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Plant Physiology 138:1896-1897 (2005) © 2005 American Society of Plant Biologists On the InsideGreen SpermAlthough a number of pollen-specific promoters have been isolated from plants, most are active only in the vegetative cell of the pollen grain and not in the sperm cells. The functional analysis of sperm proteins, however, would be greatly facilitated by promoters that direct gene expression exclusively in sperm cells. Toward this end, Engel et al. (pp. 2124–2133) selected Arabidopsis (Arabidopsis thaliana) genes that were similar to several different sperm-specific transcripts from maize (Zea mays). The putative promoter regions of these Arabidopsis genes were tested for cell specificity by expressing enhanced green fluorescent protein (eGFP) in transgenic Arabidopsis plants. The promoter of Gamete Expressed 1 (AtGEX1) directed expression in some somatic tissues, but in pollen it directed eGFP expression only in the sperm cells of tricellular pollen and not in the progenitor generative cell or in the vegetative cell. The AtGEX2 promoter directed eGFP expression in the generative cells of bicellular pollen and in the sperm cells in tricellular pollen grains, but not in other tissues. Both GEX1 and GEX2 are plasma membrane proteins of unknown function. The promoters of their genes, however, provide powerful new tools for imaging sperm dynamics. Moreover, the AtGEX1 and AtGEX2 promoters will be useful for manipulating gene expression in sperm, via antisense or overexpression constructs. A Gene Caught in the Process of Becoming Lost Gene loss is an important mechanism involved in the generation of genome diversity among eukaryotic species. Genome-scale analyses of gene loss have underscored the extent of loss between distantly related species and how this contributes to the different attributes of species. For example, comparative analyses of available protein sequences revealed that Saccharomyces cerevisiae has lost about 300 genes since its divergence from its common ancestor with Schizosaccharomyces pombe. In comparison to the number of studies that have demonstrated interspecific gene loss, examples of intraspecific gene loss are much fewer. Logically, prior to a gene being completely lost from a species, there must be a window of time when the species is composed of a mixture of individuals that have or do not have the gene. Presumably such a situation must exist for some genes in extant species. Li et al. (pp. 2386–2395) have serendipitously stumbled upon a gene of unknown function (designated Crs-1) that appears to be in the process of being lost from heterozygous populations of the turfgrass species creeping bentgrass (Agrostis stolonifera). Most individual creeping bentgrass plants examined lack Crs-1. Some individuals are hemizygous for the Crs-1 locus, indicating major haplotype noncolinearity at that locus. Crs-1 was not detected in several other Agrostis species, demonstrating it is being lost from the genus. The presence of Crs-1 homologs in grass lineages ancestral to creeping bentgrass also supports the interpretation that the Crs-1 gene is in the process of being lost from creeping bentgrass but is still present in some individuals. The Crs-1 locus in creeping bentgrass provides a rare example of the evolutionary process of gene loss occurring within a plant species. A Plant Exocyst? In many organisms, polarized exocytosis has been found to require the involvement of a protein complex known as the exocyst. The exocyst is composed of eight proteins, Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70, and Exo84, most of which were initially identified as temperature-sensitive secretory mutants of the budding yeast Saccharomyces cerevisiae. In S. cerevisiae, mutation of any one of the exocyst components leads to arrested formation of the polarly growing bud and the accumulation of secretory vesicles. Orthologs for genes encoding all eight of the exocyst components have subsequently been identified in a growing number of eukaryotes and implicated in a variety of cellular processes involving polarized exocytosis. In general, the exocyst appears to be involved in polarized exocytosis to regions of the plasma membrane experiencing rapid polarized growth or requiring tightly controlled delivery of specific membrane components. More specifically, the exocyst appears to facilitate vesicle docking at the plasma membrane during exocytosis. Orthologs for genes encoding all components of the exocyst have been identified in Arabidopsis and Oryza sativa, suggesting the existence of a plant exocyst. The genome of Arabidopsis contains single copies of genes for Sec6, Sec8, and Sec10, two copies each of Sec3, Sec5, and Sec15, three copies of Exo84, and a remarkable 23 copies of Exo70. However, a demonstrated functional role for the exocyst or any of its components in plants has not been reported. Cole et al. (pp. 2005–2018) have used T-DNA insertional mutations of AtSEC8 to test the hypothesis that the exocyst is involved in tip growth in plants. Their genetic and microscopic studies reveal that AtSEC8 is required for both pollen germination and competitive pollen tube growth, supporting the hypothesis that one function of the putative plant exocyst is to facilitate the initiation and maintenance of the polarized growth of pollen tubes. Protein Thiolation in Plants
