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Plant Physiology 139:1097-1098 (2005) © 2005 American Society of Plant Biologists On the InsideJasmonic Acid and the Establishment of Arbuscular MycorrhizaeMore than 80% of terrestrial plant species form symbiotic associations with arbuscular mycorrhizal (AM) fungi. The main feature of this mutual symbiosis is the exchange of nutrients between both partners: The fungus supplies the plant with mineral nutrients from the soil and receives carbohydrates in return. Arbuscules are the main symbiotic organs involved in the exchange of nutrients between the fungus and the plant. Although screening of several nonmycorrhizal mutants has led to the identification of putative plant receptors of fungal signals, little is known about such factors and the molecular dialogue between the two symbiotic partners that leads to the establishment of AM. During the symbiotic interaction between Medicago truncatula (Fig. 1) and the AM fungus Glomus intraradices, an endogenous increase in jasmonic acid (JA) occurs. Allene oxide cyclase (AOC) is believed to be of prime importance in JA biosynthesis. Isayenkov et al. (pp. 1401–1410) have cloned and characterized two full-length cDNAs coding for AOC from M. truncatula, designated as MtAOC1 and MtAOC2. The AOC protein was localized in plastids and found to occur constitutively in all vascular tissues of M. truncatula. MtAOCs are expressed in leaves and roots upon JA application. Enhanced expression was also observed during mycorrhization with G. intraradices. A partial suppression of MtAOC expression was achieved in roots following transformation with Agrobacterium rhizogenes harboring the MtAOC1 cDNA in the antisense direction. Roots with suppressed MtAOC1 expression exhibited lower JA levels and a remarkable delay in colonization by G. intraradices. Staining of fungal material in roots with suppressed MtAOC1 revealed a decreased number of arbuscules but no alteration in structure. These results indicate a crucial role for JA in the establishment of AM symbiosis in M. truncatula.
An Egg Apparatus-Specific Enhancer
Relatively little is known about the molecular biology underlying the development and function of the female gametophyte in flowering plants. The obstacles to progress in this area include the inaccessibility of the embryo, the brevity of the developmental stage, and the likely lethality of mutants affecting female gametophyte development. As part of efforts directed toward identifying promoters and genes that could be useful in the genetic engineering of apomixis, Yang et al. (pp. 1421–1432) have taken advantage of enhancer detector line(s) with spatially restricted Indole-3-Butyric Acid: The Other Natural Auxin
Indole-3-butyric acid (IBA) is a naturally occurring auxin that has found wide commercial application in the induction root formation in cuttings. Beyond this one application, however, IBA has received scant attention compared to indole-3-acetic acid (IAA). In a number of plant species, however, concentrations of free IBA approach the levels of free IAA. Moreover, IBA and IAA can be interconverted, an observation that has led to the suggestion that IBA may act as a precursor to IAA. Arabidopsis mutants whose roots have reduced sensitivity to growth inhibition by IBA, but normal sensitivity to IAA, have been isolated and shown to have defects in High-Affinity Manganese Transport Manganese (Mn) deficiency is a serious plant nutritional disorder in many areas of the world, often associated with high soil pHs that favor oxidation to plant unavailable MnO2. There is considerable variability among plant species and genotypes in their ability to grow in soil with low Mn availability. In a screening of 32 barley (Hordeum vulgare) genotypes in Australia, for example, it was shown that the most Mn-efficient barley genotypes were largely unaffected by a low plant availability of Mn, whereas the most inefficient ones were not able to complete a full life cycle. Despite the fact that differential Mn efficiency in plants has been known for a long time, the underlying physiological mechanism(s) is not known. Pedas et al. (pp. 1411–1420) have investigated Mn2+ influx and compartmentation in roots of the Mn-efficient genotype Vanessa and the Mn-inefficient genotype Antonia. Two separate Mn transport systems, mediating high-affinity Mn2+ influx at concentrations up to 130 nM and low-affinity Mn2+ influx at higher concentrations, were identified in both genotypes. The two genotypes differed only in high-affinity kinetics, with the Mn-efficient genotype Vanessa having almost 4 times higher Vmax than the inefficient Antonia but similar Km values. Inductively coupled plasma-mass spectrometry