The starch-deficient plastidic PHOSPHOGLUCOMUTASE mutant of the constitutive crassulacean acid metabolism (CAM) species Kalanchoë fedtschenkoi impacts diel regulation and timing of stomatal CO2 responsiveness

BACKGROUND AND AIMS

Crassulacean acid metabolism (CAM) is a specialized type of photosynthesis characterized by a diel pattern of stomatal opening at night and closure during the day, which increases water-use efficiency. Starch degradation is a key regulator of CAM, providing phosphoenolpyruvate as a substrate in the mesophyll for nocturnal assimilation of CO2. Growing recognition of a key role for starch degradation in C3 photosynthesis guard cells for mediating daytime stomatal opening presents the possibility that starch degradation might also impact CAM by regulating the provision of energy and osmolytes to increase guard cell turgor and drive stomatal opening at night. In this study, we tested the hypothesis that the timing of diel starch turnover in CAM guard cells has been reprogrammed during evolution to enable nocturnal stomatal opening and daytime closure.

METHODS

Biochemical and genetic characterization of wild-type and starch-deficient RNAi lines of Kalanchoë fedtschenkoi with reduced activity of plastidic phosphoglucomutase (PGM) constituted a preliminary approach for the understanding of starch metabolism and its implications for stomatal regulation in CAM plants.

KEY RESULTS

Starch deficiency reduced nocturnal net CO2 uptake but had negligible impact on nocturnal stomatal opening. In contrast, daytime stomatal closure was reduced in magnitude and duration in the starch-deficient rPGM RNAi lines, and their stomata were unable to remain closed in response to elevated concentrations of atmospheric CO2 administered during the day. Curtailed daytime stomatal closure was linked to higher soluble sugar contents in the epidermis and mesophyll.

CONCLUSIONS

Nocturnal stomatal opening is not reliant upon starch degradation, but starch biosynthesis is an important sink for carbohydrates, ensuring daytime stomatal closure in this CAM species.

Phosphorolytic degradation of leaf starch via plastidic α-glucan phosphorylase leads to optimized plant growth and water use efficiency over the diel phases of Crassulacean acid metabolism

In plants with Crassulacean acid metabolism (CAM), it has been proposed that the requirement for nocturnal provision of phosphoenolpyruvate as a substrate for CO2 uptake has resulted in a re-routing of chloroplastic starch degradation from the amylolytic route to the phosphorolytic route. To test this hypothesis, we generated and characterized four independent RNAi lines of the obligate CAM species Kalanchoë fedtschenkoi with a >10-fold reduction in transcript abundance of plastidic α-glucan phosphorylase (PHS1). The rPHS1 lines showed diminished nocturnal starch degradation, reduced dark CO2 uptake, a reduction in diel water use efficiency (WUE), and an overall reduction in growth. A re-routing of starch degradation via the hydrolytic/amylolytic pathway was indicated by hyperaccumulation of maltose in all rPHS1 lines. Further examination indicated that whilst operation of the core circadian clock was not compromised, plasticity in modulating net dark CO2 uptake in response to changing photoperiods was curtailed. The data show that phosphorolytic starch degradation is critical for efficient operation of the CAM cycle and for optimizing WUE. This finding has clear relevance for ongoing efforts to engineer CAM into non-CAM species as a means of boosting crop WUE for a warmer, drier future.

Kalanchoë PPC1 Is Essential for Crassulacean Acid Metabolism and the Regulation of Core Circadian Clock and Guard Cell Signaling Genes

Unlike C3 plants, Crassulacean acid metabolism (CAM) plants fix CO2 in the dark using phosphoenolpyruvate carboxylase (PPC; EC 4.1.1.31). PPC combines phosphoenolpyruvate with CO2 (as HCO3 -), forming oxaloacetate. The oxaloacetate is converted to malate, leading to malic acid accumulation in the vacuole, which peaks at dawn. During the light period, malate decarboxylation concentrates CO2 around Rubisco for secondary fixation. CAM mutants lacking PPC have not been described. Here, we employed RNA interference to silence the CAM isogene PPC1 in Kalanchoë laxiflora Line rPPC1-B lacked PPC1 transcripts, PPC activity, dark period CO2 fixation, and nocturnal malate accumulation. Light period stomatal closure was also perturbed, and the plants displayed reduced but detectable dark period stomatal conductance and arrhythmia of the CAM CO2 fixation circadian rhythm under constant light and temperature free-running conditions. By contrast, the rhythm of delayed fluorescence was enhanced in plants lacking PPC1 Furthermore, a subset of gene transcripts within the central circadian oscillator was upregulated and oscillated robustly in this line. The regulation of guard cell genes involved in controlling stomatal movements was also perturbed in rPPC1-B These findings provide direct evidence that the regulatory patterns of key guard cell signaling genes are linked with the characteristic inverse pattern of stomatal opening and closing during CAM.

