Effect of humic acid on the composition of osmolytes and lipids in a melanin-containing phytopathogenic fungus Alternaria alternata

Humic substances (HS) have a direct impact on living cells, causing a wide range of various biological effects, and stimulating or inhibiting fungal growth. Therefore, it is important to reveal cellular indicators that could indicate the nature and level of HS' effects on living organisms. Paying attention to the important role of lipid and osmolytes in adaptation to stress, the aim of this work was to study the composition of osmolytes and lipids in the growth dynamics of the phytopathogenic melanin-containing fungus Alternaria alternata under the influence of humic acid (HA). HA was isolated from a commercial peat humic product. For cultivation, liquid Czapek medium with the addition of 200 mg/l HA (HA variant) was used, and with no HA for the control variant. The main osmolytes of the fungal mycelium were glucose, mannitol, and trehalose. Both in the control and in HA variants, a general pattern was observed during the growth process; the amount of glucose increased against the background of a decrease of mannitol. In the control, the amount of trehalose increased significantly by the stage of active melanin formation (day 4), while under the influence of HA, such an increase was not observed. Membrane lipids were represented by phospholipids, sphingolipids, and sterols. A characteristic feature of A. alternata was a high proportion of phosphatidic acids (PA) in the composition of membrane lipids. The amount and proportion of PA decreased during growth in the control, while under the influence of HA such decline was not observed. The influence of HA induced the increase in the amount of sterols (St) and phosphatidylcholines (PC) that were able to stabilise the membrane lipid bilayer. We can assume that general cellular response of A. alternata to HA influence, being expressed as a higher amount of PA, PC, St and a lower level of trehalose in comparison with the control, indicates the an absence of inhibition of growth processes of this phytopathogenic melanin-producing fungus. The effect of HA on the qualitative and quantitative composition of the osmolytes and membrane lipids of A. alternata may cause changes of virulence and stress resistance of this phytopathogen.