measurements verified that the observed differences in high-affinity influx resulted in a higher Mn net uptake of Vanessa compared to Antonia. Further evidence for the importance of the differences in high-affinity uptake kinetics for Mn acquisition was obtained in a hydroponic system with mixed cultivation of the two genotypes at a continuously low Mn concentration (10 to 50 nM) similar to that occurring in soil solution. Under these conditions, Vanessa had a competitive advantage and contained 55% to 75% more Mn in the shoots than did Antonia. It is concluded that differential capacity for high-affinity Mn influx contributes to differences between barley genotypes in Mn efficiency. Repressor of Abscisic Acid Response in Arabidopsis The plant hormone abscisic acid (ABA) plays an important role in plant development and stress responses. An important step of ABA action is the activation or inactivation of gene expression. Although several transcription factors are known to positively regulate ABA-induced gene expression, little is known about the negative regulators of ABA-regulated gene expression. Pandey et al. (pp. 1185–1193) have identified an AP2 domain transcription factor that serves as a repressor of ABA response during seed germination and ABA- and stress-induced gene expression in Arabidopsis. The expression of the AP2-like ABA repressor 1 (ABR1) gene itself was responsive to ABA and stress conditions, including cold, high salt, and drought. Disruption of ABR1 led to a hypersensitive response to ABA in seed germination and root growth assays. The ABA biosynthesis inhibitor norflurazon rescued the stress hypersensitivity phenotype, suggesting that the increased stress sensitivity of the mutant may result from hypersensitivity to ABA. The abr1 mutant plants accumulated significantly higher levels of ABA- and stress-inducible gene transcripts as compared to the wild-type plants, supporting the hypothesis that this AP2 domain protein serves as a repressor of ABA-regulated gene expression. Physiological Activity of a JA Precursor Recent reports suggest that a cyclopentenone precursor of JA, 12-oxo-phytodienoic acid (OPDA), can also induce gene expression. However, little is known about the physiological significance of OPDA-dependent gene expression. Taki et al. (pp. 1268–1283) have performed a microarray analysis of approximately 21,500 Arabidopsis genes to compare responses to JAs and OPDA treatment. Although many genes responded identically to both OPDA and JAs, the authors have identified a set of genes (ORGs) that specifically responded to OPDA but not to JAs. ORGs primarily encoded signaling components, transcription factors, and stress response-related genes. Wounding induced 50% of the ORGs. Analysis using mutants deficient in the biosynthesis of JAs revealed that OPDA functions as a signaling molecule in the wounding response. The OPDA signaling pathway apparently functions independently of JAs signaling and is required for the wounding response in Arabidopsis.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900177. Related articles in Plant Physiol.:
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-glucuronidase expression patterns to isolate regulatory elements and/or their associated genes involved in female gametophyte development. Enhancer detectors typically rely on a Ds element (DsE) or T-DNA carrying a gusA reporter gene under the control of a minimal promoter. If the DsE or T-DNA inserts in the proximity of regulatory sequences (e.g. enhancers), the minimal promoter is influenced by these cis-regulatory elements and then directs gusA expression in a specific temporal and spatial pattern. Since this pattern could reflect the expression of a nearby gene controlled by the same regulatory element, it may allow for the identification of genes according to their expression patterns rather than by the mutant phenotypes caused by their disruption. Using a Ds-based enhancer detector line, the authors have cloned an egg apparatus-specific enhancer (EASE) from Arabidopsis (Arabidopsis thaliana). Although the function of the EASE in the Arabidopsis genome is unknown, its ability to control gene expression in the egg apparatus and early embryo is clear. Without the minimal promoter, the EASE itself is not functional. However, when it is fused to the cauliflower mosaic virus 35S minimal promoter, the EASE can drive gusA or GFP gene expression in a very specific and efficient manner. The potential usefulness of EASE in studying and manipulating specific gene expression in the female gametophyte of Arabidopsis was demonstrated by embryo-specific ablation by means of the transactivation of a diphtheria toxin gene under the control of the EASE.