Recombinants of Alternaria alternata endophytes enhance inorganic phosphate solubilization and plant growth hormone production

Endophytic microorganisms can enhance the growth of their host plants by solubilizing phos-phates and producing phytohormones. In some cases, spontaneous auxotrophic mutant fungi may exhibit improved solubilization and phytohormone production compared to the wild type. In this study, six endophytic strains of Alternaria alternata were molecularly identified and found to have the ability to solubilize phosphate and produce 3-indolacetic acid (IAA) in vitro. The A. alternata G3-92 strain was particularly noteworthy for its high levels of phosphate solubilization (2.2 cm) and IAA production (2.6 mu g mL(-1)). This strain was used to obtain auxotrophic mutants using the Potassium Bromate Resistance System, and the most promising mutants were then protoplasted and fused. Comparison with the wild G3-92 strain showed significant improvement in phosphate solubilization (2.9 cm) for the G3-92(arg-2), G3-92(pro-2), and G3-92(AU-2) mutants. IAA production also showed significant improvement for the G3-92(trp-3) and G3-92(ala-2) mutants, with production of 3.6 and 3.5 mu g mL(-1), respectively. The fusion of the G3-92(trp-3) and G3-92(arg-2) protoplasts resulted in eight new recombinant prototrophic strains with improved phosphate solubilization ranging from 2.4 to 2.7 cm. Seven of these recombinants also showed improved IAA production compared to the parental strains, G3-92(trp-3) and G3-92(arg-2), and the wild strain G3-92(Wild), with IAA production ranging from 12 to 22 mu g mL(-1). These results indicate that these newly obtained recombinant strains have potential for use as eco-friendly agricultural inoculants, as an alternative to chemical fertilizers.