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Plant Physiology 136:3001-3002 (2004) © 2004 American Society of Plant Biologists On the InsideCell Wall Loosening: A Role for Reactive Oxygen Intermediates?
The mechanisms underlying the dramatic growth rates of maize (Zea mays) roots are poorly understood. Liszkay et al. (pp. 3114–3123) argue that many current hypotheses concerning the underlying biochemistry of plant cell elongation are based on circumstantial and/or controversial experimental evidence. The authors discuss several inconsistencies in the in vitro versus in vivo evidence for enzyme-mediated cell wall loosening. In this paper, the authors explore an alternative possibility that the wall loosening reactions underlying root elongation growth are induced by reactive oxygen intermediates (ROI), especially hydroxyl radicals (·OH), in the cell wall. ·OH are short-lived, highly reactive molecules that cleave cell wall polysaccharides in a nonenzymatic reaction and can in this way cause wall loosening in isolated cell walls as well as in living tissues. The authors propose that the ROIs involved in cell elongation may be initiated by a NAD(P)H oxidase-catalyzed formation of superoxide radicals ( A New Proteomic Database for the Plant Cell Wall Despite the importance of cell walls to the biology of plants, not much is known about the biosynthesis and function of their major macromolecular components. Biochemical approaches have only been successful thus far in characterizing a few of the hundred of enzymes that are probably involved in cell wall function. Moreover, comparative molecular genetic studies have provided little insight because the walls of other organism groups, such as bacteria and yeast, are fundamentally different in composition, structure, and function from those of plants. Recent advances in genomics make it possible to quickly identify large numbers of genes as being putatively involved in particular plant cell processes. With these new resources for identifying candidate genes encoding biosynthetic enzymes and regulatory proteins in cell walls comes the challenge of extracting the critical information from complex data sets to guide the functional analysis of these genes and the proteins they encode. To fill this need, Girke et al. (pp. 3003–3008) have created and are maintaining Cell Wall Navigator (CWN), a web-based database that integrates cell wall-related protein families and allows easy comparison among sequences derived from fully sequenced plant genomes plus the known protein sequences from other species. The CWN database has numerous visualization and interactive mining tools and an adaptable design for organizing complex protein families across many organisms to cover the complete space of known sequences. CWN's flexible architecture and automated update and analysis features allow for the rapid integration of new families and up-to-the-minute information on cell wall proteins. Hyponasty: A Complex Growth Response
The semiaquatic plant Rumex palustris responds to complete submergence by an upward movement of the younger petioles. This hyponastic response, in combination with heightened petiole elongation, brings the leaf blade above the water surface and restores contact with the atmosphere. In this issue, Cox et al. (pp. 2948–2960) present a detailed study of this differential growth process. They show that hyponastic growth is caused by differential cell elongation across a small region of the petiole base, with cells on the abaxial (lower) surface elongating faster than cells on the adaxial (upper) surface. Pharmacological studies and endogenous hormone measurements revealed that ethylene, auxin, abscisic acid, and GA regulate different and sometimes overlapping stages of hyponastic growth. For example, the initiation of hyponastic growth and the maintenance of the maximum petiole angle are regulated by ethylene, abscisic acid, and auxin, whereas the speed of the response is influenced by ethylene, abscisic acid, and GA. A redistribution of indole-3-acetic acid (IAA) is too late to be involved in the onset of hyponastic growth, but probably plays a role in maintenance of this differential growth response. Evidence is presented that expansin genes are not expressed differentially in response to submergence, indicating that it is unlikely that this cell-wall loosening protein is the downstream target for the hormones that regulate the differential cell elongation leading to submergence-induced hyponastic growth in R. palustris.
