2012 年 29 巻 p. 51-57
A facultative halophyte Mesembryanthemum crystallinum L. changes its carbon fixation mechanism from C3 to crassulacean acid metabolism (CAM) in response to environmental stress such as drought and salinity. Induction of CAM also results in the accumulation of metabolites like malate, betacyanin, proline, and pinitol in the plant with mechanisms that are unknown as yet. In order to explore the factors that lead to the formation of these metabolites in this plant, we treated M. crystallinum L. with culture supernatants of Aspergillus oryzae, Nectria gracilipes, and Rhizopus sp. Among the fungi tested, culture supernatant from A. oryzae induced typical CAM phenotypes in this halophyte species even in the absence of salt. Endogenous malate levels in the treated plants increased 5-fold in comparison with those in the control plants. We investigated the expression patterns of genes like CAM-specific Ppck and Ppc1. Our analysis of expression patterns of these genes showed no significant increase in their expression levels in the A. oryzae-treated plant, indicating that the accumulation of malate was independent of CAM-photosynthesis. When the plant was treated simultaneously with the culture supernatant and 400 mM NaCl, circadian regulation of Ppck was disturbed. Expression patterns of a pinitol biosynthesis gene (Imt) in the treated plants also showed anomalies in the circadian rhythm. On the basis of these results, we suggest that Ppck and Imt genes may be regulated by a novel mechanism independent of CAM.