The tripeptide glutathione (GSH; Apomixis Genes Apomixis is an asexual mode of reproduction in which the seed embryo is formed autonomously by parthenogenesis from an unreduced egg of an embryo sac. The introduction of apomictic reproduction into crop plants is of interest because it could potentially be exploited to propagate superior hybrids or specific genotypes indefinitely. Albertini et al. (pp. 2185–2199) have isolated and characterized two genes of Poa pratensis, namely PpSERK and APOSTART, whose expression clearly differed in apomictic versus sexual genotypes of this species. The APOSTART homolog of Arabidopsis is exclusively upregulated during senescence. Moreover, the predicted localization of APOSTART in the mitochondrion membrane and its putative role in regulating mitochondrial membrane permeability makes it a prime candidate for a molecular player in the process of apoptosis. PpSERK is homologous to SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK), a gene previously implicated in the acquisition of embryogenic competence in other plant cells. The authors propose that in the nucellar cells of apomictic genotypes, PpSERK may serve as a developmental switch, the activation of which leads to embryo sac development. Cross Talk between Ethylene and Abscisic Acid Although abscisic acid (ABA) is the phytohormone most identified with stomatal regulation, it is conceivable that stomatal movements, like many plant processes, may be controlled by the interactions of many phytohormones, including ethylene. In order to examine cross talk between the ABA and ethylene during guard function, Tanaka et al. (pp. 2337–2343) examined ABA-induced stomatal closure in Arabidopsis wild-type plants, in an ethylene-overproducing mutant (eto1-1), as well as in two ethylene-insensitive mutants. In isolated epidermal peels, stomata of wild-type plants were found to close within a few minutes in response to ABA, whereas stomata of the eto1-1 mutant closed only half as much. The same inhibitory effects of ethylene on stomatal closure were observed in ABA-irrigated plants and drought-stressed plants, indicating that the inhibitory effects of ethylene on ABA-induced stomatal closure were also observed in planta. What is the physiological role of ethylene inhibition of ABA-mediated stomatal closure? One possibility may be that ethylene ensures the minimum supply of carbon dioxide for photosynthesis by keeping the stomata half opened. Therefore, ethylene may play a role in keeping a minimum level of photosynthesis during prolonged drought stress. Glycinebetaine and Tolerance to High Temperature Glycinebetaine (GB) is an organic compatible solute that accumulates rapidly in many plants under environmental stress. Previous studies have shown that Arabidopsis that have been genetically engineered to accumulation more GB exhibit enhanced tolerance to high temperatures during the growth of young seedlings. The physiological basis of this increased heat tolerance, however, remains unknown. In vitro studies suggest that GB is particularly effective in protecting highly complex proteins, such as PSII complex, against heat-induced inactivation. Since photosynthesis is among the plant functions most sensitive to high temperature stress, Yang et al. (pp. 2299–2309) examine the possibility that the physiological basis for enhanced tolerance of growth to high temperature stress induced by accumulation of GB in vivo may be associated with increased tolerance of photosynthesis to high temperatures. Genetically engineered tobacco (Nicotiana tabacum) with the ability to synthesis GB was established by introducing the BADH gene for betaine aldehyde dehydrogenase from spinach (Spinacia oleracea). The genetic engineering enabled the plants to accumulate GB mainly in chloroplasts and resulted in enhanced tolerance to high temperature stress during growth of young seedlings. Moreover, CO2 assimilation by transgenic plants was significantly more tolerant to high temperatures. The analyses of chlorophyll fluorescence and the activation of Rubisco indicated that the enhancement of photosynthesis to high temperatures was not related to the function of PSII but to the Rubisco activase-mediated activation of Rubisco. Under high temperature stress, GB maintains the activation of Rubisco by preventing the sequestration of Rubisco activase to the thylakoid membranes from the soluble stroma fractions. These results suggest that engineering of the biosynthesis of GB by transformation with the BADH gene is effective for enhancing the high temperature tolerance of plants.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 Related articles in Plant Physiol.:
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-glu-cys-gly) serves important functions in plants as a reductant, transiently accumulating under stress conditions as its oxidized disulfide (GSSG). As well as forming disulfides with itself, GSH can also form mixed disulfides with proteinaceous cysteines. This S-glutathionylation of proteins is commonly termed thiolation and has recently become established as a widespread reversible posttranslational modification of proteins that occurs in animal and fungal cells exposed to oxidative stress. Dixon et al. (pp.