Leaf physiology and histopathology of the interaction between the opportunistic phytopathogen Fusarium equiseti and Gossypium hirsutum plants

Abstract

Fungal diseases constitute a major challenge in the cultivation of the world's most important textile plant, cotton. Opportunistic fungi attack plants and promote physiological alterations and tissue damage, thereby causing losses in productivity. Herein, the hypothesis that Gossypium hirsutum L. plants are physiologically affected, depending on the phenological stage and site of infection (root system or leaf), when affected by the opportunistic phytopathogen Fusarium equiseti was tested. To this end, the photosynthetic pigment production, gas exchange, chlorophyll a fluorescence, and histopathological mechanisms involved in the plant-phytopathogen interaction were evaluated. This study aimed to diagnose the most aggressive route of infection by comparing infection via root with infection via the leaf, and to identify the developmental stage with the highest susceptibility by comparing different phenological stages. Cotton plants show metabolic losses at the maturation stage, resulting in low photosynthetic pigment concentrations, low photosynthetic rates, and photochemical stress. However, when infected by F. equiseti in the root region, the photosynthesis and photochemistry of these plants are more affected than plants inoculated in the leaf region. Low net photosynthesis (A) and water use efficiency (WUE) rates, but high light absorption flux (ABS/RC) and energy dissipation flux (DI0/RC) values were observed. Regardless of the phenological stage, the presence of F. equiseti in the roots of cotton plants caused high transpiratory rates in the plants, suggesting non-stomatic water loss and photochemical damage. In contrast, non-inoculated plants produced more photosynthetic pigments and showed higher WUE and photochemical efficiency, as indicated by the high PIABS values. Leaf histopathology assessment revealed more fragile regions of the epidermis, such as the contact region between the guard cells and adjacent epidermal cells, as well as the basal region of glandular trichomes as important access routes for F. equiseti to the inner leaf layers at all developmental stages. This study confirmed that the root colonization of cotton plants with F. equiseti can more intensely impact the physiological quality of the plants, regardless of the developmental stage.