Substrate-dependent biosynthesis of ethylene by rhizosphere soil fungi and its influence on etiolated pea seedlings

Several fungi were isolated from rhizosphere soils of various crops through enrichment on l-methionine (l-MET) as a sole C and N source. These fungi were examined for their C2H4 production potential in vitro. GC-FID analysis indicated that 78%, 83%, 89% and 72% of fungi isolated from rhizosphere soil of wheat, maize, potato and tomato, respectively, produced C2H4 from substrate l-MET. Eight of the most efficient C2H4-producing fungal isolates (two from each crop) were further tested for their ability to produce C2H4 from 2-keto-4-methylthiobutyric acid (KMBA) and α-ketoglutaric acid (α-KGA). Interestingly, all eight fungi produced C2H4 from KMBA, but none used α-KGA as C2H4 precursor. This result implies that the MET→KMBA→C2H4 pathway was most likely operating in these fungi. The most efficient C2H4 producer fungus (isolated from maize rhizosphere soil) was identified as Aspergillus terreus; this isolate was further studied to determine the optimal conditions for C2H4 production from l-MET. It was observed that a substrate (l-MET) concentration of 10 mmol l−1, a glucose (C-source) concentration of 6.0 g L−1, no nitrogen, a pH of 6.0, an incubation temperature of 30 °C and an incubation time of 72 h were optimal conditions for l-MET-derived C2H4 biosynthesis by Aspergillus terreus. In addition, C2H4 released from precursor (l-MET) by Aspergillus terreus significantly affected the seedling length and stem diameter of etiolated pea seedlings in both sterilized and non-sterilized soils compared to a control. C2H4-producing microflora may be ubiquitous in soil, and the availability of a substrate like l-MET could enhance C2H4 synthesis in the rhizosphere of a plant, in turn evoking a physiological response. As far as we know, this study represents the first comprehensive screening of rhizosphere fungi for their ability to produce C2H4 from l-MET and KMBA.