C4 and crassulacean acid metabolism within a single leaf: deciphering key components behind a rare photosynthetic adaptation

Although biochemically related, C₄ and crassulacean acid metabolism (CAM) systems are expected to be incompatible. However, Portulaca species, including P. oleracea, operate C₄ and CAM within a single leaf, and the mechanisms behind this unique photosynthetic arrangement remain largely unknown. Here, we employed RNA-seq to identify candidate genes involved exclusively or shared by C₄ or CAM, and provided an in-depth characterization of their transcript abundance patterns during the drought-induced photosynthetic transitions in P. oleracea. Data revealed fewer candidate CAM-specific genes than those recruited to function in C₄ . The putative CAM-specific genes were predominantly involved in night-time primary carboxylation reactions and malate movement across the tonoplast. Analysis of gene transcript-abundance regulation and photosynthetic physiology indicated that C₄ and CAM coexist within a single P. oleracea leaf under mild drought conditions. Developmental and environmental cues were shown to regulate CAM expression in stems, whereas the shift from C₄ to C₄ -CAM hybrid photosynthesis in leaves was strictly under environmental control. Moreover, efficient starch turnover was identified as part of the metabolic adjustments required for CAM operation in both organs. These findings provide insights into C₄ /CAM connectivity and compatibility, contributing to a deeper understanding of alternative ways to engineer CAM into C₄ crop species.

Undervalued potential of crassulacean acid metabolism for current and future agricultural production

The potential for crassulacean acid metabolism (CAM) to support resilient crops that meet demands for food, fiber, fuel, and pharmaceutical products far exceeds current production levels. This review provides background on five families of plants that express CAM, including examples of many species within these families that have potential agricultural uses. We summarize traditional uses, current developments, management practices, environmental tolerance ranges, and economic values of CAM species with potential commercial applications. The primary benefit of CAM in agriculture is high water use efficiency that allows for reliable crop yields even in drought conditions. Agave species, for example, grow in arid conditions and have been exploited for agricultural products in North and South America for centuries. Yet, there has been very little investment in agricultural improvement for most useful Agave varieties. Other CAM species that are already traded globally include Ananas comosus (pineapple), Aloe spp., Vanilla spp., and Opuntia spp., but there are far more with agronomic uses that are less well known and not yet developed commercially. Recent advances in technology and genomic resources provide tools to understand and realize the tremendous potential for using CAM crops to produce climate-resilient agricultural commodities in the future.

The Kalanchoë genome provides insights into convergent evolution and building blocks of crassulacean acid metabolism

Crassulacean acid metabolism (CAM) is a water-use efficient adaptation of photosynthesis that has evolved independently many times in diverse lineages of flowering plants. We hypothesize that convergent evolution of protein sequence and temporal gene expression underpins the independent emergences of CAM from C3 photosynthesis. To test this hypothesis, we generate a de novo genome assembly and genome-wide transcript expression data for Kalanchoë fedtschenkoi, an obligate CAM species within the core eudicots with a relatively small genome (~260 Mb). Our comparative analyses identify signatures of convergence in protein sequence and re-scheduling of diel transcript expression of genes involved in nocturnal CO2 fixation, stomatal movement, heat tolerance, circadian clock, and carbohydrate metabolism in K. fedtschenkoi and other CAM species in comparison with non-CAM species. These findings provide new insights into molecular convergence and building blocks of CAM and will facilitate CAM-into-C3 photosynthesis engineering to enhance water-use efficiency in crops.

