Fungi and plants are rich sources of thousands of secondary metabolites. The genetically coded possibilities for secondary metabolite production, the stimuli of the production, and the special phytotoxins basically determine the microscopic fungi-host plant interactions and the pathogenic lifestyle of fungi. The review introduces plant secondary metabolites usually with antifungal effect as well as the importance of signaling molecules in induced systemic resistance and systemic acquired resistance processes. The review also concerns the mimicking of plant effector molecules like auxins, gibberellins and abscisic acid by fungal secondary metabolites that modulate plant growth or even can subvert the plant defense responses such as programmed cell death to gain nutrients for fungal growth and colonization. It also looks through the special secondary metabolite production and host selective toxins of some significant fungal pathogens and the plant response in form of phytoalexin production. New results coming from genome and transcriptional analyses in context of selected fungal pathogens and their hosts are also discussed.

/pdf/secondary-metabolites-in-fungus-plant-interactions-15rnaz4vmj.pdf

Secondary metabolites in fungus-plant interactions

The soil-borne pathogen Rhizoctonia solani is responsible for crop losses on a wide range of important crops worldwide. The lack of effective control strategies and the increasing demand for organically grown food has stimulated research on biological control. The aim of the present study was to evaluate the rhizosphere competence of the commercially available inoculant Bacillus amyloliquefaciens FZB42 on lettuce growth and health together with its impact on the indigenous rhizosphere bacterial community in field and pot experiments. Results of both experiments demonstrated that FZB42 is able to effectively colonize the rhizosphere (7.45 to 6.61 Log 10 CFU g−1 root dry mass) within the growth period of lettuce in the field. The disease severity (DS) of bottom rot on lettuce was significantly reduced from severe symptoms with DS category 5 to slight symptom expression with DS category 3 on average through treatment of young plants with FZB42 before and after planting. The 16S rRNA gene based fingerprinting method terminal restriction fragment length polymorphism (T-RFLP) showed that the treatment with FZB42 did not have a major impact on the indigenous rhizosphere bacterial community. However, the bacterial community showed a clear temporal shift. The results also indicated that the pathogen R. solani AG1-IB affects the rhizosphere microbial community after inoculation. Thus, we revealed that the inoculant FZB42 could establish itself successfully in the rhizosphere without showing any durable effect on the rhizosphere bacterial community.

/pdf/effects-of-bacillus-amyloliquefaciens-fzb42-on-lettuce-o1nw5ahs6u.pdf

Effects of Bacillus amyloliquefaciens FZB42 on lettuce growth and health under pathogen pressure and its impact on the rhizosphere bacterial community.

Rhizoctonia solani is a soil-borne basidiomycete fungus with a necrotrophic lifestyle which is classified into fourteen reproductively incompatible anastomosis groups (AGs). One of these, AG8, is a devastating pathogen causing bare patch of cereals, brassicas and legumes. R. solani is a multinucleate heterokaryon containing significant heterozygosity within a single cell. This complexity posed significant challenges for the assembly of its genome. We present a high quality genome assembly of R. solani AG8 and a manually curated set of 13,964 genes supported by RNA-seq. The AG8 genome assembly used novel methods to produce a haploid representation of its heterokaryotic state. The whole-genomes of AG8, the rice pathogen AG1-IA and the potato pathogen AG3 were observed to be syntenic and co-linear. Genes and functions putatively relevant to pathogenicity were highlighted by comparing AG8 to known pathogenicity genes, orthology databases spanning 197 phytopathogenic taxa and AG1-IA. We also observed SNP-level “hypermutation” of CpG dinucleotides to TpG between AG8 nuclei, with similarities to repeat-induced point mutation (RIP). Interestingly, gene-coding regions were widely affected along with repetitive DNA, which has not been previously observed for RIP in mononuclear fungi of the Pezizomycotina. The rate of heterozygous SNP mutations within this single isolate of AG8 was observed to be higher than SNP mutation rates observed across populations of most fungal species compared. Comparative analyses were combined to predict biological processes relevant to AG8 and 308 proteins with effector-like characteristics, forming a valuable resource for further study of this pathosystem. Predicted effector-like proteins had elevated levels of non-synonymous point mutations relative to synonymous mutations (dN/dS), suggesting that they may be under diversifying selection pressures. In addition, the distant relationship to sequenced necrotrophs of the Ascomycota suggests the R. solani genome sequence may prove to be a useful resource in future comparative analysis of plant pathogens.

/pdf/genome-sequencing-and-comparative-genomics-of-the-broad-host-3qn7qdymxg.pdf

Genome Sequencing and Comparative Genomics of the Broad Host-Range Pathogen Rhizoctonia solani AG8

Rice sheath blight disease, caused by the basidiomycetous necrotroph Rhizoctonia solani,
became one of the major threats to the rice cultivation worldwide, especially after the adoption
of high-yielding varieties. The pathogen is challenging to manage because of its extensively
broad host range and high genetic variability and also due to the inability to find any satisfactory
level of natural resistance from the available rice germplasm. It is high time to find remedies to
combat the pathogen for reducing rice yield losses and subsequently to minimize the threat to
global food security. The development of genetic resistance is one of the alternative means to
avoid the use of hazardous chemical fungicides. This review mainly focuses on the effort of
better understanding the host–pathogen relationship, finding the gene loci/markers imparting
resistance response and modifying the host genome through transgenic development. The latest
development and trend in the R. solani–rice pathosystem research with gap analysis are
provided.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/pbi.13312

Understanding sheath blight resistance in rice: the road behind and the road ahead

Relationship between root-endophytic microbial communities and replant disease in specialized apple growing areas in Europe

Members of the form genus Rhizoctonia D.C. are considered as a complex mixture of filamentous fungi, having in common the possession of a non-spored imperfect state, usually referred to as the Rhizoctonia anamorph. The group includes several of the most devastating crop pathogens like Thanatephorus cucumeris (Frank) Donk (anamorph=Rhizoctonia solani Kuhn), the majority of orchid mycorrhizal symbionts (mainly belonging to genus Ceratobasidium D.P. Rogers) and a collection of saprotrophic organisms of different systematic placement. The Rhizoctonia anamorph is characterized by several common features present among members of the entire Rhizoctonia species complex. Taxa from the group have been rearranged into several groups of higher fungi, including both Ascomycota and Basidiomycota, and split into several genera, employing criteria such as the analysis and ultrastructural comparison of septal apparatus. Until very recently, classification for some of the groups within the complex has been exclusively based on criteria such as hyphal anastomosis, since other types of diagnostic features are usually scarce in these fungi. Phytopathological studies in the complex have represented the major contingent of contributions in the group, especially in the case of R. solani. Some members of the complex have been reported to be protective isolates against pathogenic members of Rhizoctonia and some other fungal pathogens. This review focuses on the knowledge of several aspects of the species of Rhizcotonia s. lato, such as its current taxonomic placement, the biology and systematics of some groups of the complex, and a revision of the methodologies employed in studying it.

http://digital.csic.es/bitstream/10261/15813/3/Rhizoctonia.pdf

V. Rubio Susan

Papers

Review. Biology and systematics of the form genus Rhizoctonia