Bringing CAM photosynthesis to the table: Paving the way for resilient and productive agricultural systems in a changing climate

Abstract

Modern agricultural systems are directly threatened by global climate change and the resulting freshwater crisis. A considerable challenge in the coming years will be to develop crops that can cope with the consequences of declining freshwater resources and changing temperatures. One approach to meeting this challenge may lie in our understanding of plant photosynthetic adaptations and water use efficiency. Plants from various taxa have evolved crassulacean acid metabolism (CAM), a water-conserving adaptation of photosynthetic carbon dioxide fixation that enables plants to thrive under semi-arid or seasonally drought-prone conditions. Although past research on CAM has led to a better understanding of the inner workings of plant resilience and adaptation to stress, successful introduction of this pathway into C₃ or C₄ plants has not been reported. The recent revolution in molecular, systems, and synthetic biology, as well as innovations in high-throughput data generation and mining, creates new opportunities to uncover the minimum genetic tool kit required to introduce CAM traits into drought-sensitive crops. Here, we propose four complementary research avenues to uncover this tool kit. First, genomes and computational methods should be used to improve understanding of the nature of variations that drive CAM evolution. Second, single-cell 'omics technologies offer the possibility for in-depth characterization of the mechanisms that trigger environmentally controlled CAM induction. Third, the rapid increase in new 'omics data enables a comprehensive, multimodal exploration of CAM. Finally, the expansion of functional genomics methods is paving the way for integration of CAM into farming systems.

Keywords: CAM photosynthesis; crop engineering; phylogenetics; single-cell genomics; stress resilience; synthetic biology.

Figure 1

Simplified representation of the four CAM phases over a 24-h cycle. During phase I, which takes place throughout the dark period, stomata open to allow atmospheric CO₂ to enter the cell; CO_{2 is then} hydrated by carbonic anhydrase to produce HCO₃⁻ and fixed by PPC, which is phosphorylated by PPCK. PPC uses PEP and HCO₃⁻ to form OAA, which is rapidly reduced to malate by MDH and stored inside large vacuoles as malic acid for the rest of the dark phase. Phase II represents the transition between phases I and III and occurs between the last hours of the dark phase and the first hours of the light phase. Phase III takes place during the light phase and is characterized by stomatal closure and transition to the C₃ carbon fixation mechanism, as well as inhibition of PPC via dephosphorylation by PP2A and possibly efflux of malate from the vacuole (not shown). In CAM-induced leaves of Mesembryanthemum crystallinum, malate is transported to the cytosol for decarboxylation by NADP-ME during the day. This reaction produces CO₂, which can be fixed by Rubisco in the chloroplast and integrated into the CBB cycle. Pyruvate, the other product of decarboxylation, is converted by PPDK to PEP, which can then be used for gluconeogenesis to replenish the starch pool in the chloroplast. Starch production from triose-P from the CBB cycle is not shown. Phase IV represents the transition from the light to the dark phase. PEP for the subsequent dark phase can be generated by glycolysis of the starch/glucose pool. This model represents the traditional pathway used by starch-forming CAM species and thus illustrates the decarboxylation process carried out by NADP-ME. NAD-ME and the chloroplastic isoform of NADP-ME are not shown. In some plants, malate is oxidized to OAA by MDH and decarboxylated by PEP carboxykinase (PEPCK), leading to direct production of PEP. This figure was partly adapted from Silvera et al. (2010) and produced using Biorender.

Figure 2

Discovery of CAM-plant transcriptome switches by single-cell genomics. Proposed process for analyzing expression changes during the transition from C₃ to CAM in facultative CAM species. A prolonged period without water triggers the induction of CAM in some plant species. Isolation of cells or nuclei from leaves at key points in the transition for single-cell RNA sequencing (1) and integration of the resulting datasets enable the visualization of distinct C₃ and CAM cell clusters based on the differential expression of CAM genes (3). Trajectory analyses of the transition in gene expression between cells performing C₃ and those performing CAM photosynthesis can be used to identify regulators of the induction process (4). Comparison of trajectories with those observed in datasets of closely related obligate C₃ and CAM species may help validate candidate regulators and identify additional ones (5). Figure produced using Biorender.