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Sugar metabolism and sugar signalling are not only critical for plant growth and development, but are also important for stress responses. However, how sugar homeostasis is involved in plant defence against pathogen attack in the model crop rice remains largely unknown. In this study, we observed that the grains of gif1, a loss-of-function mutant of the cell wall invertase gene GRAIN INCOMPLETE FILLING 1 ( GIF1), were hypersusceptible to postharvest fungal pathogens, with decreased levels of sugars and a thinner glume cell wall in comparison with the wild-type. Interestingly, constitutive expression of GIF1 enhanced resistance to both the rice bacterial pathogen Xanthomonas oryzae pv. oryzae and the fungal pathogen Magnaporthe oryzae. The GIF1-overexpressing ( GIF1- OE) plants accumulated higher levels of glucose, fructose and sucrose compared with the wild-type plants. More importantly, higher levels of callose were deposited in GIF1- OE plants during pathogen infection. Moreover, the cell wall was much thicker in the infection sites of the GIF1- OE plants when compared with the wild-type plants. We also found that defence-related genes were constitutively activated in the GIF1- OE plants. Taken together, our study reveals that sugar homeostasis mediated by GIF1 plays an important role in constitutive and induced physical and chemical defence. [ABSTRACT FROM AUTHOR]