Diversity of Yeasts in Vetiver Phylloplane in Thailand and Their Capability to Produce Indole- 3-acetic Acid, a Plant Growth Promoter

The phylloplane is a term for the aboveground surface of plants and usually refers to the external surface of plant leaves, a dominant aerial part of plant. It is known to be colonized by a large number of microorganisms including bacteria, yeasts and fungi. Yeasts in phylloplane belong to either phylum Ascomycota or phylum Basidiomycota. In this study diversity of yeasts in the external surface of vetiver grass leaf in Thailand was investigated by culture dependent method and yeast strains were investigated for their capability to produce indole-3-acetic acid (IAA), a plant growth promoter. Thirty-four samples of vetiver leaf were collected from eight provinces viz. Ratchaburi, Kanchanaburi, Uthai Thani, Chiang Mai, Phayao, Uttaradit, Trat and Udon Thani. Yeasts were isolated from surface of leaf by an enrichment technique using yeast extract-malt extract broth supplemented with 0.025% sodium propionate and 0.02% chloramphenicol. A total of 40 yeast strains were obtained from 29 leaf samples (85.3%). Yeasts were identified on the basis of molecular taxonomy by analysis of the D1/D2 region of the large subunit rRNA gene sequence similarity and phylogeny. Thirty-nine strains were identified to be 13 known species belong to Ascomycota viz. Ambrosiozyma monospora, Candida carpophila, Candida jaroonii, Candida michaelii, Candida tropicalis, Hyphopichia burtonii, Kodamaea ohmeri, Meyerozyma caribbica and Pichia kudriavzevii, and Basidiomycota viz. Cryptococcus laurentii, Rhodosporidium paludigenum, Rhodotorula mucilaginosa and Trichosporon asahii. One strain was found to represent a new yeast species in the genus Rhodosporidium closest to R. toruloides. The most prevalent yeast species in vetiver grass phylloplane was M. caribbica with a frequency of occurrence of 35% and followed by C. tropicalis that represented 25%, while only one to two strains were obtained from the other 10 species. Individual leaf sample was generally accommodated by only one yeast species (65.5%) and two yeast species were found in 31.0% of the samples. Determination of IAA production by the vetiver grass phylloplane yeasts revealed that only nine strains produced IAA in the range of 11.0-332.9 mg/L when cultivated in yeast extract peptone dextrose broth supplemented with 0.1% L-tryptophan. They consisted of two strains each of C. michaelii, M. caribbica and R. paludigenum, and one strain each of C. tropicalis, P. kudriavzevii and new Rhodosporidium species. The two strains of R. paludigenum produced high IAA including strain DMKU-LV61 produced the highest IAA of 332 mg/L and strain DMKU-LV56 produced 109.9 mg/L.