Abstract
Soybean is an important crop in South America, and its production is limited by fungal diseases caused by species from the genus Diaporthe, including seed decay, pod and stem blight, and soybean stem canker (SSC). In this study, we focused on Diaporthe species isolated from soybean plants with SSC lesions in different parts of Uruguay. Diaporthe diversity was determined by sequencing the internal transcribed spacer (ITS) regions of ribosomal RNA and a partial region of the translation elongation factor 1-alpha gene (TEF1α). Phylogenetic analysis showed that the isolates belong to five defined groups of Diaporthe species, Diaporthe caulivora and Diaporthe longicolla being the most predominant species present in stem canker lesions. Due to the importance of D. caulivora as the causal agent of SSC in the region and other parts of the world, we further characterized the interaction of this pathogen with soybean. Based on genetic diversity of D. caulivora isolates evaluated with inter-sequence single repetition (ISSR), three different isolates were selected for pathogenicity assays. Differences in virulence were observed among the selected D. caulivora isolates on susceptible soybean plants. Further inspection of the infection and colonization process showed that D. caulivora hyphae are associated with trichomes in petioles, leaves, and stems, acting probably as physical adhesion sites of the hyphae. D. caulivora colonized the stem rapidly reaching the phloem and the xylem at 72 h post-inoculation (hpi), and after 96 hpi, the stem was heavily colonized. Infected soybean plants induce reinforcement of the cell walls, evidenced by incorporation of phenolic compounds. In addition, several defense genes were induced in D. caulivora–inoculated stems, including those encoding a pathogenesis-related protein-1 (PR-1), a PR-10, a β-1,3-glucanase, two chitinases, two lipoxygenases, a basic peroxidase, a defensin, a phenylalanine-ammonia lyase, and a chalcone synthase. This study provides new insights into the interaction of soybean with D. caulivora, an important pathogen causing SSC, and provides information on the activation of plant defense responses.
Highlights
Soybean [Glycine max (L.) Merr.] is one of the most important sources of oil and plant protein (Masuda and Goldsmith, 2009)
The results show that Diaporthe species associated to stem canker lesions in Uruguay are mainly D. caulivora and D. longicolla
soybean stem canker (SSC) is a disease caused by Diaporthe species that affect the production of soybean in South America and other parts of the world
Summary
Soybean [Glycine max (L.) Merr.] is one of the most important sources of oil and plant protein (Masuda and Goldsmith, 2009). Several species from the fungal genus Diaporthe Nitschke (asexual morph Phomopsis) (Sacc.) cause important diseases in soybean that affect its production, including seed decay, pod and stem blight, and stem canker (Santos et al, 2011). In Uruguay, different isolates of D. aspalathi and D. caulivora were identified during 2012–2013 in soybean stems with canker lesions (Stewart, 2015). The high prevalence of D. caulivora isolates causing SSC in Uruguay is probably due to the use of soybean genotypes, mostly from Argentinian sources, carrying resistance genes that are not effective to D. caulivora (Stewart, 2015) This is consistent with previous results showing that Argentinian soybean genotypes carrying Rdm, Rdm, Rdm, and Rdm conferred resistance to D. aspalathi but not to D. caulivora (Pioli et al, 2003). Yield losses by D. caulivora can be significant, especially when canker lesions develop early, leading to plant wilting and death in the middle of the vegetative stage (Vidić et al, 2013)
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