Biochemical and Histological Insights into the Interaction Between the Canker Pathogen Neofusicoccum parvum and Prunus dulcis L.

The number of reports associated with wood dieback caused by fungi in the Botryosphaeriaceae in numerous perennial crops worldwide has significantly increased in the past years. In this study, we investigated the interactions between the canker pathogen Neofusicoccum parvum and the almond tree host (Prunus dulcis L.), with an emphasis on varietal resistance and host response at the cell wall biochemical and histological levels. Plant bioassays in a shaded house showed that among the four commonly planted commercial almond cultivars (cvs. ‘Butte’, ‘Carmel’, ‘Monterey’ and ‘Nonpareil’), there was no significant varietal difference with respect to resistance to the pathogen. Gummosis was only triggered by fungal infection, and not by wounding. A two-dimensional NMR and liquid chromatography determination of cell wall polymers showed that infected almond trees differed significantly in their glycosyl and lignin composition compared to healthy, non-infected trees. Response to fungal infection involved a significant increase in lignin, a decrease in glucans, and an overall enrichment in other carbohydrates with a profile similar to those observed in gums. Histological observations revealed the presence of guaiacyl-rich cell wall reinforcements. Confocal microscopy suggested that N. parvum mainly colonized the lumina of xylem vessels and parenchyma cells, and to a lesser extent the gum ducts. We discuss the relevance of these findings in the context of the CODIT model in almond and its potential involvement in the vulnerability of the host toward fungal wood canker diseases.

Lachnellula willkommii (European larch canker).

The European larch canker pathogen, L. willkommii, is apparently native to Japan, but established in Europe, where it became well known due to its damage to plantations of exotic and native Larix species, beginning in the nineteenth century. It attacks and spreads among the various species of Larix once it has been introduced. It was detected as an invasive to North America on two occasions; once in the northeastern USA in the 1920s (Hahn and Ayres, 1936) and once in the eastern maritime provinces of Canada in the 1980s (Magasi and Pond, 1982). Efforts to prevent its introduction across natural barriers include regulation and restriction of trade and transport of susceptible species and bark-bearing products made from them. Control by destruction of infected plants or plant parts is often made difficult by the size of the trees concerned (Tegethoff, 1965). Local spread of the fungus between trees appears to depend on dissemination and survival of airborne ascospores. Climatic conditions of humidity and temperature appear to limit natural spread from regions of establishment (Ostaff, 1985).

Identification and validation of miRNA reference genes in poplar under pathogen stress

Quantitative real time polymerase chain reaction (qRT-PCR) is a common method to analyze gene expression. Due to differences in RNA quantity, quality, and reverse transcription efficiency between qRT-PCR samples, reference genes are used as internal standards to normalize gene expression. However, few universal genes especially miRNAs have been identified as reference so far. Therefore, it is essential to identify reference genes that can be used across various experimental conditions, stress treatments, or tissues. In this study, 14 microRNAs (miRNAs) and 5.8S rRNA were assessed for expression stability in poplar trees infected with canker pathogen. Using three reference gene analysis programs, we found that miR156g and miR156a exhibited stable expression throughout the infection process. miR156g and miR156a were then tested as internal standards to measure the expression of miR1447 and miR171c, and the results were compared to small RNA sequencing (RNA-seq) data. We found that when miR156a was used as the reference gene, the expression of miR1447 and miR171c were consistent with the small RNA-seq expression profiles. Therefore, miR156a was the most stable miRNAs examined in this study, and could be used as a reference gene in poplar under canker pathogen stress, which should enable comprehensive comparisons of miRNAs expression and avoid the bias caused by different lenth between detected miRNAs and traditional referece genes. The present study has expanded the miRNA reference genes available for gene expression studies in trees under biotic stress.

Draft Genome Sequences of Nine Japanese Strains of the Kiwifruit Bacterial Canker Pathogen Pseudomonas syringae pv. actinidiae Biovar 3

ABSTRACT Pseudomonas syringae pv. actinidiae is the pathogen that causes kiwifruit bacterial canker and is categorized into several groups (biovars). In Japan, biovar 3, known as the pandemic group, was first discovered in 2014. Here, we sequenced the genomes of nine Japanese biovar 3 strains.