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Plant Physiology 144:1691-1692 (2007) © 2007 American Society of Plant Biologists On the InsidePhylloplanins: Shoot Surface Proteins Involved in Plant DefenseThe trichomes that grow along the shoot surfaces of plants and the biochemicals that accumulate there provide the first line of defense against microbial pathogens and insect herbivores. Trichomes, cuticles, and surface-accumulated biochemicals form the phylloplane that, together with surface microorganisms, constitutes a complex and diverse phyllosphere. The chemical nature and defensive properties of surface-secreted secondary metabolites (terpenoids, phenylpropanoids, etc.) have been extensively studied. In contrast, the roles of surface-secreted proteins in plants are less certain. Tobacco (Nicotiana tabacum) plants synthesize unique proteins called T-phylloplanins that are excreted only from glands of a particular procumbent trichome type that apparently does not secrete the well-known diterpenes and sugar esters produced and secreted by tall glandular trichomes. T-phylloplanins are readily washed from the leaf surface by brief water washing under conditions that remove <3% of diterpenes and sugar esters. T-phylloplanin proteins secreted to aerial surfaces of tobacco inhibit spore germination and blue mold disease caused by the oomycete pathogen Peronospora tabacina. Kroumova et al. (pp. 1843–1851) extend earlier evidence for the antifungal activity of T-phylloplanins using a reverse genetics approach. RNAi of the T-phylloplanin gene in Nicotiana resulted in loss of T-phylloplanin mRNA and protein. Leaf water washes of RNAi plants did not inhibit Peronospora spore germination or leaf infection, and young RNAi knockdown plants were susceptible to disease. They also demonstrated the glycoprotein nature and other properties of T-phylloplanins, and provide evidence that leaf surface proteins of certain non-Nicotiana species that are not susceptible to P. tabacina disease can inhibit spore germination and tobacco leaf infection by this pathogen. Ethylene and Apple Ripening Apples (Malus x domestica) produce a blend of volatile compounds upon ripening (Fig. 1) . These aroma compounds are produced from primary metabolites via at least four pathways. In apple, ethylene is central to ripening, inducing significant changes in gene expression. Good storing varieties that ripen more slowly have been linked to alleles of ACC SYNTHASE (MdACS1) that synthesize less of this ripening hormone. Schaffer et al. (pp. 1899–1912) describe a microarray approach to identify the ethylene-regulated transcriptional control points of aroma production in ripening apple fruit. They have developed a ‘Royal Gala’ line of apple containing an antisense 1-aminocyclopropane-1-carboxylic (ACC) oxidase gene that has lost its ability to synthesize ethylene. In response to external ethylene, these antisense fruit undergo a normal climacteric burst and produce increasing concentrations of ester, polypropanoid, and terpene volatile compounds over an 8-d period. When these antisense ACC oxidase lines were induced to ripen through the application of exogenous ethylene, volatiles were measured and microarrays containing 15,720 oligonucleotides derived from a nonredundant set of apple ESTs were used to detect changes in the expression of individual genes. One hundred eighty-six candidate genes that might be involved in the production of these compounds were mined from EST databases. Expression patterns of 179 of these were assessed using an apple microarray. Based on sequence similarity and gene expression patterns, they identified 17 candidate genes that are likely to be ethylene control points for aroma production in apple. Only certain points within the aroma biosynthesis pathways were regulated by ethylene. Often the first step and, in all pathways, the last steps contained enzymes that were ethylene-regulated. This analysis suggests that the initial and final enzymatic steps with the biosynthetic pathways are important transcriptional regulation points for aroma production in apple.
