Effects of a supernatant from Aspergillus oryzae culture on the regulation of crassulacean acid metabolism (CAM) and pinitol biosynthesis in the common ice plant, Mesembryanthemum crystallinum L.

A facultative halophyte Mesembryanthemum crystallinum L., also known as the common ice plant, has been attracting the attention of plant scientists for decades because of its unique carbon fi xation strategy and high tolerance towards salinity. The plant uses the C3 carbon fi xation system under normal growth conditions, however, photosynthesis by the crassulacean acid metabolism (CAM) is induced in response to environmental stress such as high salinity, drought, temperature changes, and high irradiance. Induction of CAM is also seen in some developmental stages of the plant. When the carbon fi xation metabolism is shifted to CAM, CO2 is fi xed by phosphoenolpyruvate carboxylase (PPC) during the nocturnal period, resulting in the formation of oxaloacetate, which is subsequently converted to malate to be stored in vacuoles. As the period of light commences, the plant closes its stomata to avoid dehydration and removes CO2 from the malate in the cytosol, producing pyruvate as a byproduct. The released CO2 is then re-fi xed by Rubisco in the Calvin cycle and is incorporated into carbohydrates. These events follow the circadian rhythm . The induction of CAM also manifests as a range of morphological characteristics. A dwarf plant with dark green leaves is recognized as a typical CAM phenotype. In response to environmental stress, the plant is covered with bag-shaped ‘‘bladder cells,’’ where the excess absorbed salt is accumulated. Betacyanin, an indole-derived pigment found in some plants, is also stored in these bladder cells. Environmental stress factors that induce CAM also lead to the biosynthesis of pinitol (1-D-3-O-methyl chiro-inositol). Pinitol is a well-known compatible solute for plants, just like Original Article