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

Abstract

Aim. To investigate the ability of our phytopathogenic fungal strain 502, earlier preliminarily identified as the phytopathogen Plectosphaerella 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.), garden cress (Lepidium sativum L.), cucumber (Cucumis sativus L.), and onion (Allium cepa L.). The cytotoxicity and genotoxicity of the fungus were estimated using the Allium cepa-test. The mitotic index of the, the duration of mitosis phases, and the frequency 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 gas chromatograph «Agilent Technologies 6850» (USA) fitted with a flame ionization detector, using commercial ethylene as a standard for identification and quantification Every experiment had three repeats. The reliability of experimental data was assessed by statistical methods using Statistica 12 (Stat-Soft Inc., USA). Results. Undiluted culture fluid (obtained by growing the fungus on liquid wort) of our strain 502 inhibited the growth of Z. mays seedlings by 14 %, L. sativum seedlings by 18 % (1 : 100 dilution) and stimulated the growth of L. sativum roots by 54 and 41 % (1 : 10 and 1 : 100 dilutions, respectively). The culture fluid, obtained by growing the fungus on Raulin-Thom’s synthetic agar, demonstrated a slight inhibitory effect on the seedlings and roots of L. sativum, and at the dilution of 1 : 1000 stimulated growth by 30 %. Insignificant changes in the mitotic index of the meristem of A. cepa roots were revealed at the effect of the culture fluid 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 prophase stage 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 and 1 : 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 were 5.0 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 effect on roots and seedlings of the investigated cultures, demonstrating the exclusion of phytotoxins from the possible range of effectors. No cytotoxic or genotoxic activity of the culture fluid was observed either. However, the culture fluid altered the dynamics of the cell cycle, in particular, shortened the prophase and stimulated the metaphase, anaphase, and telophase. The culture fluid of the fungus stimulated the growth of L. sativum roots depending on the nutrient medium, where the fungus was 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 demonstrates the ability of strain 502 to possibly synthesize growth promoting substances. Also, we have shown the ability of this strain to synthetize ethylene in vitro (111.78 ± 13.27 nmol/h per g), which can act as virulence factor. We consider the obtained results to be the first stage of the study on the mechanism of the interaction between pathogenic strain 502 and plants.

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“…The improvement in the growth and development of these plants may be conditioned by the synthesis of growth-regulating compounds, stimulating the ontogenesis processes in the initial stages of mycelium growth in plant tissues, but further development of the fungus in the host plant tissues is accompanied by the excess of phytohormones. Previously we have shown that P. melonis strain 502 synthesizes the ethylene phytohormone in vitro, and the cultural liquid of P. melonis strain 502 is characterized by growth-regulating properties and may inhibit or stimulate the growth of plants depending on the dilution and the test plant (Tsekhmister et al, 2021). It is generally known that many plant pathogens pretend to be mutualistic, and at fi rst, they stimulate the growth of a host plant; artifi cially infected plants look greener and healthy, then there are manifestations of the damage and even collapse of plants (van Baalen and Jansen, 2001;Bronstein, 2001).…”

“…Strain. A natural strain of fungus P. melonis strain 502, isolated from the diseased roots of cucumber plants (C. sativus cv Koroliok), cultivated in the covered soil, was used in the work (Kopilov et al, 2021;Tsekhmister et al, 2021). The strain was deposited in the Depository of the Institute of Microbiology and Virology, NAS of Ukraine, and numbered IMV F-100138.…”

“…In Ukraine, this disease was fi rst registered in 2012 in cucumber plants, cultivated in the covered soil, where P. melonis strain 502 was isolated (Kopilov et al, 2021). We have already studied the capability of the cultural liquid of P. melonis 502 to regulate the growth of plants depending on the test culture and to synthesize the phytohormone ethylene in vitro (Tsekhmister et al, 2021); we have also investigated the histotrophic localization of the pathogen and its ability to synthesize cellulase enzymes depending on environmental conditions (Patyka et al, 2022), and in this study, our work aimed to demonstrate the phylogenetic relations between P. melonis strain 502 and the isolates from other countries and to study the sensitivity of different varieties of Cucumis sativus L.…”