through the mediation of NADPH oxidase. ROS have been implicated in the responses of a wide variety of plants to both biotic and abiotic stresses. Song et al. (pp. 1222–1232) report that applied eATP, but not AMP or phosphate, induces the accumulation of superoxide in a biphasic, dose-dependent manner, with a threshold at 500 nM eATP in leaves of Arabidopsis. Inhibitors of mammalian P2-type ATP receptors abolished eATP-induced superoxide production, suggesting that the ATP effects may be mediated through P2-like receptors in plants. Moreover, ATP Transcriptome Analysis of Cytokinin Habituation The proliferation of callus, greening, and the formation of shoot structures are cytokinin-dependent processes. Habituation is a naturally occurring phenomenon in which the division and growth of plant cell cultures, upon continued passage, eventually become independent of a requirement for cytokinin. Earlier studies of calli derived from plants with higher-than-normal cytokinin content indicated that the overproduction of cytokinin by the culture tissues is a possible explanation for this acquired cytokinin independence. Pischke et al. (pp. 1255–1278) have performed a genome-wide, transcriptional analysis of a habituated Arabidopsis cell culture line to explore the changes in gene expression underlying the phenomenon of habituation. The up-regulation of several cytokinin oxidases, the down-regulation of several known cytokinin-inducible genes, and a lack of regulation of the cytokinin synthases indicated that increases in hormone concentration may not be required for habituation. Altered levels of expression of the cytokinin receptor CRE1, as well as several other genes involved in cytokinin signaling, indicated that deregulation of cytokinin-signaling components could underlie habituation. In addition, the up-regulation of the homeodomain transcription factor FWA, as well as transposon-related elements and several DNA- and chromatin-modifying enzymes, indicated that epigenetic changes may be critical to the acquisition of cytokinin habituation. Transcriptome of Pollen Transport Genes Although it is well known that nutrients such as boron and ion gradients or currents of Ca2+, H+, and K+ are critical for pollen tube growth, the molecular identities of the transporters mediating these fluxes and gradients are mostly unknown. As a first step toward integrating transport processes with pollen development and function, Bock et al. (pp. 1151–1168) have conducted a genome-wide analysis of transporter genes expressed in the male gametophyte of Arabidopsis at four developmental stages. Of 1,751 total transporter and unknown protein-encoding genes in the Arabidopsis genome, 1,511 were on the chip used by the authors, and 1,046 genes (or 69%) were expressed in developing or mature pollen. This value is remarkably high considering that only one or two cell types constitute the male gametophyte. The authors provide a working list of 124 transporter genes that are defined as specifically or preferentially expressed in developing pollen. The list includes members of well-known transport families, including P-type H+ pumps and Ca2+ pumps, several ABC transporters, and only two vacuolar H+-ATPase subunits. Among the H+-coupled cotransporters on the list are members of the monosaccharide/H+ symporter (STP), amino acid transporter (AAAP), and putative oligopeptide and nitrate transporter (OPT and POT) families. The channels listed include two K+ channels of the voltage-gated ion channel (VIC) family, four members of the putative cyclic nucleotide and calmodulin-regulated ion channel (CNGC) subfamily, and two putative tonoplast water channels of the major integral protein (MIP) family. Some genes are highly expressed in microspores and bicellular pollen, while others are activated only in tricellular or mature pollen. About 300 unknown proteins are expressed in pollen, and 9% (26 genes) are specifically or preferentially expressed in pollen. One of these unknown genes (At3g20300) is 23% similar and 12% identical to olfactory receptors from Drosophila melanogaster, and the authors speculate whether such "odorant receptors" may receive some of the diverse chemical cues that guide the pollen tube to the ovule. Scanning Raman Microscopy of Wood Cross Sections Most traditional chemical analyses of plant cell walls require disintegration of the plant tissue. In addition, sample isolation difficulties arise when small cell wall areas or single layers are of interest since they have to be carefully excised. Therefore, the ability of confocal Raman microscopy to generate images of the chemical composition from the plant