Structural and functional changes in potato leaves when damaged by potato virus M and fungus Alternaria solani

Fungi and viruses that cause plant diseases lead to a significant reduction in potato yields. The symptoms of infectious diseases differ depending on the type of infection. The fungus Alternaria solani Sorauer, the causative agent of dry spot disease of potato, forms necroses on leaves, first very small and then growing in concentric rings. One of the common viral pathogens of potato the potato virus M (PVM) causes leaf deformations (twisting and wrinkling). Many cellular processes in infection involve biological membranes, which are important constituents of plant cells. In the present study, similarities and differences in structural and functional changes in potato leaves during PVM and A. solani infections were established for the first time. The aim of the work was to evaluate changes in tissue structure, chloroplasts, membrane lipid profile and redox potential of potato leaves during infection with PVM and A. solani . The med-early potato variety Tana was grown in the experimental field of the Tulaikov Samara Research Institute of Agriculture (Samara Province) in 2022-2023. Tubers were planted in the second decade of May in four replications of 50 plants each. Potatoes were grown without irrigation and fertilization under natural light. Leaves were sampled at the beginning of flowering under natural infection load. Diagnosis of plant pathogens was carried out in the field visually and then confirmed by PCR. Samples included uninfected leaves (control) as well as those infected with latent PVM and the fungus A. solani . To determine the morphological parameters of the leaf, cuttings were taken from fresh leaves of 3-4 plants and fixed in a 3.5 % glutaraldehyde in a standard phosphate buffer (pH 7.4). Leaf parameters (structural characteristics of the mesophyll, number of stomata, number and size of chloroplasts) were measured using a microscope equipped with a C-P20 camera (B-500TPL, Optika S.r.l., Italy). For biochemical analyzes, fresh leaves were collected, combined into a joined sample, three 0.5-1.0 g specimens were formed and stored in liquid nitrogen until assays. Physiological and biochemical analyses included determination of leaf dry weight, the amount of photosynthetic pigments, proteins, lipids and fatty acids, lipid peroxidation products and low molecular weight antioxidants. It was established that the PVM causes leaf deformation, and A. solani causes tissue necrosis. The internal architecture of leaves affected by pathogens has dense spongy parenchyma, smaller intercellular space, and changes in the structure of chloroplasts. Wt revealed a 1.7-2-fold decrease for photosynthetic pigments (chlorophylls a and b, carotenoids) in leaves, especially under the action of A. solani . We also established the specificity of the effect of infections on redox potential and membrane state. Under the influence of A. solani , the level of malondialdehyde (MDA) as a marker of oxidative processes increased from 0.18 to 0.37 μmol/g dry weight, the concentration of free proline decreased approx. 5 times from 10 to 1.6 μg/g dry weight and water-soluble phenolic compounds decreased 1.4 times from 128 to 95.0 mg/g dry weight. PVM had no effect on MDA content, but resulted in a 35 % decrease in membrane protein content. The fatty acid (FA) composition of lipids during A. solani infection showed a decrease in the FA unsaturation index from 1.8 to 1.5.