Guard cell photosynthesis is critical for stomatal turgor production, yet does not directly mediate CO2 - and ABA-induced stomatal closing

The microRNA408-plantacyanin module balances plant growth and drought resistance by regulating reactive oxygen species homeostasis in guard cells

The conserved microRNA (miRNA) miR408 enhances photosynthesis and compromises stress tolerance in multiple plants, but the cellular mechanism underlying its function remains largely unclear. Here, we show that in Arabidopsis (Arabidopsis thaliana), the transcript encoding the blue copper protein PLANTACYANIN (PCY) is the primary target for miR408 in vegetative tissues. PCY is preferentially expressed in the guard cells, and PCY is associated with the endomembrane surrounding individual chloroplasts. We found that the MIR408 promoter is suppressed by multiple abscisic acid (ABA)-responsive transcription factors, thus allowing PCY to accumulate under stress conditions. Genetic analysis revealed that PCY elevates reactive oxygen species (ROS) levels in the guard cells, promotes stomatal closure, reduces photosynthetic gas exchange, and enhances drought resistance. Moreover, the miR408-PCY module is sufficient to rescue the growth and drought tolerance phenotypes caused by gain- and loss-of-function of MYB44, an established positive regulator of ABA responses, indicating that the miR408-PCY module relays ABA signaling for regulating ROS homeostasis and drought resistance. These results demonstrate that miR408 regulates stomatal movement to balance growth and drought resistance, providing a mechanistic understanding of why miR408 is selected during land plant evolution and insights into the long-pursued quest of breeding drought-tolerant and high-yielding crops.

Stomatal CO2 responses at sub- and above-ambient CO2 levels employ different pathways in Arabidopsis

Stomatal pores that control plant CO2 uptake and water loss affect global carbon and water cycles. In the era of increasing atmospheric CO2 levels and vapor pressure deficit (VPD), it is essential to understand how these stimuli affect stomatal behavior. Whether stomatal responses to sub-ambient and above-ambient CO2 levels are governed by the same regulators and depend on VPD remains unknown. We studied stomatal conductance responses in Arabidopsis (Arabidopsis thaliana) stomatal signaling mutants under conditions where CO2 levels were either increased from sub-ambient to ambient (400 ppm) or from ambient to above-ambient levels under normal or elevated VPD. We found that guard cell signaling components involved in CO2-induced stomatal closure have different roles in the sub-ambient and above-ambient CO2 levels. The CO2-specific regulators prominently affected sub-ambient CO2 responses, whereas the lack of guard cell slow-type anion channel SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1) more strongly affected the speed of above-ambient CO2-induced stomatal closure. Elevated VPD caused lower stomatal conductance in all studied genotypes and CO2 transitions, as well as faster CO2-responsiveness in some studied genotypes and CO2 transitions. Our results highlight the importance of experimental setups in interpreting stomatal CO2-responsiveness, as stomatal movements under different CO2 concentration ranges are controlled by distinct mechanisms. Elevated CO2 and VPD responses may also interact. Hence, multi-factor treatments are needed to understand how plants integrate different environmental signals and translate them into stomatal responses.

Calvin cycle and guard cell metabolism impact stomatal function

Stomatal conductance (gs) determines CO2 uptake for photosynthesis (A) and water loss through transpiration, which is essential for evaporative cooling and maintenance of optimal leaf temperature as well as nutrient uptake. Stomata adjust their aperture to maintain an appropriate balance between CO2 uptake and water loss and are therefore critical to overall plant water status and productivity. Although there is considerable knowledge regarding guard cell (GC) osmoregulation (which drives differences in GC volume and therefore stomatal opening and closing), as well as the various signal transduction pathways that enable GCs to sense and respond to different environmental stimuli, little is known about the signals that coordinate mesophyll demands for CO2. Furthermore, chloroplasts are a key feature in GCs of many species, however, their role in stomatal function is unclear and a subject of debate. In this review we explore the current evidence regarding the role of these organelles in stomatal behaviour, including GC electron transport and Calvin-Benson-Bassham (CBB) cycle activity as well as their possible involvement correlating gs and A along with other potential mesophyll signals. We also examine the roles of other GC metabolic processes in stomatal function.

