Plant Growth-promoting Ability and Pathogen Inhibitory Effect of Actinomycetes Isolated from Fecal Pellets of the Giant Millipede Thyropygus resimus (Diplopoda)

Abstract

The microbial properties of millipede fecal pellets have been studied mainly in Glomerida (pill millipedes), and much less in the significant majority of other millipede groups. Therefore, the present study examined actinomycetes isolated from the fecal pellets of the non-glomerid giant millipede Thyropygus resimus Attems, 1938 (Spirostreptida) to (1) test their plant growth-promoting ability, and (2) evaluate their potential to control and inhibit plant pathogenic microorganisms. Millipedes were collected from Phu Kum Khao, Kalasin Province, Thailand. A total of 59 actinomycete isolates were obtained and identified as belonging to the genus Streptomyces using 16S rRNA sequencing. The plant growth-promoting properties of the isolates were tested by screening four characteristics: nitrogen fixation, phosphate solubility, siderophore production, and indole-3-acetic acid (IAA) production. A nitrogen-fixation test on nitrogen-free solid malate media (NFM) showed that 54 isolates were capable of fixing nitrogen. Phosphate solubility was tested on double-layered glucose yeast extract agar (GYA) medium containing tricalcium phosphate. This showed that 42 isolates formed a clear zone around the colonies due to phosphate dissolution. Siderophore production was tested on chrome azurol sulfate (CAS) agar. This showed that 55 isolates could grow on this medium and form clear yellow to orange zones around their colonies. IAA production tests revealed that 41 isolates could produce IAA. Based on the combined results of these four tests, eight of the 59 isolates were the most effective in promoting plant growth: KLS-AC04, KLD-AC01, KLD-AC02-1, KLD-AC08, KLD-AC09, KLD-AC16, KLD-AC29-1, and KLD-AC30. Seventeen isolates inhibited the growth of Xanthomonas oryzae pv. oryzae, the causal agent of bacterial leaf blight disease in rice, more effectively than rifampicin (100 ppm), with isolate KLS-AC02-1-1 being the most effective (inhibition zone, 58.25 mm in diameter). Therefore, these isolates can be used for growth promotion and rice disease control in the future.