Biochemical and histological characterization of succulent plant Tacitus bellus response to Fusarium verticillioides infection in vitro.

We present changes in Tacitus bellus antioxidative system that specifically correspond to subsequent phases of hemibiotroph Fusarium verticillioides infection revealed by histological analysis. T. bellus response to spore germination 6 h post inoculation (hpi), manifested as first oxidative burst, was characterized by transient decrease in malondialdehyde (MDA) content, transient increase in catalase (CAT), low level of superoxide dismutase (SOD) and peroxidase (POD) activity, as well as with transient decrease in total antioxidant capacity (TAC), total phenol content (TPC) and phenylalanine ammonium lyase activity (PAL), and no changes in polyphenol oxidase (PPO) activity, or phenolic profile. During the biotrophic phase of F. verticillioides infection, characterized by hyphae spread intercellularly in epidermal and mesophyll tissue, the host antioxidative system was suppressed. The transition to necrotrophic phase of F. verticillioides infection (inter- and intracellular colonization and sporulation), occurred 3-4 days post inoculation (dpi). During the necrotrophic phase, 5-7 dpi, slowed progression of colonization of T. bellus mesophyll cells occurred and it coincided with sharp increase in MDA content and CAT, SOD and POD activities, but the drop in TAC, TPC content, and PPO activity, as well as the production of phytotoxin fusaric acid. Presented results add to the knowledge of events and mechanisms related to the transition from biotrophy to necrotrophy in F. verticillioides.