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Plant Physiology 132:1117-1118 (2003) © 2003 American Society of Plant Biologists A Promising Insecticidal LectinLeaves of ground ivy (Glechoma hederacea; see Fig. 1), a member of the Lamiaceae, contain a lectin called Gleheda that is structurally and evolutionary related to the classical legume lectins. In addition to adding a new layer of mystery concerning the molecular evolution of legume-type lectins, this discovery also raises the questions of what are the functions of Gleheda in planta and in which part of the plant does it occur? Wang et al. (pp. 1322–1334) report that Gleheda accounts for over one third of the total leaf protein in some clones, whereas it cannot be detected in other clones pf G. hederacea growing in the same environment. Amazingly, there can be a 5,000-fold difference in the Gleheda content of different clones. Gleheda is predominantly and constitutively expressed in the leaves where it accumulates during early leaf maturation. Wounding, insect feeding, or the application of phytohormones have no effect on its concentration. The lectin is not uniformly distributed over the leaves but exhibits a unique localization pattern characterized by an almost exclusive confinement to a single layer of palisade parenchyma cells. Insect feeding trials demonstrated that Gleheda is a potent insecticidal protein against the larvae of the Colorado potato beetle (Leptinotarsa decemlineata). The observed insecticidal activity cannot be ascribed to an aspecific cytotoxicity since the lectin does not affect the viability of human and murine cells in vitro. Most probably, the observed adverse effects on the L. decemlineata larvae are related to the pronounced specificity of Gleheda toward a structurally simple O-linked glycan that is quite common in lower animals but is normally not present in higher vertebrates. The specific insecticidal activity of Gleheda makes it an interesting candidate for eventual introduction into transgenic crops.
Dwarf Aspens The advantages of dwarf or semi-dwarf varieties in orchard and cereal crops are well known, but surprisingly semi-dwarf varieties may also prove to be advantageous in forestry. Some models suggest that semi-dwarf trees, due to reduced investment in root mass, could potentially have higher biomass productivity than non-dwarf trees. Dwarfism may also reduce the propensity of tree crop progeny to spread in the wild as well as the chances of linked transgenes successfully introgressing into wild populations. The recent identification and functional analysis of the genes encoding the major catabolic enzyme, gibberellic acid (GA) 2-oxidase (GA2ox), have revealed that GA2ox is important in controlling of GA levels. In this issue, Busov et al. (pp. 1283–1291) report that they have identified a dwarf transgenic hybrid poplar (Populus tremula X Populus alba) possessing a hyperactivated gene encoding GA2ox. The mutation resulted from insertion of a strong transcriptional enhancer near the transcription start site. Overexpression of the poplar GA2ox-encoding gene (PtaGA2ox1) caused hyperaccumulation of mRNA transcripts and a quantitative shift in the spectrum but not the total levels of GAs. Exogenous application of GA3, which is resistant to catabolism by GA2ox, rapidly restored normal development to the mutant, strongly supporting the hypothesis that the mutant phenotype is a result of the deficiency of the bioactive GAs, GA1 and GA4. Calmodulin and Heat Shock Cells synthesize heat shock proteins (HSPs) in response to elevated temperature. Using a Ca2+ indicator dye and confocal microscopy, Liu et al. (pp. 1405–1414) investigated the involvement of Ca2+ and Ca2+-activated calmodulin (Ca2+-CaM) in heat shock signal transduction in wheat (Triticum aestivum). They measured an increase of intracellular free Ca2+ ([Ca2+] cyt) started within 1 min after a 37°C heat shock. The levels of CaM mRNA and protein increased during HS at 37°C in the presence of Ca2+. The expression of hsp26 and hsp70 genes was up-regulated by the addition of CaCl2 and down-regulated by the Ca2+ ion chelator EGTA, Ca2+ ion channel blockers, or by CaM antagonists. These results indicate that Ca2+-CaM is directly involved in the HS signal transduction pathway. A Cold-Tolerant C4 Plant C4 photosynthesis has a higher theoretical efficiency and potential productivity than C3 photosynthesis, but this efficiency is normally realized only in humid, warm environments. Both the early season growth and the growing range of many C4 crops, including maize (Zea mays), sorghum (Sorghum bicolor), and sugar cane (Saccharum officinarum), are limited by poor performance at low temperatures. The rhizomatous perennial grass Miscanthus