A smut fungus Ustilago esculenta causes enlargement of infected culms of Zizania latifolia, a perennial aquatic grass. The swollen culm is used as a vegetable in China and some southeastern countries. Interaction between the fungus and the host plant plays a vital role in the enlargement of infected culms. U. esculenta is a biotrophic endophyte that completes its life cycle in host tissues. In our previous studies, we have observed that Z. latifolia plants infected by U. esculenta showed higher net photosynthetic rate and chlorophyll content, which is different from the results obtained after induction by pathogens. However, in this plant-fungus association, detailed biochemical, physiological or morphological changes, and the role of reactive oxygen species (ROS) and their scavenging system as well as photosystem (PS) Ⅱ efficiency during this interactive process, have not been studied. To ascertain the actual relationship between U. esculenta and Z. latifolia, we determined the antioxidative system and chlorophyll fluorescence in leaves of Z. latifolia.
The experiment was carried out at Zhejiang University, Hangzhou. Z. latifolia plants (cv. "Zhejiao No.2", a double-harvest variety) were used in this study. There were 16 pots (10 plants each pot) in each treatment. These plants were regularly watered to keep a 10 cm-layer of water above soil. Pesticides were sprayed for the prevention of pests and diseases if needed.
In this study, the response of Z. latifolia plants to U. esculenta infection was investigated, and activities of antioxidant enzymes, superoxide radical (O₂^·-) formation rate, hydrogen peroxide (H₂O₂) content, malondialdehyde (MDA) content, and chlorophyll fluorescence were determined. Our results showed that plant height, leaf length, leaf width, leaf thickness, root length, biomass of culm, root and aboveground were reduced, but tiller number was increased by U. esculenta infection. Meanwhile,U. esculenta infection caused increase in activities of ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD), glutathione reductase (GR), superoxide dismutase (SOD) and H₂O₂ content, but decrease in O₂^·- formation rate and MDA content (P<0.05). The decrease of O₂^·- formation rate in U. esculenta infected plants might be due to the higher SOD activity, which transformed O₂^·- to H₂O₂ and O₂. In this study, we also observed that quantum efficiency of PS Ⅱ (Φ_PSⅡ) and electron transport rate (ETR) were slightly increased (P > 0.05), but non-photochemical quenching (NPQ) was decreased by U. esculenta infection (P > 0.05). It is observed that maximum quantum efficiency of PSⅡ (F_v/F_m) was not altered by U. esculenta infection, indicating that U. esculenta infection may not be a biotic stress for Z. latifolia plants. Therefore, based on the changes of the plant growth indices, PS Ⅱ efficiency and membrane lipid peroxidation in infected plants, we found the interactive relationship between U. esculenta and Z. latifolia was neither pathogenic nor mutualistic.