Bacterial community associated with Acacia crassicarpa seeds and cotyledons

Abstract

Acacia crassicarpa is widely planted in timber plantations for the pulp and paper industry but, this species is susceptible to diseases such as bacterial leaf blight. Xanthomonas campestris was described as the causal agent of this disease and was reported to be seedborne. However, isolation was not performed from the seeds and Koch’s postulate was not fulfilled Therefore, the aim of this study, was to isolate, identify, and characterize the bacterial leaf blight pathogen on A. crassicarpa and to study the bacterial community associated with A. crassicarpa seeds. Bacterial species were isolated from surface sterilized diseased leaves, symptomatic cotyledons, asymptomatic cotyledons, and seeds. Culture-dependent and independent methods were used to identify and characterize bacterial species of A. crassicarpa. Pantoea and Enterobacter species were the most isolated from diseased leaves. Pantoea species were also commonly isolated from symptomatic cotyledons. The dominant genera in asymptomatic cotyledons and seeds were Bacillus and Sphingobacterium species, respectively. Rhizobium was dominant in majority of A. crassicarpa seed genotypes based on the relative abundance plot. Statistical analysis showed that the difference between and within genotypes is not significant even though the observed species, community evenness and dominance between genotypes were slightly different. Although Xanthomonas species was not isolated, a potential pathogen was also isolated from both diseased leaves and symptomatic cotyledons using the culture-based method and identified with multi-locus sequence analysis (MLSA). Based on the sequences of two housekeeping genes (atpD and infB), the potential pathogen was identified as Pantoea stewartii subsp. indologenes. However, no disease symptoms were observed when this bacterium was inoculated into healthy A. crassicarpa seedlings. Several bacterial endophytic species were isolated and identified from different tissues of A. crassicarpa. These species could potentially promote the growth of woody species. These findings from the study will aid in future production and growth of healthy A. crassicarpa seedlings.