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The association of the cell wall degrading enzyme endo-beta-1,4-xylanase (EC 3.2.1.8) with pathogenicity of Mycosphaerella graminicola was examined in planta. The enzyme production of two M. graminicola isolates (T0372 and T0491), as well as their ability to infect seedlings of susceptible wheat cv. Scorpion, was first compared. No significant difference was found between the two isolates regarding spore germination rates, mycelial growth on the leaf surface or direct and stomatal penetrations. However, restricted hyphal growth was observed inside leaves inoculated with T0372, whereas successful mesophyll colonization with a strong intercellular fungal growth was found in leaves infected with T0491. Likewise, T0372 was unable to induce lesions bearing pycnidia and to produce endo-beta-1,4-xylanase activity until 22 days post-inoculation (d.p.i.). On the other hand, significant high increases of both diseased leaf area bearing pycnidia and endo-beta-1,4-xylanase activity were observed between 16 and 22 d.p.i. for T0491 (r = 0AE98). The investigation of 24 additional isolates, including the IPO323 and IPO94269 reference isolates, highlighted a strong correlation between endo-beta-1,4-xylanase activity and disease development levels (r = 0AE94). This study demonstrates that differences in pathogenicity in M. graminicola are not linked to events on the leaf surface or to frequency of leaf penetration, but to the ability of the fungus to colonize the mesophyll and to produce the cell wall degrading enzyme endo-1,4-beta-xylanase during the necrotrophic phase.

Catch crops can effectively decrease nitrate leaching in arable cropping systems but their longterm impacts on nitrogen mineralization are not well known. This study quantified the effects of continuous catch crops on net N mineralization, crop N uptake, crop N use efficiency and N leaching in three long-term (13-17 years) field experiments in northern France. Mustard was grown every year at Boigneville, radish every year at Thibie and ryegrass every 2 years at Kerlavic. The mean N content of catch crop residues at these sites was 33, 36 and 35 kg ha −1 yr −1 and their mean C:N ratio was 13, 17 and 28, respectively. Net mineralization was calculated with a mass balance of soil mineral N using measured inputs and outputs. Catch crop establishment enhanced annual mineralization by on average 26, 18 and 9 kg N ha −1 yr −1 respectively during the 13-17 year period. The difference in mineralization rate between catch crop and control treatments (extra mineralization) was positive from the first year at Boigneville, whereas it was negative or nil during the first 3-5 years at Thibie and Kerlavic. At the latter sites, the extra mineralization rate increased significantly over time at a rate of 2.0 and 2.6 kg N ha −1 yr −2 . At the end of the experiment, cumulative extra mineralization represented 72%, 60% and 23% of the total N added by catch crop residues at Boigneville, Thibie and Kerlavic, respectively. Repeated catch crops significantly increased N uptake and N use efficiency by main crops at Kerlavic and Thibie, but not at Boigneville. The efficiency of catch crops in reducing N leaching persisted over the years at all sites. A model simulating C and N dynamics during catch crop decomposition was able to reproduce the pattern of extra N mineralization kinetics with the various catch crop species, but underestimated the range of variation between sites. Better predictions were obtained when C or N inputs due to catch crops were increased by 10-57%, suggesting that the actual inputs could be markedly greater than those measured in catch crop residues. According to the model, the C: N ratio of catch crop residues largely explained differences in mineralization due to different catch crop species.

In wheat, little is known about disease resistance inducers and, more specifically, about the biological activities from those derived from endogenous elicitors, such as oligogalacturonides (OGAs). Therefore, we tested the ability of two fractions of OGAs, with polymerization degrees (DPs) of 2-25, to induce resistance to Blumeria graminis f. sp. tritici and defense responses in wheat. One fraction was unacetylated (OGAs-Ac) whereas the second one was 30% chemically acetylated (OGAs+Ac). Infection level was reduced to 57 and 58% relative to controls when OGAs-Ac and OGAs+Ac, respectively, were sprayed 48 h before inoculation. Activities of various defense-related enzymes were then assayed in noninoculated wheat leaves infiltrated with OGAs. Oxalate oxidase, peroxidase, and lipoxygenase were responsive to both OGAs-Ac and OGAs+Ac, which suggests involvement of reactive oxygen species and oxilipins in OGAs-mediated responses in wheat. In inoculated leaves, both fractions induced a similar increase in H₂O₂ accumulation at the site of fungal penetration. However, only OGAs+Ac led to an increase in papilla-associated fluorescence and to a reduction of formed fungal haustoria. Our work provides the first evidence for elicitation and protection effects of preventive treatments with OGAs in wheat and for new properties of acetylated OGAs.

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