Biocontrol potential of an endophytic Bacillus pumilus JK-SX001 against poplar canker

Several endophytes play a vital role in plant protection and plant growth promotion. In China, poplar canker disease caused by three pathogens (Cytospora chrysosperma, Phomopsis macrospora and Fusicoccum aesculi) is difficult to control by use of chemicals; therefore, use of biocontrol agents is desirable. The aim of this study was to examine the effects of strain JK-SX001 on plant growth and poplar canker caused by three pathogens. It is clearly demonstrated that strain JK-SX001 described as Bacillus pumilus by the Biolog identification system combined with morphological, physiological and biochemical tests and phylogenetic (16S rDNA) studies. Strain JK-SX001 produced lytic enzymes such as cellulases and protease. In vitro antagonistic assays showed that secondary metabolites of JK-SX001 extracted using methylbenzene could suppress the growth of three pathogens effectively. A derivative of this strain, labeled with gfp, was also used to study the colonization of its poplar hosts. This strain was verified as an endophytic bacterium of poplar by confocal laser scanning microscopy. The population of the GFP-labeled JK-SX001 inoculant was larger and more stable in roots and stems than that in leaves. The evaluation of the antagonistic strains against poplar canker indicated that JK-SX001 effectively reduced disease incidence. In greenhouse studies, poplar seedlings inoculated with this strain showed high increase in biomass production, shoot length, stem diameter and photosynthetic activity. Strain JK-SX001 colonized poplar roots, stems and leaves endophytically, promoting plant growth and suppressing pathogenic activities on seedling of poplar. Our results demonstrate JK-SX001 as a promising candidate for biocontrol of poplar canker.