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.
Rice blast caused by Magnaporthe oryzae is one of the most destructive diseases of rice worldwide. The fungal pathogen is notorious for its ability to overcome host resistance. To better understand its genetic variation in nature, we sequenced the genomes of two field isolates, Y34 and P131. In comparison with the previously sequenced laboratory strain 70-15, both field isolates had a similar genome size but slightly more genes. Sequences from the field isolates were used to improve genome assembly and gene prediction of 70-15. Although the overall genome structure is similar, a number of gene families that are likely involved in plant-fungal interactions are expanded in the field isolates. Genome-wide analysis on asynonymous to synonymous nucleotide substitution rates revealed that many infection-related genes underwent diversifying selection. The field isolates also have hundreds of isolate-specific genes and a number of isolate-specific gene duplication events. Functional characterization of randomly selected isolate-specific genes revealed that they play diverse roles, some of which affect virulence. Furthermore, each genome contains thousands of loci of transposon-like elements, but less than 30% of them are conserved among different isolates, suggesting active transposition events in M. oryzae. A total of approximately 200 genes were disrupted in these three strains by transposable elements. Interestingly, transposon-like elements tend to be associated with isolate-specific or duplicated sequences. Overall, our results indicate that gain or loss of unique genes, DNA duplication, gene family expansion, and frequent translocation of transposon-like elements are important factors in genome variation of the rice blast fungus.
Wall-associated protein kinases (WAKs) are a new group of receptor-like kinases (RLK) recently identified in Arabidopsis. A cDNA encoding a novel WAK was isolated from rice and was named OsWAK1 (Oryza sativa WAK). The deduced amino acid sequence of OsWAK1 showed 27.6% identity to WAK2 from Arabidopsis. OsWAK1 not only has the ability of autophosphorylation but also can phosphorylate OsRFP1, a putative transcription regulator recently identified in rice. OsRFP1 strongly interacts with the kinase domain of OsWAK1. This demonstrated that OsWAK1 is a functional protein kinase. A fusion protein of OsWAK1 with GFP was found to be localized on the cell surface. Plasmolysis experiments further revealed OsWAK1 is associated with the cell wall. Northern blotting analysis showed that infection of the rice blast fungus, Magnaporthe oryzae significantly induced the OsWAK1 transcripts, and the accumulation of OsWAK1 mRNA occurred earlier and was more abundant in rice leaves infected with an incompatible race than with a compatible race of the blast fungus. OsWAK1 was also induced after treatment by mechanical wounding, SA and MeJA, but not by ABA. These results imply that OsWAK1 is a novel gene involved in plant defense. Furthermore, six transgenic rice lines with constitutive expression of OsWAK1 became resistant to the compatible race. However, OsWAK1 expression was undetectable in leaves, stems and flowers but very weak in roots under normal growth conditions. This provides functional evidence that induction of OsWAK1 as a novel RLK plays important roles in plant disease resistance.
The authors of paper “Two new species of Pestalotiopsis from Southern China” regret that the affiliation of Bin Zhang is incorrect. His affiliation is “Plant Protection and Quarantine Station, Guiyang City, 550081, People’s Republic of China”, thus the superscript of his name should be changed from “1” to “3”.
Fruit length is a prominent agricultural trait during cucumber (Cucumis sativus) domestication and diversifying selection; however, the regulatory mechanisms of fruit elongation remain elusive. We identified two alleles of the FRUITFULL (FUL)-like MADS-box gene CsFUL1 with 3393 C/A Single Nucleotide Polymorphism variation among 150 cucumber lines. Whereas CsFUL1 A was specifically enriched in the long-fruited East Asian type cucumbers (China and Japan), the CsFUL1 C allele was randomly distributed in cucumber populations, including wild and semiwild cucumbers. CsFUL1 A knockdown led to further fruit elongation in cucumber, whereas elevated expression of CsFUL1 A resulted in significantly shorter fruits. No effect on fruit elongation was detected when CsFUL1 C expression was modulated, suggesting that CsFUL1 A is a gain-of-function allele in long-fruited cucumber that acts as a repressor during diversifying selection of East Asian cucumbers. Furthermore, CsFUL1 A binds to the CArG-box in the promoter region of SUPERMAN, a regulator of cell division and expansion, to repress its expression. Additionally, CsFUL1 A inhibits the expression of auxin transporters PIN-FORMED1 (PIN1) and PIN7, resulting in decreases in auxin accumulation in fruits. Together, our work identifies an agriculturally important allele and suggests a strategy for manipulating fruit length in cucumber breeding that involves modulation of CsFUL1 A expression.
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