Characterization of resistance mechanisms to powdery mildew (Erysiphe betae) in beet (Beta vulgaris)

Microbial Inoculation for Productivity Improvements and Potential Biological Control in Sugar Beet Crops

Used mainly for sucrose production, sugar beet is one of the most important crops in Castilla y León (Spain). Several studies have demonstrated the benefits of microorganisms in different crop management programs, among which Plant Growth Promoting Rhizobacteria (PGPR). This research aims to assess the beneficial effects of two PGPRs strains (Pseudomonas fluorescens Pf0-1 and Pseudomonas chlororaphis CECT 462) on sugar beet (Beta vulgaris) production. Three treatments: a PGPRs co-inoculation assay of untreated seeds without any chemical treatment (TB), a conventional treatment with commercial seeds and fungicide application (TT); and a control with seeds without protective coating, bacterial inoculation and chemical treatment (ST). The efficacy of PGPRs inoculation on sugar beet production was determined measuring periodically the photosynthetic status of plants, and the final yield and quality of tubers. Aerial and root plant biomass, maximum beet perimeter, polarization, and sugar values of the sugar beet plants inoculated with PGPRs showed higher values and significant differences to sugar beet subjected to other treatments. We could see that PGPRs inoculation (TB treatment) produced significant differences in the quantum yield of PSII (ΦPSII). TB showed the highest value for ΦPSII and the NPQ (non-photochemical quenching), the lowest value, even though the PSII (maximum quantum yield of photosystem II) was very similar in all treatments. The two assayed PGPR strains triggered a significant increase in sugar beet production yield and quality. PGPRs inoculation techniques could be used in different crops and they could be applied as biofertilizers, improving the agricultural production.

Dual RNA-Seq analysis of Medicago truncatula and the pea powdery mildew Erysiphe pisi uncovers distinct host transcriptional signatures during incompatible and compatible interactions and pathogen effector candidates

Powdery mildew (PM) is a serious fungal disease of legumes. To gain novel insights into PM pathogenesis and host resistance/susceptibility, we used dual RNA-Seq to simultaneously capture host and pathogen transcriptomes at 1 d post-inoculation of resistant and susceptible Medicago truncatula genotypes with the PM Erysiphe pisi (Ep). Differential expression analysis indicates that R-gene mediated resistance against Ep involves extensive transcriptional reprogramming. Functional enrichment of differentially expressed host genes and in silico analysis of co-regulated promoters suggests that amplification of PTI, activation of the JA/ET signaling network, and regulation of growth-defense balance correlate with resistance. In contrast, processes that favor biotrophy, including suppression of defense signaling and programmed cell death, and weaker cell wall defenses are important susceptibility factors. Lastly, Ep effector candidates and genes with known/putative virulence functions were identified, representing a valuable resource that can be leveraged to improve our understanding of legume-PM interactions.

Histochemical characterization of early response to Cochliobolus sativus infection in selected barley genotypes

Much effort is being made to breed barley with durable resistance to leaf spot blotch incited by Bipolaris sorokiniana (teleomorph: Cochliobolus sativus). We hypothesized that susceptibility and resistance traits in 11 diverse barley genotypes inoculated with a single C. sativus isolate might specify a range of distinct host cell responses. Quantitative descriptions of interaction microphenotypes exhibited by different barley genotype seedlings after infection with C. sativus are provided. Early oxidative responses occurring in epidermis and mesophyll leaf tissue were monitored by histochemical analysis of H2O2 accumulation at 8, 24, and 48 h after inoculation. Cell wall apposition (CWA) in epidermal cells and hypersensitive reaction (HR) of epidermal or mesophyll tissue were early defenses in both resistant and susceptible genotypes. There were differences in level, duration, and frequency of occurrence for CWA and HR for the different barley genotypes. Occurrence of HR in epidermal cells at post-penetration stages was indicative of compatibility. Patterns of cell responses were microphenotypically diverse between different resistant and susceptible genotypes. This suggests that timing and level of response are key features of microphenotypic diversity that distinguish different functional mechanisms of resistance and susceptibility present in barley.