Induction of stomatal opening following a night-chilling event alleviates physiological damage in mango trees

Chilling events have become more frequent with climate change and are a significant abiotic factor causing physiological damage to plants and, consequently, reducing crop yield. Like other tropical and subtropical plants, mango (Mangifera indica L.) is particularly sensitive to chilling events, especially if they are followed by bright sunny days. It was previously shown that in mango leaves stomatal opening is restricted in the morning following a night-chilling event. This impairment results in restraint of carbon assimilation and subsequently, photoinhibition and reactive oxygen species production, which leads to chlorosis and in severe cases, cell death. Our detailed physiological analysis showed that foliar application of the guard cell H+-ATPase activator, fusicoccin, in the morning after a cold night, mitigates the physiological damage from 'cold night-bright day' abiotic stress. This application restored stomatal opening, thereby enabling gas exchange, releasing the photosynthetic machinery from harmful excess photon energy, and improving the plant's overall physiological state. The mechanisms by which plants react to this abiotic stress are examined in this work. The foliar application of compounds that cause stomatal opening as a potential method of minimizing physiological damage due to night chilling is discussed.

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.

Diel dynamics of multi-omics in elkhorn fern provide new insights into weak CAM photosynthesis

Crassulacean acid metabolism (CAM) has high water-use efficiency (WUE) and is widely recognized to have evolved from C3 photosynthesis. Different plant lineages have convergently evolved CAM, but the molecular mechanism that underlies C3-to-CAM evolution remains to be clarified. Platycerium bifurcatum (elkhorn fern) provides an opportunity to study the molecular changes underlying the transition from C3 to CAM photosynthesis because both modes of photosynthesis occur in this species, with sporotrophophyll leaves (SLs) and cover leaves (CLs) performing C3 and weak CAM photosynthesis, respectively. Here, we report that the physiological and biochemical attributes of CAM in weak CAM-performing CLs differed from those in strong CAM species. We investigated the diel dynamics of the metabolome, proteome, and transcriptome in these dimorphic leaves within the same genetic background and under identical environmental conditions. We found that multi-omic diel dynamics in P. bifurcatum exhibit both tissue and diel effects. Our analysis revealed temporal rewiring of biochemistry relevant to the energy-producing pathway (TCA cycle), CAM pathway, and stomatal movement in CLs compared with SLs. We also confirmed that PHOSPHOENOLPYRUVATE CARBOXYLASE KINASE (PPCK) exhibits convergence in gene expression among highly divergent CAM lineages. Gene regulatory network analysis identified candidate transcription factors regulating the CAM pathway and stomatal movement. Taken together, our results provide new insights into weak CAM photosynthesis and new avenues for CAM bioengineering.

Leaf starch metabolism sets the phase of stomatal rhythm

In leaves of C3 and C4 plants, stomata open during the day to favor CO2 entry for photosynthesis and close at night to prevent inefficient transpiration of water vapor. The circadian clock paces rhythmic stomatal movements throughout the diel (24-h) cycle. Leaf transitory starch is also thought to regulate the diel stomatal movements, yet the underlying mechanisms across time (key moments) and space (relevant leaf tissues) remain elusive. Here, we developed PhenoLeaks, a pipeline to analyze the diel dynamics of transpiration, and used it to screen a series of Arabidopsis (Arabidopsis thaliana) mutants impaired in starch metabolism. We detected a sinusoidal, endogenous rhythm of transpiration that overarches days and nights. We determined that a number of severe mutations in starch metabolism affect the endogenous rhythm through a phase shift, resulting in delayed stomatal movements throughout the daytime and diminished stomatal preopening during the night. Nevertheless, analysis of tissue-specific mutations revealed that neither guard-cell nor mesophyll-cell starch metabolisms are strictly required for normal diel patterns of transpiration. We propose that leaf starch influences the timing of transpiration rhythm through an interplay between the circadian clock and sugars across tissues, while the energetic effect of starch-derived sugars is usually nonlimiting for endogenous stomatal movements.

Hydrogen peroxide sensor HyPer7 illuminates tissue-specific plastid redox dynamics

The visualization of photosynthesis-derived reactive oxygen species has been experimentally limited to pH-sensitive probes, unspecific redox dyes, and whole-plant phenotyping. Recent emergence of probes that circumvent these limitations permits advanced experimental approaches to investigate in situ plastid redox properties. Despite growing evidence of heterogeneity in photosynthetic plastids, investigations have not addressed the potential for spatial variation in redox and/or reactive oxygen dynamics. To study the dynamics of H2O2 in distinct plastid types, we targeted the pH-insensitive, highly specific probe HyPer7 to the plastid stroma in Arabidopsis (Arabidopsis thaliana). Using HyPer7 and glutathione redox potential (EGSH) probe for redox-active green fluorescent protein 2 genetically fused to the redox enzyme human glutaredoxin-1 with live cell imaging and optical dissection of cell types, we report heterogeneities in H2O2 accumulation and redox buffering within distinct epidermal plastids in response to excess light and hormone application. Our observations suggest that plastid types can be differentiated by their physiological redox features. These data underscore the variation in photosynthetic plastid redox dynamics and demonstrate the need for cell-type-specific observations in future plastid phenotyping.

