Defense responses induced by ulvan in wheat against powdery mildew caused by Blumeria graminis f. sp. tritici.

Ulvan is a water-soluble sulfated heteropolysaccharide extracted from the cell walls of the green seaweeds Ulva spp. This polysaccharide is known to induce resistance and protect plants against a broad range of plant pathogenic fungi, such as Blumeria graminis f. sp. tritici (Bgt), the causal agent of powdery mildew in wheat. We aimed to study the defense mechanisms induced by ulvan against Bgt in susceptible wheat by investigating the defense-related gene expression, enzymes activity, accumulation of phenolic compounds and hydrogen peroxide (H2O2), as well as the development of Bgt infection structures in vitro and in planta. Symptoms were reduced by 42% in ulvan-treated plants. In vitro, ulvan did not inhibit conidial germination of Bgt but in planta, increased the appressorial germ tubes without haustorium. Ulvan increased the presence of fluorescent papillae and accumulation of H2O2 at the penetration sites of Bgt, as well as the content of phenolic compounds. POX, PAL and LOX activities were stimulated in ulvan-treated plants during the first 48 h after inoculation. However, few of defense-related genes studied were differentially expressed in infected plants after ulvan treatment. By contrast, in non-infected conditions, ulvan up-regulated the expression of genes involved in phenylpropanoid metabolism, i.e. PAL, CHS, COMT, ANS and FLS, genes encoding pathogenesis-related proteins, i.e. PR1, PR9, PR15, and LOX during the first 96 h after treatment. This study provides new insights about the multiple ulvan effects on wheat defense responses, and especially the elicitation of the phenylpropanoid pathway leading to phenolic compounds accumulation, which could be involved in cell wall reinforcement.

Reclassification of the Main Causal Agent of Glomerella Leaf Spot on Apple into Colletotrichum chrysophilum in Southern Brazil and Uruguay.

Glomerella leaf spot (GLS) is one of the most important diseases of apple, affecting a wide range of economically important cultivars, particularly Golden Delicious and its descendants. Caused mainly by species of the Colletotrichum gloeosporioides species complex (CGSC), C. fructicola has been described as the most prevalent and aggressive species associated with GLS and apple bitter rot (ABR) in Brazil and Uruguay. Recently, new CGSC species, closely related to C. fructicola, have been identified causing ABR. To verify the accuracy of species identification within the CGSC, we aimed to reevaluate the identity of representative GLS-causing isolates from Brazilian and Uruguayan populations, previously identified as C. fructicola. Multilocus phylogenetic analysis based on APN2, ApMAT, CAL, GAPDH, GS, ITS, and TUB2 allocated these isolates in a monophyletic clade with C. chrysophilum. This species was first described as the causal agent of anthracnose in banana fruits in Brazil, and recent reports indicate its association with ABR in the United States. This is the first report of C. chrysophilum causing GLS disease on apple worldwide.

A Laminarin-Based Formulation Protects Wheat Against Zymoseptoria tritici via Direct Antifungal Activity and Elicitation of Host Defense-Related Genes.

The present study aimed to evaluate the potential of the laminarin-based formulation Vacciplant to protect and induce resistance in wheat against Zymoseptoria tritici, a major pathogen on this crop. Under greenhouse conditions, a single foliar spraying of the product 2 days before inoculation with Z. tritici reduced disease severity and pycnidium density by 42 and 45%, respectively. Vacciplant exhibited a direct antifungal activity on Z. tritici conidial germination both in vitro and in planta. Moreover, it reduced in planta substomatal colonization as well as pycnidium formation on treated leaves. Molecular investigations revealed that Vacciplant elicits but did not prime the expression of several wheat genes related to defense pathways, including phenylpropanoids (phenylalanine ammonia-lyase and chalcone synthase), octadecanoids (lipoxygenase and allene oxide synthase), and pathogenesis-related proteins (ß-1,3-endoglucanase and chitinase). By contrast, it did not modulate the expression of oxalate oxidase gene involved in the reactive oxygen species metabolism. Ultrahigh-performance liquid chromatography-mass spectrometry analysis indicated limited changes in leaf metabolome after product application in both noninoculated and inoculated conditions, suggesting a low metabolic cost associated with induction of plant resistance. This study provides evidence that the laminarin-based formulation confers protection to wheat against Z. tritici through direct antifungal activity and elicitation of plant defense-associated genes.