A Cell Plate Protein That Binds Phosphoinositides Ultrastructural evidence clearly indicates that membrane trafficking is central to the centrifugal development of the cell plate during plant cytokinesis. Our understanding of the molecular events underlying membrane trafficking and fusion during cell plate formation has recently been aided by the analysis of mutants that are defective in cell plate construction and by studies of proteins that localize to the cell plate. Not surprisingly many of the molecules that function in cell plate biogenesis are related to proteins involved in membrane trafficking in other eukaryotes. In this issue, Peterman et al. (pp. 3080–3094) present the first biochemical and functional characterization of patellin1 (PATL1), a novel cell plate-associated protein that is related in sequence to phosphatidylinositol (PtdIns) lipid transfer proteins (PITPs) in other eukaryotes. In these other eukaryotic systems, PITPs are known to play diverse roles in membrane trafficking, phosphoinositide signaling, and the regulation of phospholipid metabolism. In this study, analysis of the Arabidopsis genome indicates that PATL1 is one of a small family of Arabidopsis proteins characterized by a variable N-terminal domain followed by two domains found in other membrane trafficking proteins. Immunolocalization and biochemical fractionation studies indicate that PATL1 is recruited from the cytoplasm to the expanding and maturing cell plate. In vesicle binding assays, PATL1 bound to specific phosphoinositides that are important regulators of membrane trafficking. These findings suggest a role for PATL1 in the membrane trafficking events associated with cell plate expansion or maturation and emphasize the involvement of phosphoinositides in cell plate formation. A Role for Endocytosis in Gravitropism? The perception of gravity by plant cells is mediated by the sedimentation of amyloplasts that results in mechanical pressure on cellular components such as the endoplasmic reticulum (ER), cytoskeleton, or internal membranes. This mechanical stimulus is thought to trigger a signal-transduction cascade and, ultimately, to regulate auxin flux and differential growth. In this issue, Silady et al. (pp. 3095–3103) characterize the gravitropism defective 2 (grv2) mutant of Arabidopsis. The inflorescence shoots of grv2 mutants exhibit three abnormalities related to tropisms: reduced response to gravity, extended horizontal growth of lateral shoots, and enhanced response to light. Amyloplasts in the shoot endodermal cells of grv2 do not sediment to the same degree as in wild type. The GRV2 gene encodes a polypeptide that is 42% similar to the nematode Caenorhabditis elegans RME-8 protein, which is required for the receptor-mediated endocytosis of yolk protein into oocytes. Since there are no structurally similar genes to GRV2 in the Arabidopsis genome that could compensate for the loss of GRV2 function, the authors speculate that the grv2 mutation may result in generally inefficient endocytosis and membrane recycling. Conceivably, a defect in GRV2 may prevent proper cycling of proteins from the plasma membrane to the internal membrane network. This could, for example, affect the normal turnover of auxin efflux carriers via endocytosis and polarized secretion as well as their relocalization during gravitropism. It will be interesting to determine whether the cycling of auxin efflux carriers or other membrane proteins is affected in the grv2 mutant. Ethylene as an Air Pollutant? Ethylene is a volatile hormone that diffuses freely in air and can potentially affect neighboring plants. In rural areas, the ethylene concentrations in free air are typically below 2 nL L–1. In heavily industrialized areas, airborne ethylene may reach an annual average of 20 nL L–1 and hourly maximum concentrations of 1 µL L–1. Field studies of the effects of anthropogenic ethylene have shown that plants downwind from major industrial sources often show symptoms of ethylene "poisoning," including epinasty and effects on seed germination, flowering, and fruit ripening. In this issue, Munné-Bosch et al. (pp. 2937–2947) examine the effects of airborne ethylene at concentrations found in polluted areas on the response of plants to other environmental stresses. More specifically, the authors evaluated the extent of oxidative stress, photo- and antioxidant protection, and visual leaf area damage in ethylene-treated and control holm oak (Quercus ilex) plants exposed to heat stress, or to a combination of heat and drought stress. Ethylene-treated plants showed symptoms of oxidative stress at lower temperatures (35°C) than the controls in drought. In addition, ethylene-treated plants showed higher visual leaf area damage and greater reductions in the maximum efficiency of the PSII photochemistry than controls in response to heat stress or to a combination of heat and drought stress. These results demonstrate for the first time that airborne ethylene at concentrations similar to those found in polluted areas may reduce plant stress tolerance by altering, among other possible mechanisms, antioxidant defenses.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900123. Related articles in Plant Physiol.:
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) at the plasma membrane and culminated by the generation of polysaccharide-cleaving ·OH by cell wall peroxidase. In support of their hypothesis, the authors demonstrate that ROIs are produced in the elongation zone of maize roots and that the auxin-induced inhibition of growth is accompanied by a reduction of 