Plant growth regulatory activity in the phytopathogenic fungus Plectosphaerella melonis strain 502

Aim. To investigate the ability of our phytopathogenic fungal strain 502, earlier preliminarily identified as the phytopathogenPlectosphaerella melonis (syn. Acremonium cucurbitacearum), to have phytotoxic and/or plant growth regulatory activity.Methods. The phytotoxicity of strain 502, was studied by bioassays using the test cultures of corn (Zea mays L.), gardencress (Lepidium sativum L.), cucumber (Cucumis sativus L.), and onion (Allium cepa L.). The cytotoxicity and genotoxicityof the fungus were estimated using the Allium cepa-test. The mitotic index of the, the duration of mitosis phases, and thefrequency of aberrant ana-telophases of Allium cepa L. roots meristem was also investigated. For this purpose, strain 502,was grown in the following culture media: synthetic Raulin-Thom medium for 10 days at 26 ± 2 °С. Cell-free filtrate(culture fluid) was used for the study. Ethylene production was quantified in culture filtrate using gas-chromatography meth-od. Ethylene measurement was performed every 7 days during 8 weeks. The determination was carried out using a gaschromatograph «Agilent Technologies 6850» (USA) fitted with a flame ionization detector, using commercial ethyleneas a standard for identification and quantification Every experiment had three repeats. The reliability of experimental datawas assessed by statistical methods using Statistica 12 (Stat-Soft Inc., USA). Results. Undiluted culture fluid (obtainedby growing the fungus on liquid wort) of our strain 502 inhibited the growth of Z. mays seedlings by 14 %, L. sativumseedlings by 18 % (1 : 100 dilution) and stimulated the growth of L. sativum roots by 54 and 41 % (1 : 10 and 1 : 100dilutions, respectively). The culture fluid, obtained by growing the fungus on Raulin-Thom’s synthetic agar, demonstrateda slight inhibitory effect on the seedlings and roots of L. sativum, and at the dilution of 1 : 1000 stimulated growth by30 %. Insignificant changes in the mitotic index of the meristem of A. cepa roots were revealed at the effect of the culturefluid of P. melonis, strain 502, diluted at the ratio of 1 : 100 and 1 : 1000. At the same time, the number of cells at the prophasestage decreased 1.7 times (1 : 100 dilution). There is a significant increase in the number of cells at the metaphase stage –1.3 and 1.4 times (dilution 1 : 100 and 1 : 1000, respectively), the anaphase stage – 2.1 and 1.8 times (dilution 1 : 100 and1 : 1000, respectively) and the telophase stage – 1.8 times (1 : 100 dilution), as compared with the positive control(culture medium). The frequencies of aberrant ana-telophases in the apical meristems of the initial roots were5.0 and 2.2 % (at the culture fluid dilution of 1 : 100 and 1 : 1000, respectively). We researched the abil-ity of P. melonis 502 to synthesize ethylene and the highest level of it was registered after 5 weeks of cultivation(111.78 nmol/h g). Conclusions: It was demonstrated by us that the culture fluid of strain 502 showed no phytotoxic effecton roots and seedlings of the investigated cultures, demonstrating the exclusion of phytotoxins from the possible range ofeffectors. No cytotoxic or genotoxic activity of the culture fluid was observed either. However, the culture fluid altered thedynamics of the cell cycle, in particular, shortened the prophase and stimulated the metaphase, anaphase, and telophase. Theculture fluid of the fungus stimulated the growth of L. sativum roots depending on the nutrient medium, where the funguswas grown and cultivated. In particular, when growing the fungus on the liquid wort, the growth was higher by 54 and 41 %(dilution 1 : 10 and 1 : 100, respectively), when growing on synthetic Raulin-Thom’s medium – by 30 %. This demonstratesthe ability of strain 502 to possibly synthesize growth promoting substances. Also, we have shown the ability of this strain tosynthetize ethylene in vitro (111.78 ± 13.27 nmol/h per g), which can act as virulence factor. We consider the obtained resultsto be the first stage of the study on the mechanism of the interaction between pathogenic strain 502 and plants.