Phosphorylation of Phosphoenolpyruvate Carboxylase Is Essential for Maximal and Sustained Dark CO2 Fixation and Core Circadian Clock Operation in the Obligate Crassulacean Acid Metabolism Species Kalanchoë fedtschenkoi

Phosphoenolpyruvate carboxylase (PPC; EC 4.1.1.31) catalyzes primary nocturnal CO2 fixation in Crassulacean acid metabolism (CAM) species. CAM PPC is regulated posttranslationally by a circadian clock-controlled protein kinase called phosphoenolpyruvate carboxylase kinase (PPCK). PPCK phosphorylates PPC during the dark period, reducing its sensitivity to feedback inhibition by malate and thus enhancing nocturnal CO2 fixation to stored malate. Here, we report the generation and characterization of transgenic RNAi lines of the obligate CAM species Kalanchoë fedtschenkoi with reduced levels of KfPPCK1 transcripts. Plants with reduced or no detectable dark phosphorylation of PPC displayed up to a 66% reduction in total dark period CO2 fixation. These perturbations paralleled reduced malate accumulation at dawn and decreased nocturnal starch turnover. Loss of oscillations in the transcript abundance of KfPPCK1 was accompanied by a loss of oscillations in the transcript abundance of many core circadian clock genes, suggesting that perturbing the only known link between CAM and the circadian clock feeds back to perturb the central circadian clock itself. This work shows that clock control of KfPPCK1 prolongs the activity of PPC throughout the dark period in K. fedtschenkoi, optimizing CAM-associated dark CO2 fixation, malate accumulation, CAM productivity, and core circadian clock robustness.

Emerging model systems for functional genomics analysis of Crassulacean acid metabolism

Crassulacean acid metabolism (CAM) is one of three main pathways of photosynthetic carbon dioxide fixation found in higher plants. It stands out for its ability to underpin dramatic improvements in plant water use efficiency, which in turn has led to a recent renaissance in CAM research. The current ease with which candidate CAM-associated genes and proteins can be identified through high-throughput sequencing has opened up a new horizon for the development of diverse model CAM species that are amenable to genetic manipulations. The adoption of these model CAM species is underpinning rapid advances in our understanding of the complete gene set for CAM. We highlight recent breakthroughs in the functional characterisation of CAM genes that have been achieved through transgenic approaches.

A roadmap for research on crassulacean acid metabolism (CAM) to enhance sustainable food and bioenergy production in a hotter, drier world

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that features nocturnal CO2 uptake, facilitates increased water-use efficiency (WUE), and enables CAM plants to inhabit water-limited environments such as semi-arid deserts or seasonally dry forests. Human population growth and global climate change now present challenges for agricultural production systems to increase food, feed, forage, fiber, and fuel production. One approach to meet these challenges is to increase reliance on CAM crops, such as Agave and Opuntia, for biomass production on semi-arid, abandoned, marginal, or degraded agricultural lands. Major research efforts are now underway to assess the productivity of CAM crop species and to harness the WUE of CAM by engineering this pathway into existing food, feed, and bioenergy crops. An improved understanding of CAM has potential for high returns on research investment. To exploit the potential of CAM crops and CAM bioengineering, it will be necessary to elucidate the evolution, genomic features, and regulatory mechanisms of CAM. Field trials and predictive models will be required to assess the productivity of CAM crops, while new synthetic biology approaches need to be developed for CAM engineering. Infrastructure will be needed for CAM model systems, field trials, mutant collections, and data management.

Transgenic perturbation of the decarboxylation phase of Crassulacean acid metabolism alters physiology and metabolism but has only a small effect on growth

Mitochondrial NAD-malic enzyme (ME) and/or cytosolic/plastidic NADP-ME combined with the cytosolic/plastidic pyruvate orthophosphate dikinase (PPDK) catalyze two key steps during light-period malate decarboxylation that underpin secondary CO(2) fixation in some Crassulacean acid metabolism (CAM) species. We report the generation and phenotypic characterization of transgenic RNA interference lines of the obligate CAM species Kalanchoë fedtschenkoi with reduced activities of NAD-ME or PPDK. Transgenic line rNAD-ME1 had 8%, and rPPDK1 had 5% of the wild-type level of activity, and showed dramatic changes in the light/dark cycle of CAM CO(2) fixation. In well-watered conditions, these lines fixed all of their CO(2) in the light; they thus performed C(3) photosynthesis. The alternative malate decarboxylase, NADP-ME, did not appear to compensate for the reduction in NAD-ME, suggesting that NAD-ME was the key decarboxylase for CAM. The activity of other CAM enzymes was reduced as a consequence of knocking out either NAD-ME or PPDK activity, particularly phosphoenolpyruvate carboxylase (PPC) and PPDK in rNAD-ME1. Furthermore, the circadian clock-controlled phosphorylation of PPC in the dark was reduced in both lines, especially in rNAD-ME1. This had the consequence that circadian rhythms of PPC phosphorylation, PPC kinase transcript levels and activity, and the classic circadian rhythm of CAM CO(2) fixation were lost, or dampened toward arrhythmia, under constant light and temperature conditions. Surprisingly, oscillations in the transcript abundance of core circadian clock genes also became arrhythmic in the rNAD-ME1 line, suggesting that perturbing CAM in K. fedtschenkoi feeds back to perturb the central circadian clock.

