Previous studies of IAA-producing yeast were reported in
Lindera (Williopsis) saturnus (Nassar et al. 2005), Rhodotorula graminis and R. mucilaginosa (Xin et al. 2009), Candida tropicalis
(Amprayn et al. 2012), Candida valida, Rhodotorula glutinis, and
Trichosporon asahii (El-Tarabily 2004), Cryptococcus sp. (Deng
et al. 2012) and 20 yeast species isolated from the phylloplanes
of various plant species collected from seven provinces in
Thailand (Limtong & Koowadjanakul 2012). Although a higher
IAA level was reported in the yeast Rhodotorula spp. by Xin et al.
(2009), no other plant growth-promoting traits have yet been
proposed by the authors. It must be noted that the lower
NH3 and siderophore production found in R. paludigenum
DMKU-RP301 might not recommend this yeast as a good candidate for plant growth promotion. However, a higher level of
IAA production was foreseen through optimisation of either
medium composition or growth conditions. Experimental design has been set up and placed under investigation not only
for R. paludigenum DMKU-RP301 but also for Torulaspora globosa
DMKU-RP31. The latter has received more attention than the
others because of its capability to produce antifungal volatile
compounds, hence showing significant antagonistic effects
against three molds on a solid medium. This is due to the
fact that not only phytohormones such as IAA are necessary
to support plant growth, but also other indirect plant
growth-promoting traits are required to obtain good plant
yield.