x giganteus is from the same taxonomic group as sugar cane, sorghum and maize, and uses the same C4 photosynthetic pathway (NADP-ME form). In contrast to Z. mays, field observations of M. x giganteus show that it maintains high quantum yields of CO2 assimilation under chilling conditions (i.e. below 12°C). This species, therefore, represents an invaluable resource for understanding how its agronomically important relatives might be altered to similarly achieve high photosynthetic capacity at low growth temperatures. In this issue, Naidu et al. (pp. 1688–1697) report that a low temperature growth regimen had no effect on photosynthesis in M. x giganteus, but decreased rates by 80% at all measurement temperatures in Z. mays. Neither the amounts nor the expression of phosphenolpyruvate (PEP) carboxylase were affected by growth temperature in either species. However, pyruvate orthophosphate dikinase (PPDK) and the large subunit of Rubisco decreased approximately 50% and approximately 30%, respectively, in cold-grown Z. mays, while remaining unaffected by temperature in M. x giganteus. Although a key role for PPDK in controlling C4 photosynthesis at low temperature has been previously suggested, there were no obvious differences in the sequence of M. x giganteus C4-PPDK relative to S. officinarum and Z. mays that could explain increased protein stability of this enzyme at low temperature. Redox Control of Protein Tyr Phosphatases (PTPs) and MAP Kinase The oxidative stress factor H2O2 has been shown to serve as a critical messenger in many signal transduction pathways including those involved in the responses of plants to pathogen invasion, to the hormone abscisic acid, and to various abiotic stresses. In one class of the receptor-like PTPs of animal cells, H2O2 induces a rapid and reversible catalytic Cys-dependent conformation change in vivo. The catalytic Cys must be in the reduced form for a PTP to be active. Because redox regulation is critically important in plant cell regulation, Gupta and Luan (pp. 1149–1152) examined whether plant PTPs are also regulated by the redox state of Cys residues and thereby serve as a molecular target for oxidative stress. To study the effect of H2O2 on plant PTPs, they analyzed the phosphatase activity of purified AtPTP1 and determined that the activity of AtPTP1 was arrested within 1 min after addition of H2O2. Recent studies have shown that PTPs function in the regulation of MAP kinases in plants as they do in animal cells. Interestingly, oxidative stress such as H2O2 treatment also activates MAP kinases in plant cells. The authors show that AtPTP1 inactivation was strongly correlated with MAPK (AtMPK6) activation by H2O2, suggesting that AtPTP1 may represent a primary target for oxidative stress in plants. Copines: Ca2+-Depenent Phospholipid-Binding Proteins The copines are a class of highly conserved proteins present in organisms ranging from protozoans to mammals to plants. These proteins are named copines (the French feminine noun meaning "friends") because of their tight association with lipid membranes. Although several biochemical studies of copines have revealed that they have a calcium-dependent phospholipid-binding activity, their specific biological functions are unknown. In this issue, Jambunathan and McNellis (pp. 1370–1381) report that the mutation of the Arabidopsis CPN1 (COPINE 1) gene is associated with an increased resistance to bacterial and oomycetous pathogens. CPN1 transcript accumulation was induced specifically by pathogenic bacteria, and this induction required a functional bacterial type III protein secretion system. Salicylic acid, a key chemical inducer of plant defense responses required for the development of systemic acquired resistance (SAR), also induced an increase in CPN1 gene transcript. In toto, these results suggest that the CPN1 gene product could act as a suppressor of defense-related cell death and defense responses. The mutant also shows an altered phenotype, including small size, curled leaves, and minute lesions (a hyper-sensitive cell death response) under conditions of low humidity or low temperature. The authors speculate the apparent involvement of CPN1 in plant responses to both biotic and abiotic stimuli may be related to calcium-dependent phospholipid binding. CPN1, as a Ca2+-dependent membrane-associated protein, may be involved in determining the specificity of Ca2+ signaling and preventing inappropriate defense responses under conditions of low temperature or low humidity.
Department of Natural Sciences Mercy College Dobbs Ferry, NY 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.900082. Related articles in Plant Physiol.:
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