2 Carbon Concentrating Mechanisms: CAM and C4photosynthesis
Dry habitats are often also sunny, placing further specific restraints
on plant physiology and specifically photosynthesis: high light
intensity and high temperature. As plants acclimate to drought by
restricting their water loss through closing their stomata, they also
restrict CO2 uptake, reducing the available
CO2 for photosynthesis. High light becomes damaging if
the energy obtained from light absorbtion is not used by the
photosynthetic electron transport chain due to the absence of
CO2. Simultaneously, high temperature reduces the
solubility of gases so that RuBisCO activity shifts away from
carboxylation towards more oxygenation, increasing photorespiration and
leading to wasted energy (Edwards, 2019). To adapt to these constraints,
carbon concentrating mechanisms (CCMs) have evolved in numerous lineages
to enable efficient photosynthesis in dry, hot, and high light
environments while improving water use efficiency.
CCMs work by separating the initial carbon fixing away from the
photosynthetic carbon fixing (Calvin cycle) either temporally across the
diurnal cycle, or spatially across different cell types or compartments.
Both the temporal separation, as seen in Crassulacean Acid Metabolism
(CAM), and spatial separation, as seen in C4photosynthesis, have evolved independently in over 60 lineages (Edwards
& Ogburn, 2012). The evolutionary paths to both CAM and
C4 photosynthesis have been recently reviewed and
discussed in great detail (Bräutigam, Schlüter, Eisenhut, & Gowik,
2017; Chen, Xin, Wai, Liu, & Ming, 2020; Edwards, 2019; Heyduk,
Moreno-Villena, Gilman, Christin, & Edwards, 2019; Niklaus & Kelly,
2019; Schlüter & Weber, 2020; Sedelnikova, Hughes, & Langdale, 2018)
and in short, both require two main aspects: 1) an anatomical adaptation
and 2) co-option of the carbon concentrating metabolic pathway to the
correct spatiotemporal location. The main anatomical adaptation for CAM
is enlarged storage vacuole to store the malate synthesized during the
night and enable the day-night CCM (Luttge, 1987). In many
C4 plants, the spatial separation is across two cell
types, mesophyll and bundle sheath, and to achieve this,
C4 leaves adapt with so-called Kranz anatomy with
enlarged bundle sheath cells with increased plastid numbers and
increased vein density (Haberlandt, 1904). The regulation of Kranz
anatomy is proving to be a complex process (Sedelnikova et al., 2018),
and although it has readily evolved convergently in some plant clades,
it is starting to appear that not all plant clades are pre-conditioned
for the C4 photosynthesis to evolve (Edwards & Ogburn,
2012). Conversely to complex leaf anatomy and its regulation, all the
enzymes required for both the CAM and C4 metabolic
pathways, such as phosphoenolpyruvate (PEP) carboxylase and malate
dehydrogenase, are present in all plants serving other functions. To
co-opt these enzymes for CCMs, the expression and regulatory patterns
have evolved to be spatially and temporally specific (Brown et al.,
2011; Burgess et al., 2016; Christin et al., 2013; Dunning et al., 2019;
Gowik et al., 2004; Heyduk, Ray, et al., 2019; Kajala et al., 2012; Ming
et al., 2015; Rondeau et al., 2005; Schulze et al., 2013; Williams et
al., 2016; Yang et al., 2017).
Whole genome sequencing has enabled a level of understanding of how
these CCMs evolved in plants and contributed to their drought tolerance.
The first C4 (Sorghum bicolor ) (Paterson et al.,
2009) and CAM (Phalaenopsis equestris ) (Cai et al., 2015)
genomes provided insights about
redirection of genes involved in C3 photosynthesis and
expansion of ancient and recent gene families. Recent genome sequences
and transcriptomic approaches are also offering new evidence about
convergent evolution of genes and regulatory pathways underlying these
CCMs. For example, the genome and temporal transcriptome sequencing of
the CAM species Kalanchoë fedtschenkoi revealed that the
independent emergences of CAM from C3 have been based on
rewiring of diel gene expression patterns along with protein sequence
mutations (Yang et al., 2017). Furthermore, the pineapple genome
(Ananas comosus (L.) Merr.), another CAM species, indicated that
the transition from C3 to CAM was based on regulatory
neofunctionalization of preexisting genes and regulation of circadian
clock components through evolution of novel cis -regulatory
elements (Ming et al., 2015).
To resolve how gene expression patterns and regulatory networks have
evolved in convergent C4 lineages,
comparative leaf transcriptomics have been utilized, including
comparisons of C3, C4 and intermediate
C3-C4 leaves, developmental gradients,
specific cell types and environmental cues (Aubry, Kelly, Kümpers,
Smith-Unna, & Hibberd, 2014; Bräutigam et al., 2011; Burgess et al.,
2016; Gowik et al., 2004; Li et al., 2010). Transcriptomic comparison
across the monocot-dicot divide revealed deep evolutionary conservation
of C4 leaf development pathways and that certain
homologous cell type-specific regulators were co-opted during the
independent evolutions of C4 photosynthesis (Aubry et
al., 2014). The understanding of C4 enzymes’ regulatory
networks in ancestral C3 state was elucidated also by a
transcriptomics approach: comparison of how light and chloroplasts
regulate C4 enzymes in closely related
C3 and C4 plants. This linked the
C4 enzymes into a pre-existing C3regulatory network, explaining the readiness of C4 to
evolve at the molecular level (Burgess et al., 2016).
Counter-intuitively, C4 and CAM can exist in the same
leaf. Earlier this year, a transcriptomic approach was taken to dissect
the behavior of both C4 and drought-induced CAM in the
same plant, Portulaca oleracea , offering insight how the
regulatory networks of shared enzymes might be able to coexist while
responding to different environmental and temporal cues (Ferrari et al.,
2020). With more C4 and CAM genomes and transcriptomes
becoming recently available (at least six C4 and three
CAM genomes at the moment - Phytozome v.12.1, Goodstein et al. (2012)),
more information about the basis of convergent evolution of CCMs and
other parallel drought adaptations in these species will become possible
in future research.