Molecular genetics of disease resistance in cereals

AIMS: This Botanical Briefing attempts to summarize what is currently known about the molecular bases of disease resistance in cereal species and suggests future research directions. SCOPE: An increasing number of resistance (R) genes have been isolated from rice, maize, wheat and barley that encode both structurally related and unique proteins. This R protein diversity may be attributable to the different modus operandi employed by pathogen species in some cases, but it is also a consequence of multiple defence strategies being employed against phytopathogens. Mutational analysis of barley has identified additional genes required for activation of an R gene-mediated defence response upon pathogen infection. In some instances very closely related barley R proteins require different proteins for defence activation, demonstrating that, within a single plant species, multiple resistance signalling pathways and different resistance strategies have evolved to confer protection against a single pathogen species. Despite the apparent diversity of cereal resistance mechanisms, some of the additional molecules required for R protein function are conserved amongst cereal and dicotyledonous species and even other eukaryotic species. Thus the derivation of functional homologues and interacting partner proteins from other species is contributing to the understanding of resistance signalling in cereals. The potential and limit of utilizing the rice genome sequence for further R gene isolation from cereal species is also considered, as are the new biotechnological possibilities for disease control arising from R gene isolation. CONCLUSIONS: Molecular analyses in cereals have further highlighted the complexity of plant-pathogen co-evolution and have shown that numerous active and passive defence strategies are employed by plants against phytopathogens. Many advances in understanding the molecular basis of disease resistance in cereals have focused on monogenic resistance traits. Future research targets are likely to include less experimentally tractable, durable polygenic resistances and nonhost resistance mechanisms.     

Purification and characterization of aminopeptidase (pumAPE) from Ustilago maydis

The aminopeptidase pumAPE was purified from the haploid fungus Ustilago maydis FB1 strain. The purification procedure consisted of ammonium sulfate fractionation and three chromatographic steps, which included anion-exchange, hydrophobic interaction, and gel filtration chromatography, resulting in a 23% recovery. The molecular mass of the dimeric enzyme was estimated to be 110 kDa and 58 kDa by gel filtration chromatography and SDS-PAGE, respectively. Enzymatic activity was optimal at pH 7.0 and at 35 degrees C toward Lys-pNA and the pI was determined to be 5.1. The enzyme was inhibited by EDTA-Na2, 1,10- phenanthroline, bestantin, PMSF and several divalent cations (Cu2+, Hg2+ and Zn2+). The aminopeptidase showed a preference for lysine and arginine in the N-position. The K(m) value was 54.4 microM and the Vmax value was 408 micromolmin(-1)mg(-1) for Lys-pNA.     

诱发玉米抗小斑病突变体的研究——Ⅳ.从玉米单倍体胚性细胞无性系筛选抗玉米小斑病突变体

将经γ射线和EMS诱变处理的玉米八趟白单倍体胚性细胞无性系为试验材料,以玉米小斑病菌Helminthosporium maydis单胞系342号菌株产生的致病毒素为选择剂,采用正选择法进行筛选,已获得抗玉米小斑病的突梦体。这个突变体脱离选择剂5代(5个月)后性状稳定,用小斑病菌的分生孢于直接接种鉴定仍具有很强的抗病力。过氧化物酶同工酶的分析,抗病突变体与不抗病对照间出现明显差异。     

Detection of a Bacterium Associated with a Leaf Spot Disease of Maize in Brazil

A leaf spot disease of maize occurring in Brazil in the 1980s was described as being caused by the ascomycete Phaeosphaeria maydis (P. Henn) Rane. Payak and Renfro (imperfect form Phyllosticta sp.). Disease symptoms were dark-green water-soaked spots that later became necrotic lesions. There are no reports at present in the literature of re-infection by the fungus under controlled conditions, casting doubt on the true identity of the pathogen. In this study, cytological analyses of lesions at the initial stages did not detect the presence of fungal structures. Bacterial colonies with yellow pigmentation were isolated from the lesions, which reacted positively in hypersensitivity tests in tobacco plants. Maize plants were inoculated with the isolated bacteria. After 72 h incubation in a dew chamber, plants were transferred to a greenhouse, where they remained until evaluation. Typical symptoms of the disease were observed 5–7 days after inoculation of plants, only on treatments inoculated with the bacteria. The bacterium was re-isolated, which suggests its involvement in the initial phases of disease. The bacterium was identified as Pantoea ananas (synonym Erwinia ananas ).     

Is diversification history of maize influencing selection of soil bacteria by roots?

A wide range of plant lines has been propagated by farmers during crop selection and dissemination, but consequences of this crop diversification on plant-microbe interactions have been neglected. Our hypothesis was that crop evolutionary history shaped the way the resulting lines interact with soil bacteria in their rhizospheres. Here, the significance of maize diversification as a factor influencing selection of soil bacteria by seedling roots was assessed by comparing rhizobacterial community composition of inbred lines representing the five main genetic groups of maize, cultivated in a same European soil. Rhizobacterial community composition of 21-day-old seedlings was analysed using a 16S rRNA taxonomic microarray targeting 19 bacterial phyla. Rhizobacterial community composition of inbred lines depended on the maize genetic group. Differences were largely due to the prevalence of certain Betaproteobacteria and especially Burkholderia, as confirmed by quantitative PCR and cloning/sequencing. However, these differences in bacterial root colonization did not correlate with plant microsatellite genetic distances between maize genetic groups or individual lines. Therefore, the genetic structure of maize that arose during crop diversification (resulting in five main groups), but not the extent of maize diversification itself (as determined by maize genetic distances), was a significant factor shaping rhizobacterial community composition of seedlings.     

Immunolocalization of chitin synthases in the phytopathogenic dimorphic fungus Ustilago maydis

Conserved polypeptides of the chitin synthase genes UmCHS3 and UmCHS6 from the phytopathogenic fungus Ustilago maydis were utilized as immunogens to obtain polyclonal antibodies that were purified by affinity procedures. Because of their similarities at the regions encoded by either polypeptide, it was concluded that anti-Chs3 antibodies recognized both Chs3 and Chs4 chitin synthases, whereas anti-Chs6 antibodies recognized Chs6 and Chs8 polypeptides. These antibodies were used to analyze the localization of the corresponding chitin synthases in U. maydis cells, using both indirect immunofluorescence microscopy and immunoelectron microscopy with colloidal-gold-labeled secondary antibodies. It was observed that chitin synthase proteins were accumulated both in the surface and in the cytoplasm of the fungal cells. Electron microscopy images revealed the accumulation of clusters of gold particles in vesicles, providing evidence for the possible origin and destination of chitin synthases in the fungal cells.