The participation of phytochrome in the signal transduction pathway of salt stress responses in Mesembryanthemum crystallinum L

Continuous irradiation of Mesembryanthemum crystallinum plants with light of equal amounts of photosynthetically active radiation, but widely different red:far red ratios was used to intervene in phytochrome-mediated signal transduction pathways in the presence and absence of salt stress. Light with a low ratio of red:far red (in contrast to light with a high ratio of red:far red), caused induction of PEP carboxylase activity, accumulation of the CAM isoform of PEP carboxylase, and the accumulation of malate anion. Taking these as indicators of CAM induction it is concluded that phytochrome can participate in the signal transduction pathway leading to CAM in M. crystallinum. A low ratio of red: far red light acted synergystically with salt stress in the induction of these CAM indicators. The simplest interpretation of this interaction is that the phytochrome-mediated effects and salt stress effects acted on the same signal transduction pathway. The accumulation of pinitol was also increased by light with a low ratio of red: far red, consistent with the existence of a stress syndrome in M. crystallinum which utilizes a common transduction pathway. A low ratio of red: far red light induced a strong shade avoidance response and, compared to light with a high red:far red ratio, modified chlorophyll content and betacyanin pigment complement. Plants grown in light with a low ratio of red:far red flowered earlier than plants grown in light with a high red:far red ratio. It is concluded that phytochrome can participate in the signal transduction pathway leading to the induction of both CAM and the processes which result in pinitol accumulation and pigmentation in M. crystallinum, as well as in the mediation of shade avoidance and flowering responses.