A Moss Sodium ATPase The bryophyte Physcomitrella patens is unlike any other plant identified to date in that it possesses a gene that encodes an ENA-type Na+-ATPase (PpENA1). PpENA1 was shown to act as a Na+ pump when expressed heterologously in yeast (Saccharomyces cerevisiae) and complemented a salt-sensitive yeast strain deficient in Na+ and K+ efflux. This implies that Physcomitrella has either gained the gene or the gene has been lost during the evolution of higher plants. Lunde et al. (pp. 1786–1796) have determined the importance of a functional Na+-ATPase in planta by conducting physiological analyses of PpENA1 in Physcomitrella. Expression studies showed that PpENA1 is up-regulated by NaCl and to a lesser degree by osmotic stress. No other abiotic stress tested led to significant increases in PpENA1 expression. In the gametophyte, strong expression was confined to the rhizoids, stem, and the basal part of the leaf. Wild-type plants were able to maintain a higher K+ to Na+ ratio than the PpENA1 (ena1) gene knockout at moderately high NaCl concentrations, but at higher NaCl concentrations no difference was observed. Although no difference in chlorophyll content was observed between ena1 and wild type under moderate salt conditions, the impaired Na+ exclusion in ena1 plants led to an approximately 40% decrease in growth. Thus, PpENA1 plays an essential role in Physcomitrella under moderate salt stress by improving the K+/Na+ homeostasis and allowing normal growth. How Synergids Die During angiosperm reproduction, a pollen tube grows into and effects the death of one of the female gametophyte's two synergid cells. The pollen tube then ceases growth and releases its contents, including the two sperm cells, into the degenerating synergid cytoplasm. Ultimately, the two sperm cells migrate to and fuse with the egg cell and central cell to effect double fertilization. In many species, including Arabidopsis (Arabidopsis thaliana), synergid cell death is pollination dependent, indicating that pollen tubes induce death of the synergid. Pollen tubes could cause synergid cell death either through physical disruption or by inducing programmed cell death by means of a diffusible or a contact-mediated signal. Sandaklie-Nikolova et al. (pp. 1753–1762) have carefully examined the temporal relationship between pollen tube arrival at the female gametophyte and synergid cell death in Arabidopsis using light and transmission electron microscopy, as well as real-time imaging of these two events in an in vitro pollen tube growth assay. Their data suggest that the synergid cell initiates cell death after the pollen tube arrives at the female gametophyte but before pollen tube discharge. These observations suggest that the pollen tube triggers cell death by directly interacting with the synergid cell. The authors hypothesize that a signaling cascade triggered by contact between the pollen tube and the synergid cell may induce the death of the synergid cell in Arabidopsis. Improving Salt Tolerance High levels of Na+ ratios can disrupt various enzymatic processes in plant cells owing to the ability of Na+ to compete with K+ for binding sites in the cytoplasm. The sensitivity of cytosolic enzymes to Na+ is similar in both glycophytes and halophytes, indicating that the maintenance of a low cytosolic Na+ to K+ ratio is a key requirement of plant growth in saline soil. Two contributions in this issue concern the genetic modification of plasma membrane transport proteins to alter salt tolerance in plants. To identify novel ion transporter genes participating in salt tolerance in rice (Oryza sativa) cells, Obata et al. (pp. 1978–1985) conducted functional complementation screening of a rice cDNA library for suppressors of a salt-sensitive phenotype of yeast mutant strain G19. The strain displays salt sensitivity as a result of disruptions in its Ena1 to 4 genes, which encode Na+ export pumps. Using this screen, the authors identified the rice gene OsKAT1, which encodes a Shaker family K+ channel protein, as a suppressor of Na+ sensitivity in G19. The cellular Na+ to K+ ratio of OsKAT1-expressing G19 cells remained lower than nonexpressing cells under saline conditions. Moreover, rice cells overexpressing OsKAT1 also showed enhanced salt tolerance and increased cellular K+ content. The expression of OsKAT1 was restricted to internodes and rachides of wild-type rice, whereas other Shaker family genes were expressed in various organs. These results suggest that OsKAT1, in cooperation with other K+ channels, is involved in salt tolerance in rice, and that it participates in the maintenance of cytosolic cation homeostasis during salt stress.
Gévaudant et al. (pp. 1763–1776) provide evidence that plasma membrane proton pumping ATPases (H+-ATPases) also play an important role in reducing Na+ toxicity. To study the physiological consequence of this activation, the authors analyzed transgenic tobacco (Nicotiana plumbaginifolia) plants expressing either wild-type plasma membrane H+-ATPase 4 (wtPMA4) or a PMA4 mutant lacking the autoinhibitory domain ( Regulation of Leaf Size in Arabidopsis Evolutionary innovations in xylem structure that increase hydraulic efficiency while maintaining the continuity of the water column have been crucial to the success of the vascular plants. What remains unclear is how leaf structural evolution has influenced the hydraulic efficiency and subsequent photosynthetic performance of land plants. Leaf veins are at the core of leaf structural diversity, forming the transport network for water, nutrients, and carbon for nearly all plants. Following the logic that leaf veins evolved to bypass inefficient water transport through living mesophyll tissue, Brodribb et al. (pp. 1890–1898) examined the possibility that the hydraulic pathway beyond the distal ends of the vein system may be a possible limiter of water transport in leaves. They tested a mechanistic hypothesis that the length of this final traverse, as water moves from veins across the mesophyll to where it evaporates from the leaf, governs the hydraulic efficiency and photosynthetic carbon assimilation of any leaf. Sampling 43 species across the breadth of plant diversity from mosses to flowering plants, they found that the post-vein traverse, as determined by characters such as vein density, leaf thickness, and cell shape, was strongly correlated with the hydraulic conductivity and maximum photosynthetic rate of foliage. Since leaf photosynthetic performance is coupled to the capacity of the leaf vascular system to supply water to photosynthesizing mesophyll cells, these data support the hypothesis that vein positioning limits photosynthesis via its influence on leaf hydraulic efficiency. The concept that vein positioning sets the upper limits for water flow and gas exchange in the leaf is profound as it suggests the development and evolution of leaf productivity are molded by the structural properties of the leaf hydraulic system.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900233 Related articles in Plant Physiol.:
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PMA4), thereby generating a constitutively activated enzyme. They provide evidence that, in contrast to wtPMA4 overexpression, which did not induce any phenotypic modification, expression of the constitutively activated