Effects of systemic pesticides imidacloprid and metalaxyl on the phyllosphere of pepper plants

Microbes inhabiting the phyllosphere of crops are exposed to pesticides applied either directly onto plant foliage or indirectly through soil. Although, phyllosphere microbiology has been rapidly evolving, little is still known regarding the impact of pesticides on the epiphytic microbial community and especially on fungi. We determined the impact of two systemic pesticides (metalaxyl and imidacloprid), applied either on foliage or through soil, on the epiphytic fungal and bacterial communities via DGGE and cloning. Both pesticides induced mild effects on the fungal and the bacterial communities. The only exception was the foliage application of imidacloprid which showed a more prominent effect on the fungal community. Cloning showed that the fungal community was dominated by putative plant pathogenic ascomycetes (Erysiphaceae and Cladosporium), while a few basidiomycetes were also present. The former ribotypes were not affected by pesticides application, while selected yeasts (Cryptococcus) were stimulated by the application of imidacloprid suggesting a potential role in its degradation. A less diverse bacterial community was identified in pepper plants. Metalaxyl stimulated an Enterobacteriaceae clone which is an indication of the involvement of members of this family in fungicide degradation. Further studies will focus on the isolation of epiphytic microbes which appear to be stimulated by pesticides application.

Durable broad-spectrum powdery mildew resistance in pea er1 plants is conferred by natural loss-of-function mutations in PsMLO1

Loss-of-function alleles of plant-specific MLO (Mildew Resistance Locus O) genes confer broad-spectrum powdery mildew resistance in monocot (barley) and dicot (Arabidopsis thaliana, tomato) plants. Recessively inherited powdery mildew resistance in pea (Pisum sativum) er1 plants is, in many aspects, reminiscent of mlo-conditioned powdery mildew immunity, yet the underlying gene has remained elusive to date. We used a polymerase chain reaction (PCR)-based approach to amplify a candidate MLO cDNA from wild-type (Er1) pea. Sequence analysis of the PsMLO1 candidate gene in two natural er1 accessions from Asia and two er1-containing pea cultivars with a New World origin revealed, in each case, detrimental nucleotide polymorphisms in PsMLO1, suggesting that PsMLO1 is Er1. We corroborated this hypothesis by restoration of susceptibility on transient expression of PsMLO1 in the leaves of two resistant er1 accessions. Orthologous legume MLO genes from Medicago truncatula and Lotus japonicus likewise complemented the er1 phenotype. All tested er1 genotypes showed unaltered colonization with the arbuscular mycorrhizal fungus, Glomus intraradices, and with nitrogen-fixing rhizobial bacteria. Our data demonstrate that PsMLO1 is Er1 and that the loss of PsMLO1 function conditions durable broad-spectrum powdery mildew resistance in pea.

Association of leaf micro-morphological characters with powdery mildew resistance in field-grown mulberry (Morus spp.) germplasm

BACKGROUND AND AIMS

Micro-morphological characteristics can influence fungal infectivity. We sought links between micro-morphology and resistance to powdery mildew in mulberry with the intention of assisting selection of disease-resistant lines.

METHODOLOGY

Over 3 years and under field conditions, we evaluated 30 lines of mulberry with contrasting susceptibilities to powdery mildew (15 resistant and 15 susceptible). Disease severity was related statistically to stomatal area, stomatal density, stomatal index, upper and lower cuticular thicknesses, leaf thickness and trichome density.

PRINCIPAL RESULTS

Differences between lines were significant (P <0.05) for all characters studied. Variation between the resistant and susceptible groups was statistically highly significant (P <0.01) for stomatal index, stomatal area and trichome density. The powdery mildew-resistant group was distinguished by  17.4 % lower stomatal density, 12.5 % smaller stomatal index per unit leaf area, 20.0 % greater trichome density and 18.0 % higher stomatal area compared with the susceptible group. Trichome density was negatively correlated with disease severity index and with the accumulative area under disease progression curves. Stomatal density was positively correlated with both measures of disease severity. Although stomatal area was negatively related to disease severity index (r = -0.28; P <0.05), the correlation was weak. There was no statistically significant relationship between stomatal area and the accumulative area under disease progression curves. The germplasm was partitioned into seven sub-groups based on hierarchical cluster analysis derived from pooled disease severity index scores and three highly significant micro-morphological characters. Eighty per cent of the resistant germplasm accumulated in three cluster components (A1, A2 and B2) characterized by high trichome densities and a high stomatal density and stomatal index.

CONCLUSIONS

Resistance to powdery mildew in mulberry is associated with trichome and stomatal features rather than leaf and epidermal thicknesses. Trichome density, stomatal density and stomatal index are shown to be promising markers for screening powdery mildew resistance in breeding programmes.