Virulence of Two Entomophthoralean Fungi, Pandora neoaphidis and Entomophthora planchoniana, to Their Conspecific (Sitobion avenae) and Heterospecific (Rhopalosiphum padi) Aphid Hosts.

Pandora neoaphidis and Entomophthora planchoniana (phylum Entomophthoromycota) are important fungal pathogens on cereal aphids, Sitobion avenae and Rhopalosiphum padi. Here, we evaluated and compared for the first time the virulence of these two fungi, both produced in S. avenae cadavers, against the two aphid species subjected to the same exposure. Two laboratory bioassays were carried out using a method imitating entomophthoralean transmission in the field. Healthy colonies of the two aphid species were exposed to the same conidial shower of P. neoaphidis or E. planchoniana, in both cases from a cadaver of S. avenae. The experiments were performed under LD 18:6 h at 21 °C and a successful transmission was monitored for a period of nine days after initial exposure. Susceptibility of both S. avenae and R. padi to fungal infection showed a sigmoid trend. The fitted nonlinear model showed that the conspecific host, S. avenae, was more susceptible to E. planchoniana infection than the heterospecific host R. padi, was. In the case of P. neoaphidis, LT50 for S. avenae was 5.0 days compared to 5.9 days for R. padi. For E. planchoniana, the LT50 for S. avenae was 4.9 days, while the measured infection level in R. padi was always below 50 percent. Our results suggest that transmission from conspecific aphid host to heterospecific aphid host can occur in the field, but with expected highest transmission success to the conspecific host.

Steviol glycosides targeted analysis in leaves of Stevia rebaudiana (Bertoni) from plants cultivated under chilling stress conditions.

Stevia rebaudiana is an important agricultural crop for the production of a high-potency natural sweetener, sensitive to low temperature during the developmental stage. Stimulation of chilling stress with a pre-treatment with endogenous signalling components and in particular with salicylic acid (SA), hydrogen peroxide (H2O2), 6-benzylaminopurine (BAP) and calcium chloride (CaCl2) could induce tolerance to chilling and could constitute a suitable way to maintain quality and quantity of steviol glycosides under controlled artificial environment. In the present work the effects of different putative signalling molecules on the morpho-physiological parameters were evaluated, and a specific method for the quali-quantitative analysis of steviol glycosides in S. rebaudiana plants cultivated under controlled conditions was developed, by using LC-ESI-FT (Orbitrap) MS, LC-ESI-QqQ-MS/MS and multivariate data analysis. This approach underlined that the pre-treatment has influence on the production of secondary metabolites. In particular Stevia plants characterised by higher contents of rebaudioside A and stevioside, were identified.

Role of arbuscular mycorrhizal symbiosis in root mineral uptake under CaCO3 stress.

This study investigated the effects of increasing CaCO(3) concentrations (0, 5, 10, 20 mM) on arbuscular mycorrhizal (AM) symbiosis establishment as well as on chicory root growth and mineral nutrient uptake in a monoxenic system. Although CaCO(3) treatments significantly decreased root growth and altered the symbiosis-related development steps of the AM fungus Rhizophagus irregularis (germination, germination hypha elongation, root colonization rate, extraradical hyphal development, sporulation), the fungus was able to completely fulfill its life cycle. Even when root growth decreased more drastically in mycorrhizal roots than in non-mycorrhizal ones in the presence of high CaCO(3) levels, the AM symbiosis was found to be beneficial for root mineral uptake. Significant increases in P, N, Fe, Zn and Cu concentrations were recorded in the mycorrhizal roots. Whereas acid and alkaline phosphatase enzymatic activities remained constant in mycorrhizal roots, they were affected in non-mycorrhizal roots grown in the presence of CaCO(3) when compared with the control.

Calcareous impact on arbuscular mycorrhizal fungus development and on lipid peroxidation in monoxenic roots.

