Chitin is a major component of fungal cell wall and is synthesized by chitin synthases (Chs). Plant pathogenic fungi normally have multiple chitin synthase genes. To determine their roles in development and pathogenesis, we functionally characterized all seven CHS genes in Magnaporthe oryzae. Three of them, CHS1, CHS6, and CHS7, were found to be important for plant infection. While the chs6 mutant was non-pathogenic, the chs1 and chs7 mutants were significantly reduced in virulence. CHS1 plays a specific role in conidiogenesis, an essential step for natural infection cycle. Most of chs1 conidia had no septum and spore tip mucilage. The chs6 mutant was reduced in hyphal growth and conidiation. It failed to penetrate and grow invasively in plant cells. The two MMD-containing chitin synthase genes, CHS5 and CHS6, have a similar expression pattern. Although deletion of CHS5 had no detectable phenotype, the chs5 chs6 double mutant had more severe defects than the chs6 mutant, indicating that they may have overlapping functions in maintaining polarized growth in vegetative and invasive hyphae. Unlike the other CHS genes, CHS7 has a unique function in appressorium formation. Although it was blocked in appressorium formation by germ tubes on artificial hydrophobic surfaces, the chs7 mutant still produced melanized appressoria by hyphal tips or on plant surfaces, indicating that chitin synthase genes have distinct impacts on appressorium formation by hyphal tip and germ tube. The chs7 mutant also was defective in appressorium penetration and invasive growth. Overall, our results indicate that individual CHS genes play diverse roles in hyphal growth, conidiogenesis, appressorium development, and pathogenesis in M. oryzae, and provided potential new leads in the control of this devastating pathogen by targeting specific chitin synthases. Chitin is one of the major components of cell wall that plays vital roles in hyphal tip growth and fungal morphogenesis. Biosynthesis of chitin is catalyzed by chitin synthases, a well-known fungicide target. However, systematic characterization of chitin synthase genes has not yet been reported in plant pathogenic ascomycetes. To determine their roles in development and pathogenesis, in this study we functionally characterized all the seven chitin synthase genes (CHS1-CHS7) in Magnaporthe oryzae, a model for studying fungal development and pathogenesis. While CHS2, CHS3, CHS4, and CHS5 are dispensable for plant infection, CHS6 is essential for pathogenesis. The chs6 mutant failed to penetrate plant cells and develop infectious hyphae. Two other chitin synthase genes, CHS1 and CHS7, also are important for virulence. The chs1 and chs7 mutants caused only rare lesions on rice seedlings. Other than plant infection, CHS1 and CHS7 also play specific roles during conidiogenesis and appressorium formation, respectively. Interestingly, the chs7 mutant was blocked in appressorium formation by germ tubes on artificial hydrophobic surfaces but was normal by hyphal tips and on plant surfaces. Different chitin synthase genes may be involved during appressorium formation by different fungal tissues or on different surfaces in M. oryzae.

In this study, field-grown wheat (Triticum aestivum L.) was treated with normal (Nn) and excessive (Ne) levels of fertilizer N. Results showed that Ne depressed the activity of superoxide dismutase and peroxidase and increased the accumulation of reactive oxygen species (ROS) and malondialdehyde. The normalized difference vegetation index (NDVI) was higher under Ne at anthesis and medium milk but similar at the early dough stage and significantly lower at the hard dough stage than that under Nn. The metabolomics analysis of the leaf responses to Ne during grain filling showed 99 metabolites that were different between Ne and Nn treatments, including phenolic and flavonoid compounds, amino acids, organic acids and lipids, which are primarily involved in ROS scavenging, N metabolism, heat stress adaptation and disease resistance. Organic carbon (C) and total N contents were affected by the Ne treatment, with lower C/N ratios developing after medium milk. Ultimately, grain yields decreased with Ne. Based on these data, compared with the normal N fertilizer treatment, we concluded that excessive N application decreased the ability to scavenge ROS, increased lipid peroxidation and caused significant metabolic changes disturbing N metabolism, secondary metabolism and lipid metabolism, which led to reduced grain filling in wheat.

Background: In wheat (Triticum aestivum L), the flag leaf has been thought of as the main source of assimilates for grain growth, whereas the peduncle has commonly been thought of as a transporting organ. The photosynthetic characteristics of the exposed peduncle have therefore been neglected. In this study, we investigated the anatomical traits of the exposed peduncle during wheat grain ontogenesis, and we compared the exposed peduncle to the flag leaf with respect to chloroplast ultrastructure, photosystem II (PSII) quantum yield, and phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) activity. Results: Transmission electron microscope observations showed well-developed chloroplasts with numerous granum stacks at grain-filling stages 1, 2 and 3 in both the flag leaf and the exposed peduncle. In the exposed peduncle, the membranes constituting the thylakoids were very distinct and plentiful, but in the flag leaf, there was a sharp breakdown at stage 4 and complete disintegration of the thylakoid membranes at stage 5. PSII quantum yield assays revealed that the photosynthetic efficiency remained constant at stages 1, 2 and 3 and then declined in both organs. However, the decline occurred more dramatically in the flag leaf than in the exposed peduncle. An enzyme assay showed that at stages 1 and 2 the PEPCase activity was lower in the exposed peduncle than in the flag leaf; but at stages 3, 4 and 5 the value was higher in the exposed peduncle, with a particularly significant difference observed at stage 5. Subjecting the exposed part of the peduncle to darkness following anthesis reduced the rate of grain growth. Conclusion: Our results suggest that the exposed peduncle is a photosynthetically active organ that produces photosynthates and thereby makes a crucial contribution to grain growth, particularly during the late stages of grain-filling.

Crop residue retention is a key component of sustainable cropping systems. In recent years, retention of crop residue is a means of improving soil quality and nutrient capacity and reducing the adverse effects of residue burning. Maize–wheat rotation is a major double-cropping system practiced on more than one fifth of agricultural lands worldwide. Currently, more than 50 % of maize residues are retained immediately after harvesting. In China, this practice has beneficial effects on soil properties and productivity of the succeeding wheat crop. However, increasing the retention of maize residues without proper soil management has also led to a series of concerns, such as short-term nitrogen immobilization, rapid moisture loss, and high susceptibility to freezing during winter as a result of increased soil porosity. Therefore, other practices for maize residue retention are needed to promote wheat growth and the release of nutrients. This article reviews recent developments in China concerning the consequences of maize residue retention on the physical, chemical, and biological properties of soil quality and the growth, productivity, and quality of wheat. Major patterns of maize residue retention are discussed. In particular, this review provides three complementary approaches for maize residue management.