Responses of tomato genotypes to avirulent and Mi-virulent Meloidogyne javanica isolates occurring in Israel.

The behavior of naturally virulent Meloidogyne isolates toward the tomato resistance gene Mi in major tomato-growing areas in Israel was studied for the first time. Virulence of seven selected isolates was confirmed over three successive generations on resistant (Mi-carrying) and susceptible (non-Mi-carrying) tomato cultivars. Diagnostic markers verified the predominance of Meloidogyne javanica among virulent isolates selected on resistant tomato cultivars or rootstocks. To better understand the determinants of nematode selection on Mi-carrying plants, reproduction of Mi-avirulent and virulent isolates Mjav1 and Mjv2, respectively, measured as eggs per gram of root, on non-Mi-carrying, heterozygous (Mi/mi) and homozygous (Mi/Mi) genotypes was evaluated. Although no reproduction of Mjav1 was observed on Mi/Mi genotypes, some reproduction was consistently observed on Mi/mi plants; reproduction of Mjv2 on the homozygous and heterozygous genotypes was similar to that on susceptible cultivars, suggesting a limited quantitative effect of the Mi gene. Histological examination of giant cells induced by Mi-virulent versus avirulent isolates confirmed the high virulence of Mjv2 on Mi/mi and Mi/Mi genotypes, allowing the formation of well-developed giant-cell systems despite the Mi gene. Analysis of the plant defense response in tomato Mi/Mi, Mi/mi, and mi/mi genotypes to both avirulent and virulent isolates was investigated by quantitative real-time polymerase chain reaction. Although the jasmonate (JA)-signaling pathway was clearly upregulated by avirulent and virulent isolates on the susceptible (not carrying Mi) and heterozygous (Mi/mi) plants, no change in signaling was observed in the homozygous (Mi/Mi) resistant line following incompatible interaction with the avirulent isolate. Thus, similar to infection promoted by the avirulent isolate on the susceptible genotype, the Mi-virulent isolate induced the JA-dependent pathway, which might promote tomato susceptibility during the compatible interaction with the homozygous (Mi/Mi) resistant line. These results have important consequences for the management of Mi resistance genes for ensuring sustainable tomato farming.

Accumulation of the mycotoxin patulin in the presence of gluconic acid contributes to pathogenicity of Penicillium expansum.

Penicillium expansum, the causal agent of blue mold rot, causes severe postharvest fruit maceration through secretion of D-gluconic acid (GLA) and secondary metabolites such as the mycotoxin patulin in colonized tissue. GLA involvement in pathogenicity has been suggested but the mechanism of patulin accumulation and its contribution to P. expansum pathogenicity remain unclear. The roles of GLA and patulin accumulation in P. expansum pathogenicity were studied using i) glucose oxidase GOX2-RNAi mutants exhibiting decreased GOX2 expression, GLA accumulation, and reduced pathogenicity; ii) IDH-RNAi mutants exhibiting downregulation of IDH (the last gene in patulin biosynthesis), reduced patulin accumulation, and no effect on GLA level; and iii) PACC-RNAi mutants exhibiting downregulation of both GOX2 and IDH that reduced GLA and patulin production. Present results indicate that conditions enhancing the decrease in GLA accumulation by GOX2-RNAi and PACC-RNAi mutants, and not low pH, affected patulin accumulation, suggesting GLA production as the driving force for further patulin accumulation. Thus, it is suggested that GLA accumulation may modulate patulin synthesis as a direct precursor under dynamic pH conditions modulating the activation of the transcription factor PACC and the consequent pathogenicity factors, which contribute to host-tissue colonization by P. expansum.

Fatty acid-and retinol-binding protein, Mj-FAR-1 induces tomato host susceptibility to root-knot nematodes.

Plant-parasitic nematodes produce at least one structurally unique class of small helix-rich retinol- and fatty-acid-binding proteins that have no counterparts in their plant hosts. Herein we describe a protein of the plant-parasitic root-knot nematode Meloidogyne javanica, which is a member of the nematode-specific fatty-acid- and retinol-binding (Mj-FAR-1) family of proteins. The mj-far-1 mRNA was detected through M. javanica pre-parasitic J2s, migratory and sedentary parasitic stages by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). Immunolocalization assays demonstrate that the FAR protein of Meloidogyne is secreted during sedentary stages, as evidenced by the accumulation of FAR at the nematode cuticle surface and along the adjacent host root tissues. Tomato roots constitutively expressing mj-far-1 demonstrated an increased susceptibility to root-knot nematodes infection as observed by accelerated gall induction and expansion, accompanied by a higher percentage of nematodes developing into mature females compared to control roots. RNA interference assays that expressed double-stranded RNA complementary to mj-far-1 in transgenic tomato lines specifically reduced nematode infection levels. Histological analysis of nematode-infested roots indicated that in roots overexpressing mj-far-1, galls contained larger feeding cells and might support a faster nematode development and maturation. Roots overexpressing mj-far-1 suppressed jasmonic acid responsive genes such as the proteinase inhibitor (Pin2) and γ-thionin, illustrating the possible role of Mj-FAR-1 in manipulating the lipid based signaling in planta. This data, suggests that Meloidogyne FAR might have a strategic function during the interaction of the nematode with its plant host. Our study present the first demonstration of an in planta functional characterization and localization of FAR proteins secreted by plant-parasitic nematodes. It provides evidence that Mj-FAR-1 facilitates infection most likely via the manipulation of host lipid-based defenses, as critical components for a successful parasitism by plant-parasitic nematodes.

A Penicillium expansum glucose oxidase-encoding gene, GOX2, is essential for gluconic acid production and acidification during colonization of deciduous fruit.

Penicillium expansum, the causal agent of blue mold rot, causes severe postharvest maceration of fruit through secretion of total, d-gluconic acid (GLA). Two P. expansum glucose oxidase (GOX)-encoding genes, GOX1 and GOX2, were analyzed. GOX activity and GLA accumulation were strongly related to GOX2 expression, which increased with pH to a maximum at pH 7.0, whereas GOX1 was expressed at pH 4.0, where no GOX activity or extracellular GLA were detected. This differential expression was also observed at the leading edge of the decaying tissue, where GOX2 expression was dominant. The roles of the GOX genes in pathogenicity were further studied through i) development of P. expansum goxRNAi mutants exhibiting differential downregulation of GOX2, ii) heterologous expression of the P. expansum GOX2 gene in the nondeciduous fruit-pathogen P. chrysogenum, and iii) modulation of GLA production by FeSO(4) chelation. Interestingly, in P. expansum, pH and GLA production elicited opposite effects on germination and biomass accumulation: 26% of spores germinated at pH 7.0 when GOX activity and GLA were highest whereas, in P. chrysogenum at the same pH, when GLA did not accumulate, 72% of spores germinated. Moreover, heterologous expression of P. expansum GOX2 in P. chrysogenum resulted in enhanced GLA production and reduced germination, suggesting negative regulation of spore germination and GLA production. These results demonstrate that pH modulation, mediated by GLA accumulation, is an important factor in generating the initial signal or signals for fungal development leading to host-tissue colonization by P. expansum.