Physiological and biochemical insights into the basal level of resistance of two maize hybrids in response to Fusarium verticillioides infection.

Fusarium stalk rot (FSR), caused by Fusarium verticillioides, is one of the most destructive diseases impacting maize yield worldwide. In this study, net carbon assimilation rate (A), stomatal conductance to water vapor (g), transpiration rate (E), and internal CO concentration (C) were evaluated on leaves and the activities of enzymes (chitinase (CHI), β-1-3-glucanase (GLU), phenylalanine ammonia-lyase (PAL), polyphenoloxidase (PPO), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POX)) as well the concentrations of total soluble phenolics (TSP), lignin-thioglycolic acid (LTGA) derivatives, and malondialdehyde (MDA) were evaluated in the internodes and nodes of plants from maize hybrids moderately resistant (BRS 1035) and susceptible (30F35Y) to FSR. The upward relative lesion length (URLL) and radial fungal colonization (RFC) were 46 and 29% lower for the BRS 1035 hybrid in comparison to 30F35Y hybrid, respectively, at 30 after inoculation (dai). For both hybrids, A, g, and E values significantly decreased while the C values increased on infected leaves compared to noninoculated plants. Inoculated plants from BRS 1035 hybrid showed an increase in A compared to inoculated plants from 30F35Y hybrid, and the increase in C values was greater for plants from 30F35Y hybrid at 30 dai compared to plants from BRS 1035 hybrid. The CHI, GLU, PPO, CAT, APX, and POX activities increased for inoculated plants from both hybrids compared to the noninoculated plants. In the internodes region, the increase in the activities of CHI (during the infection process of F. verticillioides) and GLU (at earlier stages of F. verticillioides infection) was more pronounced for plants from BRS 1035 hybrid than for plants from 30F35Y hybrid. In the region of the nodes, activities of CHI (during the infection process of F. verticillioides), PAL (at 20 dai), PPO (at 30 dai), and CAT and POX (both at three dai) were more pronounced for plants from BRS 1035 hybrid than for plants from 30F35Y hybrid. In the internodes region, the lower TSP concentration at 30 dai was linked to a high concentration of LTGA derivatives for inoculated plants from BRS 1035 hybrid compared to inoculated plants from 30F35Y hybrid. Taking together, the results of the present study allowed to conclude that the infection by F. verticillioides triggered physiological and biochemical changes on the stalk of maize plants influencing photosynthesis on leaves. A more robust antioxidative metabolism for reactive oxygen species removal in association with an efficient and strong activity of defense enzymes helped to minimize the cellular damage caused by F. verticillioides infection resulting, therefore, in an increase in maize resistance to FSR.