Characterisation of seeds of a C4 phosphoenolpyruvate carboxylase-deficient mutant of Amaranthus edulis

Seeds from the C(4) plant Amaranthus edulis were studied as part of the characterisation of a mutant (designated LaC(4) 2.16), which contains reduced amounts (5% of wild type) of the photosynthetic leaf form of phosphoenolpyruvate carboxylase (PEPC). On a per seed basis, the amount of PEPC activity was not significantly altered, while the weight and protein content of the mutant seeds were 34% lower than that of the wild type. Western gel blot analysis detected two PEPC polypeptides with molecular masses of 105 kDa (minor) and 100 kDa (major). The determination of in vitro phosphorylation in reconstituted assays revealed the presence of both calcium-dependent and calcium-independent PEPC-kinase activities in protein extracts of wild-type and mutant seeds. However, PEPC proteins were phosphorylated in dry seeds, and PEPC phosphorylation did not occur in vivo during seed imbibition in the presence of (32) P-phosphate. In contrast, (32) P-phosphate was incorporated into a range of proteins in wild-type seeds, but not in mutant seeds. In addition, ATP content was much reduced in germinating mutant seeds and this did not increase following the supply of phosphate. Collectively, these data suggest that the deficiency in C(4) PEPC in mutant A. edulis leaves has no effect on C(3) -type PEPC content and phosphorylation state in seeds, but causes impairment of energy production, thereby accounting for the reduced germination of the mutant.

Control of C4 photosynthesis: effects of reduced activities of phosphoenolpyruvate carboxylase on CO2 assimilation in Amaranthus edulis L

Heterozygous mutants of Amaranthus edulis deficient in PEP carboxylase (PEPC) have been used to study the control of photosynthetic carbon assimilation. A reduction in PEPC activity led to a decrease in the initial slope of the relationship between the CO2 assimilation rate and the intercellular CO2 concentration and to a decrease in photosynthesis at high light intensities, consistent with a decrease in the capacity of the C4 cycle in high light. PEPC exerted appreciable control on photosynthetic flux in the wild-type, with a relatively high flux control coefficient of 0.35 in saturating light and ambient CO2. The flux control coefficient was decreased in low light or increased in low CO2 or in plants containing lower PEPC activity. However, the rate of CO2 assimilation decreased down to about 55% PEPC, followed by an up-turn in the light-saturated photosynthetic rate as PEPC was further reduced, suggesting the existence of a mechanism that compensates for the loss of PEPC activity. The amounts of photosynthetic metabolites, including glycine and serine, also showed a biphasic response to decreasing PEPC. There was a linear relationship between the activity of PEPC and the activation state of the enzyme. A possible mechanism of compensation involving photorespiratory intermediates is discussed.

A splice site mutation gives rise to a mutant of the C4 plant Amaranthus edulis deficient in phosphoenolpyruvate carboxylase activity

The molecular nature of a mutant of the C4 plant Amaranthus edulis that has been shown to contain only 5% of the normal activity and protein of phosphoenolpyruvate carboxylase (PEPC) (Dever et al., 1995) has been investigated. Using Northern blot analysis, it has been shown here that the PEPC transcripts are produced in the mutant. In-vitro translation of these transcripts generated two products immunoprecipitable by a PEPC N-terminus-specific antibody. One of these products has the size of the complete PEPC polypeptide, the other is 9kDa smaller and was not revealed when using a PEPC C-terminus-specific antibody. In the mutant plant, using the same N- and C-terminus-specific antibodies, only the larger polypeptide was immunodetected, whilst at a very low level. A sequence analysis of the suspected faulty region of the mRNA revealed incorrect splicing of the last intron of the PEPC pre-mRNA. Two mis-splicings have been identified, both occurring after an AG site, one leading to a protein lacking five amino acids, the other to a truncated protein due to a stop codon generated by a frame shift in the translation. Finally, the sequencing of the boundary between the last intron and exon showed that these inaccurate splicings result from a mutation in the genuine canonical 3'AG splicing site.