The present work underlined the negative effects of increasing CaCO(3) concentrations (5, 10 and 20 mM) both on the chicory root growth and the arbuscular mycorrhizal fungus (AMF) Glomus irregulare development in monoxenic system. CaCO(3) was found to reduce drastically the main stages of G. irregulare life cycle (spore germination, germinative hyphae elongation, root colonization, extraradical hyphae development and sporulation) but not to inhibit it completely. The root colonization drop was confirmed by the decrease in the arbuscular mycorrhizal fungal marker C16:1ω5 amounts in the mycorrhizal chicory roots grown in the presence of CaCO(3). Oxidative damage evaluated by lipid peroxidation increase measured by (i) malondialdehyde (MDA) production and (ii) the antioxidant enzyme peroxidase (POD) activities, was highlighted in chicory roots grown in the presence of CaCO(3). However, MDA formation was significantly higher in non-mycorrhizal roots as compared to mycorrhizal ones. This study pointed out the ability of arbuscular mycorrhizal symbiosis to enhance plant tolerance to high levels of CaCO(3) by preventing lipid peroxidation and so less cell membrane damage.

Pathotype and microsatellite analyses reveal new sources of resistance to barley scald in Tunisia.

We examined the variation and relationships between pathogenicity and a microsatellite-based haplotype in 79 Tunisian Rhynchosporium secalis isolates that were collected from the most commonly cultivated barley populations in Tunisia, Rihane cv. and local landraces, with the goal of finding genes that might be used to monitor resistance to scald. Isolates could be classified into three distinct virulence groups based on artificial inoculation of 19 differential cultivars with known scald resistance genes. The resistance gene BRR2 carried by the Astrix differential cultivar appeared to be the most effective in Tunisia. Pathotypes sampled from the Rihane host were more virulent than those sampled from local barley landraces. Because some differential cultivars that carried the same resistance genes showed different reaction patterns to 48 of the isolates, we postulated that other unknown resistance gene(s) specific to Tunisian isolates may be prevalent and could be used in Tunisian barley breeding programs. Microsatellite fingerprinting allowed the detection of 11 alleles linked to the virulence and pathogenic identification of 52% of the tested isolates. Thus, microsatellite analysis may provide a rapid tool for pathogen detection, without an inoculation step that requires long incubation periods before ultimate disease assessment.

Genetic diversity of Rhynchosporium secalis in Tunisia as revealed by pathotype, AFLP, and microsatellite analyses.

Genetic variability among 122 Rhynchosporium secalis isolates collected from barley in three regions of Tunisia was investigated using host differentials, amplified fragment length polymorphism (AFLP), and microsatellite markers. The isolates were collected from a widely grown scald-susceptible barley cultivar Rihane and a range of local landrace cultivars in geographically distinct regions with different agroclimatic conditions. Pathotypic diversity (the proportion of unique pathotypes) was high in R. secalis populations from the high (100% diversity), moderate (95%), and low (100%) rainfall areas of Tunisia, and from both Rihane (which is the sole variety grown in the high rainfall region) and local landraces (which predominate in the low rainfall area). This may reflect a general adaptability for aggressiveness and suggests that the widely grown cultivar Rihane has exerted little or no selection pressure on the pathogen population since its release in 1983. Genotypic diversity (GD), defined as the probability that two individuals taken at random had different genotypes, was high for populations from Rihane, local landraces, and different agro-ecological zones (GD = 0.96-0.99). There was low genetic differentiation among pathogen populations from different host populations (G(ST) < or = 0.08, theta < or = 0.12) and agro-ecological zones (G(ST) < or = 0.05, theta < or = 0.04), which may be partly explained by gene flow due to the movement of infected stubble around the country. There was no correlation (r = 0.06, P = 0.39) between virulence phenotype and AFLP haplotype. A phenetic tree revealed groups with low bootstrap values that did not reflect the grouping of isolates based on host, pathotype, or agro-ecological region. The implications of these findings for R. secalis evolutionary potential and scald-resistance breeding in Tunisia are discussed.