cell wall in situ and nondestructively represents a significant advance. Gierlinger and Schwanninger (pp. 1246–1254) have used confocal Raman microscopy to illustrate changes in the molecular composition in secondary plant cell wall tissues of poplar (Populus nigra x Populus deltoides) wood. Two-dimensional spectral maps were acquired and chemical images calculated by integrating the intensity of characteristic spectral bands. This enabled direct visualization of the spatial variation of the lignin content without any chemical treatment or staining of the cell wall. A small (0.5 µm) lignified border toward the lumen was observed in the gelatinous layer of poplar tension wood. The variable orientation of the cellulose also was characterized, leading to visualization of the S1 layer with dimensions smaller than 0.5 µm. Scanning Raman microscopy should prove to be a powerful, nondestructive tool for imaging changes in molecular cell wall organization with high spatial resolution. A Polyadenylation Factor That Binds Calmodulin It has been estimated that as many as 25% of all Arabidopsis genes are alternatively polyadenylated. The Arabidopsis genome possesses genes that are capable of encoding orthologs of virtually the entire suite of known eukaryotic polyadenylation factor subunits. Among these is a gene that encodes a protein similar to a subunit of the mammalian cleavage and polyadenylation specificity factor (CPSF30). CPSF30 and its yeast counterpart, Yth1p, are CCCH-type zinc-finger proteins and bind RNA with characteristics consistent with an involvement in mRNA 3'-end formation. Delaney et al. (pp. 1507–1521) describe several biochemical properties of an Arabidopsis ortholog of CPSF30 (AtCPSF30). AtCPSF30 is shown to be a nucleus-localized, RNA-binding protein that binds calmodulin. AtCPSF30 is also shown to be capable of interacting with itself. Many of the properties of these proteins can be mapped to an evolutionarily conserved core of three CCCH-type zinc-finger motifs, and an adjacent domain that is unique to plant CPSF30-like proteins. These results show that the central portion of AtCPSF30 is involved in a number of important functions, and they raise interesting possibilities for both the interplay between splicing and polyadenylation and the regulation of these processes by stimuli that act through calmodulin. Lactone-Inducible Promoter System Chemically inducible gene expression systems are useful molecular tools to tightly control temporal and spatial gene expression in cases where the constitutive expression of transgenes or RNAi constructs is not desirable or detrimental to cells. It is also useful for switching on regulator genes in specific tissues and observing whole-genome expression patterns as well as constructing genetic regulatory networks. You et al. (pp. 1205–1212) describe a novel and efficient inducible system to regulate gene expression in plants based on quorum-sensing components found in Gram-negative bacteria such as Agrobacterium tumefaciens. These bacteria monitor their own population density by utilizing members of the N-acyl homoserine lactone family as inducers and a transcriptional activator as its receptor. The authors utilize the components from A. tumefaciens, i.e. 3-oxooctanyl-L-homoserine lactone (OOHL) synthesized by the TraI protein and its receptor TraR. When OOHL binds to TraR, it recognizes its specific cis-element, the tra box. The authors translationally fused the eukaryotic VP16 activation domain to the N terminus of TraR. In the presence of OOHL, the chimeric VP16:TraR transcriptional regulator induces reporter gene expression in cells from a broad spectrum of plant species. The lactone-inducible system has a quick response, adjustable switch-off response, inducer specificity, a rapid foliar uptake of the inducer, and no effect on endogenous gene expression. A major advantage of this system is the tight specificity between the inducer and receptor.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900189. Related articles in Plant Physiol.:
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S, a nonhydrolyzable P2 receptor agonist, mimicked the effect of eATP. Superoxide production induced by eATP was abolished in mutants that had a disrupted subunit of NADPH oxidase. A Ca2+ channel blocker, a Ca2+ chelator, and a calmodulin antagonist all reduced eATP-induced superoxide accumulation. Transgenic plants with heightened levels of apyrase, an enzyme that hydrolyzes ATP, had reduced superoxide production in response to applied ATP and wounding. Together, these data suggest a role for eATP as a signal in wound and stress responses.