The Algal Polysaccharide Ulvan Induces Resistance in Wheat Against Zymoseptoria tritici Without Major Alteration of Leaf Metabolome.

This study aimed to examine the ability of ulvan, a water-soluble polysaccharide from the green seaweed Ulva fasciata, to provide protection and induce resistance in wheat against the hemibiotrophic fungus Zymoseptoria tritici. Matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry (MALDI-TOF-MS) analysis indicated that ulvan is mainly composed of unsaturated monosaccharides (rhamnose, rhamnose-3-sulfate, and xylose) and numerous uronic acid residues. In the greenhouse, foliar application of ulvan at 10 mg.ml-1 2 days before fungal inoculation reduced disease severity and pycnidium density by 45 and 50%, respectively. Ulvan did not exhibit any direct antifungal activity toward Z. tritici, neither in vitro nor in planta. However, ulvan treatment significantly reduced substomatal colonization and pycnidium formation within the mesophyll of treated leaves. Molecular assays revealed that ulvan spraying elicits, but does not prime, the expression of genes involved in several wheat defense pathways, including pathogenesis-related proteins (ß-1,3-endoglucanase and chitinase), reactive oxygen species metabolism (oxalate oxidase), and the octadecanoid pathway (lipoxygenase and allene oxide synthase), while no upregulation was recorded for gene markers of the phenylpropanoid pathway (phenylalanine ammonia-lyase and chalcone synthase). Interestingly, the quantification of 83 metabolites from major chemical families using ultra-high-performance liquid chromatography-mass spectrometry (UHPLC-MS) in both non-infectious and infectious conditions showed no substantial changes in wheat metabolome upon ulvan treatment, suggesting a low metabolic cost associated with ulvan-induced resistance. Our findings provide evidence that ulvan confers protection and triggers defense mechanisms in wheat against Z. tritici without major modification of the plant physiology.

Genetic Structure of Colletotrichum fructicola Associated to Apple Bitter Rot and Glomerella Leaf Spot in Southern Brazil and Uruguay.

Colletotrichum fructicola is the main species causing apple bitter rot (ABR) and Glomerella leaf spot (GLS) in southern Brazil, and ABR in Uruguay where GLS remains unnoticed. Thus, this work aimed to determine the genetic structure of C. fructicola isolates of both the countries. A total of 28 out of 31 Brazilian isolates (90.3%) caused typical symptoms of GLS, while only 6 of 25 Uruguayan isolates (24.0%) originating from fruits were able to infect leaves, but causing atypical symptoms. Both populations showed similar levels of Nei's gene diversity (h = 0.088 and 0.079, for Brazilian and Uruguayan populations, respectively), and Bayesian cluster analysis inferred two genetic clusters correlated with the geographical origin of isolates. A principal coordinates analysis scatter plot and an unweighted pair group method with arithmetic mean-based dendrogram also grouped Brazilian and Uruguayan isolates into two groups. By pairwise comparison of nitrate-nonutilizing (nit) mutants with a proposed set of testers, all Uruguayan isolates were grouped into a unique vegetative compatibility group (namely VCG 1), while Brazilian isolates were grouped into four VCGs (VCG 1 to 4). Brazilian and Uruguayan populations of C. fructicola were found to be genetically distinct. Our results suggest that isolates of C. fructicola from Brazil capable of causing GLS and ABR arose independently of those from Uruguay. Possible causes leading to the evolutionary differences between populations are discussed.