No abstract

Genomic loci related with resistance to gall-inducing insects have not been identified in any plants. Here, association mapping was used to identify molecular markers for resistance to the gall wasp Leptocybe invasa in two Eucalyptus species. A total of 86 simple sequence repeats (SSR) markers were screened out from 839 SSRs and used for association mapping in E. grandis. By applying the mixed linear model, seven markers were identified to be associated significantly (P ≤ 0.05) with the gall wasp resistance in E. grandis, including two validated with a correction of permutation test (P ≤ 0.008). The proportion of the variance in resistance explained by a significant marker ranged from 3.3% to 37.8%. Four out of the seven significant associations in E. grandis were verified and also validated (P ≤ 0.073 in a permutation test) in E. tereticornis, with the variation explained ranging from 24.3% to 48.5%. Favourable alleles with positive effect were also mined from the significant markers in both species. These results provide insight into the genetic control of gall wasp resistance in plants and have great potential for marker-assisted selection for resistance to L. invasa in the important tree genus Eucalyptus.

Identification of loci or genes under natural selection is important for both understanding the genetic basis of local adaptation and practical applications, and genome scans provide a powerful means for such identification purposes. In this study, genome-wide simple sequence repeats markers (SSRs) were used to scan for molecular footprints of divergent selection in Eucalyptus grandis, a hardwood species occurring widely in costal areas from 32° S to 16° S in Australia. High population diversity levels and weak population structure were detected with putatively neutral genomic SSRs. Using three FST outlier detection methods, a total of 58 outlying SSRs were collectively identified as loci under divergent selection against three non-correlated climatic variables, namely, mean annual temperature, isothermality and annual precipitation. Using a spatial analysis method, nine significant associations were revealed between FST outlier allele frequencies and climatic variables, involving seven alleles from five SSR loci. Of the five significant SSRs, two (EUCeSSR1044 and Embra394) contained alleles of putative genes with known functional importance for response to climatic factors. Our study presents critical information on the population diversity and structure of the important woody species E. grandis and provides insight into the adaptive responses of perennial trees to climatic variations.

Cryphonectriaceae is a diaporthalean family containing important plant pathogens of which Cryphonectria parasitica is the most notorious one. An emerging stem blight disease on Elaeocarpus apiculatus (Elaeocarpaceae) and E. hainanensis was observed in Guangdong Province of China recently. Typical Cryphonectria blight-like symptoms including cankers on tree barks with obvious orange conidial tendrils were observed. Forty-eight isolates were obtained from diseased tissues and conidiomata formed on the hosts E. apiculatus and E. hainanensis. These isolates were further identified based on both morphology and molecular methods using the combined sequence data of the internal transcribed spacer (ITS) region, large subunit of the nrDNA (LSU), the translation elongation factor 1-alpha (tef1) and DNA-directed RNA polymerase II second largest subunit (rpb2) genes. As a result, the fungus represents an undescribed genus and species within the family Cryphonectriaceae. Hence, Pseudocryphonectria elaeocarpicolagen. et sp. nov. is proposed herein to represent these isolates from diseased barks of E. apiculatus and E. hainanensis. Pseudocryphonectria differs from the other genera of Cryphonectriaceae in having dimorphic conidia. Further inoculation results showed that P. elaeocarpicola is the causal agent of this emerging blight disease in China, which can quickly infect and kill the hosts E. apiculatus and E. hainanensis.

Poplar anthracnose is one of the most serious diseases caused by the fungal pathogen Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. Biocontrol is an efficient green way for the disease control, and numerous researches have focused on exploring the potential biocontrol bacteria strains against C. gloeosporioides. In this study, antifungal activities against C. gloeosporioides of 108 rhizosphere soil isolates from healthy polar plants were investigated in vitro by the dual culture assay. The results suggested that strain ZSH-1 showed the highest level of antifungal activity, as it inhibited C. gloeosporioides at a distance of 10.00 mm. Based on the morphological, physiological-biochemical characteristics, and phylogeny analysis, strain ZSH-1 was identified as Bacillus subtilis. The sterile culture filtrate, crude protein, and crude lipopeptide extracts from the culture filtrate, and volatile compound(s) of ZSH-1 displayed a strong antagonism towards 7 fungal phytopathogens (C. gloeosporioides, Fusarium oxysporum, Alternaria tenuissima, Cytospora chrysosperma, Botryosphaeria dothidea, Mucor sp., and Absidia sp.), with inhibition rates ranging from 44.0 to 89.1%, 26.7 to 85.4%, 11.6 to 89.7%, and 7.8 to 63.2%, respectively. Moreover, ZSH-1 exhibited cell wall-degrading traits by producing 3 lytic enzymes (cellulose, β-1,3-glucanase, and protease). Finally, the greenhouse studies also revealed that strain ZSH-1 had a 47.6% (12 days) efficacy in controlling poplar anthracnose when compared with the control. In concluding, obtained results demonstrate the potential biocontrol effect of B. subtilis ZSH-1, and it can be used as a promising biocontrol agent against poplar anthracnose and other fungal phytopathogens.