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Plant Physiology 137:1171-1172 (2005) © 2005 American Society of Plant Biologists On the InsideRNAi Silencing of Isoflavone SynthaseThe isoflavones play diverse roles in the plant-microbe interactions of the Papilionoideae. Isoflavones function as antibiotics as well as precursors for defense-related phytoalexins. They also serve as signal molecules in the induction of microbial genes involved in soybean (Glycine max) nodulation. Although isoflavones are normally present at relatively low levels in mature soybean tissues, their accumulation is strongly induced in response to pathogen attack or defense elicitors. Isoflavone synthase (IFS) is a key enzyme for the formation of the isoflavones. Subramanian et al. (pp. 1345–1353) show that the silencing of both copies of IFS genes in roots initiated from soybean cotyledons leads to the effective spread of silencing throughout the nontransformed cotyledon tissues. Distal silencing was established within 5 d of transformation and was highly efficient for a 3- to 4-d period, after which it faded. Distal silencing effected a nearly complete suppression of mRNA accumulation for both the IFS1 and IFS2 genes and of isoflavone production induced by wounding or treatment with the cell wall glucan elicitor from Phytophthora sojae. Preformed isoflavone conjugates were not reduced in distal tissues, suggesting little turnover of these stored isoflavone pools. Silencing of IFS led to enhanced susceptibility to P. sojae. The soybean cotyledon system may prove to be a convenient and effective system for the functional analysis of other plant genes through gene silencing. Oxalate Oxidase Confers Resistance to Sclerotinia Blight in Peanut Sclerotinia minor is the causal agent of Sclerotinia blight, a highly destructive disease of peanut (Arachis hypogaea). Much evidence indicates that oxalic acid serves as a pathogenicity factor during Sclerotinia infection. Direct application of oxalic acid to stem or leaf tissue causes tissue injury and wilting, similar to plant responses to fungal infection by S. sclerotiorum. Mutant isolates of S. sclerotiorum, deficient in oxalic acid production, are not pathogenic on bean (Phaseolus vulgaris), but revertants became pathogenic once they regain the ability to produce oxalic acid. Oxalic acid may aid in fungal infection through a number of proposed routes, including the facilitation of cell wall-degrading enzyme activity, through pH-mediated tissue damage, or via sequestration of Ca2+ ions. There is also evidence that oxalic acid can directly suppress the oxidative burst associated with the detection of pathogens by plants and disturb guard cell function during infection by S. sclerotiorum by inducing stomatal opening and inhibiting abscisic acid (ABA)-induced stomatal closure. Oxalate oxidase (OO) has received considerable attention for its possible utility in plant defense. OO catalyzes the degradation of oxalic acid to produce carbon dioxide and hydrogen peroxide. It has been proposed that through the production of hydrogen peroxide, OO may cause cross-linking of plant cell wall proteins at the site of infection or play a role in the plant hypersensitive response. Livingstone et al. (pp. 1354–1362) report that the size of S. minor-induced lesions were reduced by 75% to 97% in OO-transformed peanut plants, providing evidence that OO confers enhanced resistance to Sclerotinia blight in peanut. Kinematic Analysis of an Aphid Infestation The pea aphid (Acyrthosiphon pisum) is a phloem-sap feeder known to reduce growth and dry-mass production under field conditions (Fig. 1). Pea aphids preferentially settle on the elongating internodes of alfalfa (Medicago sativa) and only rarely colonize leaves. Thus, their impact on stem elongation is particularly pronounced. Because pea aphids neither transmit viruses nor deliver toxic substances by salivary secretions, it is generally assumed that their effect on growth is mainly due to removal of phloem sap from their host plants. Kinematic studies offer a powerful tool for assessing the fluxes and deposition rates that sustain the growth of plant organs. Although kinematic studies have previously been used to study the effects of a wide variety of abiotic stresses on growth, Girousse et al. (pp. 1474–1484) provide the first kinematic study of a biotic stress, namely pea aphid infestation of alfalfa. They report that severe short-term aphid infestation induces a strong and synchronized reduction in elongation and in water and carbon deposition. Reduced nitrogen contents were observed in some parts of the infested stems, especially in the apex. This suggests that aphid infestation converts the shoot apex, normally a sink tissue, into a source tissue. Because aphid infestation reduced longitudinal elongation 1.4 times more than radial expansion, and in a manner similar to mechanical stimulation, the authors speculate that aphid infestation stress may also involve a thigmomorphogenic component.
