We investigated the effect of treatment with hot water extracts from the spent mushroom substrates (SMSs) of Lentinula edodes and Hypsizygus marmoreus on the resistance of rice leaves to Pyricularia oryzae infection. The spraying of the SMS extracts clearly suppressed the development of lesions caused by Py. oryzae infection. The accumulation of phytoalexins momilactones A and B, oryzalexin A, and sakuranetin was markedly induced by the spraying of extracts. The enhanced expression of defense related genes PR1b and PBZ was also found in leaves sprayed with the extracts. Treatments with the extracts also affected phytohormone levels. The levels of N 6 -(Δ 2 -isopentenyl)adenine and trans-zeatin markedly increased in response to treatment, whereas the levels of salicylic and jasmonic acids were largely unchanged.

Plant defense responses are activated by various exogenous stimuli. We found that an aqueous extract of spent mushroom substrate used for the cultivation of Hypsizygus marmoreus induced defense responses in rice. Fractionation of the spent mushroom substrate extract indicated that the compounds responsible for this induction were neutral and hydrophilic molecules with molecular weights lower than 3 kDa. Compounds with these characteristics, namely glucose, fructose, and sucrose, were detected in the extract at concentrations of 17.4, 3.3, and 1.6 mM, respectively, and the treatment of rice leaves with these sugars induced defense responses. Furthermore, microarray analysis indicated that the genes involved in defense responses were commonly activated by the treatment of leaves with spent mushroom substrate extract and glucose. These findings indicate that the induction of defense responses by treatment with spent mushroom substrate extract is, at least in part, attributable to the sugar constituents of the extract.