In this chapter, we aimed to cover the major advances in defining the signaling mechanisms controlling CAM expression in facultative species, with an emphasis on the interplay between plant hormones and second messengers. The signaling cascades implicated in transducing environmental cues such as water availability, salinity, light intensity, and photo- and thermoperiod into changes in CAM expression were discussed separately in an attempt to highlight differences and similarities among the signal transduction routes triggered by each of these external stimuli. As noted in our discussion, important aspects of the molecular and signaling processes that underpin the induction of CAM by water deficit have been recently discovered, revealing a coordinated and complex set of parallel, yet overlapping signaling pathways, which apparently involve plant hormones, intracellular calcium, nitric oxide, protein phosphatases, and kinases. Comparatively less is known about the signaling events implicated in the modulation of CAM expression by salinity, light, and temperature. Even scarcer is the available information regarding the signaling events mediating the regulation of CAM by other environmental factors such as nutrient availability, extremes in temperature, or anoxia.
Taking into consideration that, in the field, the distinct environmental factors that modulate CAM operation never act individually but always interactively, a significant challenge for the future will be to uncover how different external stimuli are integrated to adjust CAM expression at levels compatible with the environmental conditions. On the contrary, since changes in the distinct environmental parameters are frequently challenging to separate, even in laboratory experiments, special attention is also required to clarify whether a given external stimulus is able to modulate CAM expression per se or acts indirectly by affecting other environmental parameters (e.g., effect of temperature and salinity on water availability).
Besides discovering new components of the signaling cascades leading to CAM expression, another future challenge will be to compare the signaling networks implicated in the environmental control of CAM expression in facultative species with contrasting C3-CAM switch dynamics. For instance, the signaling cascades implicated in the gradual and virtually irreversible establishment of CAM under certain conditions (e.g., photoperiod induction of CAM in ice plant) may possibly differ from those required for the rapid and freely reversible C3-CAM switches observed under some circumstances (e.g., drought-induced CAM expression in some Clusia spp.). Such studies, if conducted in phylogenetically distant species, will also be essential to evaluate the degree of conservation in the signaling mechanisms controlling CAM expression in plants with independent evolutionary origins of the CAM pathway.
Another important goal for the future will be the generation of mutants and transgenic CAM plants with alterations in perception or biosynthesis of specific signaling elements (e.g., plant hormones, NO, ROS, protein phosphatases, and kinases), which could represent an important tool to investigate complex questions such as the molecular and biochemical processes required to promote CAM expression in response to developmental and external factors. In this context, efficient mutant screening protocols, and transformation and regeneration systems for CAM plant models are sorely needed.
Moreover, it appears to be highly worthwhile to study how environmental and developmental factors interact at the signaling level in order to determine the degree of CAM expression. In this sense, comparisons between the changes in endogenous signaling compounds in facultative CAM plants at different developmental stages, and, consequently, with distinct responsiveness to environmental signals that stimulate CAM, could possibly improve our understanding of the regulatory cascades that govern interactions between the developmental and environmental control of CAM photosynthesis.
Ultimately, given the ample range of environmental stimuli implicated in the modulation of CAM and the virtually infinite ways that they can interact, not only among themselves but also with developmental and tissue-specific inputs, many other aspects of the signaling processes leading to CAM expression are still to be elucidated. Therefore, a combination of biochemical, genetic, and molecular strategies in different CAM models might be needed to improve our understanding of the signal transduction events required to orchestrate the complex array of physiological modifications necessary for fine-tuning CAM expression according to environmental conditions.
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