Novel Anion Transporter in the Symbiosome Membrane During development of a legume-Rhizobium symbiosis, the bacterium ultimately becomes enclosed in a specialized, plant-derived organelle known as the symbiosome. It is across the symbiosome that reduced carbon compounds from the plant cytosol are exchanged for reduced nitrogen from the bacteroid. Vincill et al. (pp. 1435–1444) have isolated a cDNA from soybean nodules that encodes a putative transporter (GmN70) of the Major Facilitator Superfamily. GmN70 is expressed predominantly in the symbiosome membrane of mature nitrogen-fixing root nodules. Outward currents, carried by anions and with a selectivity of nitrate > nitrite >> chloride, were observed in Xenopus laevis oocytes expressing GmN70. No apparent transport of organic anions was observed. Half maximal currents were induced by nitrate concentrations between 1 to 3 mM. These currents showed little sensitivity to pH or the nature of the counter cation in the oocyte bath solution. Voltage clamp records of an ortholog of GmN70 from Lotus japonicus (LjN70) also showed anion currents with a similar selectivity profile. These findings suggest that GmN70 and LjN70 are inorganic anion transporters of the symbiosome membrane with enhanced preference for nitrate. Previously, it has been shown that the membrane potential generated by the H+-ATPase in isolated intact soybean symbiosomes can be collapsed by the addition of anion salts, suggesting the existence of an anion transporter in the symbiosome membrane. Interestingly, the effectiveness of the various anion salts to dissipate the soybean symbiosome membrane potential is similar to the permeability profile of GmN70. Proteomics of Seed Filling
During a 4- to 5-week period of seed filling, most of the storage reserves in soybeans are synthesized. At maturation, approximately 41% of soybean seed dry weight is storage protein. The two most prevalent seed storage proteins are glycinin and Aphid Resistance in Medicago truncatula Phloem feeding insects such as aphids harm plants by direct feeding damage and by serving as vectors for the spread of microbial pathogens. Bluegreen aphid or blue alfalfa aphid (Acyrthosiphon kondoi) is an important pest of pasture legumes, particularly Medicago spp. Klingler et al. (pp. 1445–1455) provide insights into aphid resistance by means of a side-by-side comparison of two closely related M. truncatula cultivars, Jemalong/A17 and Jester. They report that alatae (the winged, migratory morphs) prefer susceptible line A17 over the resistant line Jester, suggesting that antixenotic (deterrent) factors are present in aphid-resistant plants. The proportion of time that aphids spent ingesting phloem sap was dramatically reduced for aphids on previously infested Jester plants. Antibiosis against A. kondoi is enhanced by prior infestation, indicating induction of this phloem-specific defense. The finding that shoot excision eliminates A. kondoi resistance in M. truncatula raises the possibility that a resistance factor(s) is imported to the feeding site and that resistance may not be tissue autonomous. Aphid resistance segregates as a single dominant gene, AKR (Acyrthosiphon kondoi resistance), in two mapping populations, which have been used to map the locus to a region flanked by resistance gene analogs predicted to encode the CC-NBS-LRR subfamily of resistance proteins. These results suggest that AKR may reside within a cluster of defense-related genes.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 Related articles in Plant Physiol.:
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-conglycinin. Despite the importance of seed filling, systematic proteomic analyses of this phase of seed development are only beginning to be initiated in legumes. Hajduch et al. (pp.