No abstract

In plant-microbe interactions, plant sugars produced by photosynthesis are not only a carbon source for pathogens, but may also act as signals that modulate plant defense responses. Here, we report that decreasing sorbitol synthesis in apple () leaves by antisense suppression of () leads to downregulation of 56 () genes and converts the phenotypic response to from resistant to susceptible. We identified a resistance protein encoded by the apple gene and a small protein encoded by the fungal gene that interact in both a yeast two-hybrid assay and a bimolecular fluorescence complementation assay. Deletion of in enables gain of virulence on the wild-type control plant. Overexpression of in two antisense lines increases resistance, whereas RNAi suppression of in the wild-type control decreases resistance against MdWRKY79 transcriptionally regulates by binding to the promoter of in response to sorbitol, and exogenous sorbitol feeding partially restores resistance of the antisense lines to These findings indicate that sorbitol modulates resistance to via the MdNLR16 protein that interacts with the fungal effector in a classic gene-for-gene manner in apple.

Apple bitter rot caused by Colletotrichum species is a growing problem worldwide. Colletotrichum spp. are economically important but taxonomically un-resolved. Identification of Colletotrichum spp. is critical due to potential species-level differences in pathogenicity-related characteristics. A 400-isolate collection from New York apple orchards were morphologically assorted to two groups, C. acutatum species complex (CASC) and C. gloeosporioides species complex (CGSC). A sub-sample of 44 representative isolates, spanning the geographical distribution and apple varieties, were assigned to species based on multi-locus phylogenetic analyses of nrITS, GAPDH and TUB2 for CASC, and ITS, GAPDH, CAL, ACT, TUB2, APN2, ApMat and GS genes for CGSC. The dominant species was C. fioriniae, followed by C. chrysophilum and a novel species, C. noveboracense, described in this study. This study represents the first report of C. chrysophilum and C. noveboracense as pathogens of apple. We assessed the enzyme activity and fungicide sensitivity for isolates identified in New York. All isolates showed amylolytic, cellulolytic and lipolytic, but not proteolytic activity. C. chrysophilum showed the highest cellulase and the lowest lipase activity, while C. noveboracense had the highest amylase activity. Fungicide assays showed that C. fioriniae was sensitive to benzovindiflupyr and thiabendazole, while C. chrysophilum and C. noveboracense were sensitive to fludioxonil, pyraclostrobin and difenoconazole. All species were pathogenic on apple fruit with varying lesion sizes. Our findings of differing pathogenicity-related characteristics among the three species demonstrate the importance of accurate species identification for any downstream investigations of Colletotrichum spp. in major apple growing regions.

Arbuscular mycorrhizal fungi (AMF, subphylum Glomeromycotina) are symbionts of most terrestrial plants. They commonly harbour endobacteria of a largely unknown biology, referred to as MRE (Mollicutes/mycoplasma-related endobacteria). Here, we propose to accommodate MRE in the novel genus 'Candidatus Moeniiplasma.' Phylogeny reconstructions based on the 16S rRNA gene sequences cluster 'Ca.Moeniiplasma' with representatives of the class Mollicutes, whereas phylogenies derived from amino acid sequences of 19 genes indicate that it is a discrete lineage sharing ancestry with the members of the family Mycoplasmataceae. Cells of 'Ca.Moeniiplasma' reside directly in the host cytoplasm and have not yet been cultivated. They are coccoid, ~500 nm in diameter, with an electron-dense layer outside the plasma membrane. However, the draft genomes of 'Ca.Moeniiplasma' suggest that this structure is not a Gram-positive cell wall. The evolution of 'Ca.Moeniiplasma' appears to be driven by an ultrarapid rate of mutation accumulation related to the loss of DNA repair mechanisms. Moreover, molecular evolution patterns suggest that, in addition to vertical transmission, 'Ca.Moeniiplasma' is able to transmit horizontally among distinct Glomeromycotina host lineages and exchange genes. On the basis of these unique lifestyle features, the new species 'Candidatus Moeniiplasma glomeromycotorum' is proposed.