Genetic dissection of maize (Zea mays L.) chlorophyll content using multi-locus genome-wide association studies

BACKGROUND

The chlorophyll content (CC) is a key factor affecting maize photosynthetic efficiency and the final yield. However, its genetic basis remains unclear. The development of statistical methods has enabled researchers to design and apply various GWAS models, including MLM, MLMM, SUPER, FarmCPU, BLINK and 3VmrMLM. Comparative analysis of their results can lead to more effective mining of key genes.

RESULTS

The heritability of CC was 0.86. Six statistical models (MLM, BLINK, MLMM, FarmCPU, SUPER, and 3VmrMLM) and 1.25 million SNPs were used for the GWAS. A total of 140 quantitative trait nucleotides (QTNs) were detected, with 3VmrMLM and MLM detecting the most (118) and fewest (3) QTNs, respectively. The QTNs were associated with 481 genes and explained 0.29-10.28% of the phenotypic variation. Additionally, 10 co-located QTNs were detected by at least two different models or methods, three co-located QTNs were identified in at least two different environments, and six co-located QTNs were detected by different models or methods in different environments. Moreover, 69 candidate genes within or near these stable QTNs were screened based on the B73 (RefGen_v2) genome. GRMZM2G110408 (ZmCCS3) was identified by multiple models and in multiple environments. The functional characterization of this gene indicated the encoded protein likely contributes to chlorophyll biosynthesis. In addition, the CC differed significantly between the haplotypes of the significant QTN in this gene, and CC was higher for haplotype 1.

CONCLUSION

This study's results broaden our understanding of the genetic basis of CC, mining key genes related to CC and may be relevant for the ideotype-based breeding of new maize varieties with high photosynthetic efficiency.

Abscisic acid increases hydrogen peroxide in mitochondria to facilitate stomatal closure

Abscisic acid (ABA) drives stomatal closure to minimize water loss due to transpiration in response to drought. We examined the subcellular location of ABA-increased accumulation of reactive oxygen species (ROS) in guard cells, which drive stomatal closure, in Arabidopsis (Arabidopsis thaliana). ABA-dependent increases in fluorescence of the generic ROS sensor, dichlorofluorescein (DCF), were observed in mitochondria, chloroplasts, cytosol, and nuclei. The ABA response in all these locations was lost in an ABA-insensitive quintuple receptor mutant. The ABA-increased fluorescence in mitochondria of both DCF- and an H2O2-selective probe, Peroxy Orange 1, colocalized with Mitotracker Red. ABA treatment of guard cells transformed with the genetically encoded H2O2 reporter targeted to the cytoplasm (roGFP2-Orp1), or mitochondria (mt-roGFP2-Orp1), revealed H2O2 increases. Consistent with mitochondrial ROS changes functioning in stomatal closure, we found that guard cells of a mutant with mitochondrial defects, ABA overly sensitive 6 (abo6), have elevated ABA-induced ROS in mitochondria and enhanced stomatal closure. These effects were phenocopied with rotenone, which increased mitochondrial ROS. In contrast, the mitochondrially targeted antioxidant, MitoQ, dampened ABA effects on mitochondrial ROS accumulation and stomatal closure in Col-0 and reversed the guard cell closure phenotype of the abo6 mutant. ABA-induced ROS accumulation in guard cell mitochondria was lost in mutants in genes encoding respiratory burst oxidase homolog (RBOH) enzymes and reduced by treatment with the RBOH inhibitor, VAS2870, consistent with RBOH machinery acting in ABA-increased ROS in guard cell mitochondria. These results demonstrate that ABA elevates H2O2 accumulation in guard cell mitochondria to promote stomatal closure.

Genome-Wide Association Study Identified Novel SNPs Associated with Chlorophyll Content in Maize

Chlorophyll is an essential component that captures light energy to drive photosynthesis. Chlorophyll content can affect photosynthetic activity and thus yield. Therefore, mining candidate genes of chlorophyll content will help increase maize production. Here, we performed a genome-wide association study (GWAS) on chlorophyll content and its dynamic changes in 378 maize inbred lines with extensive natural variation. Our phenotypic assessment showed that chlorophyll content and its dynamic changes were natural variations with a moderate genetic level of 0.66/0.67. A total of 19 single-nucleotide polymorphisms (SNPs) were found associated with 76 candidate genes, of which one SNP, 2376873-7-G, co-localized in chlorophyll content and area under the chlorophyll content curve (AUCCC). Zm00001d026568 and Zm00001d026569 were highly associated with SNP 2376873-7-G and encoded pentatricopeptide repeat-containing protein and chloroplastic palmitoyl-acyl carrier protein thioesterase, respectively. As expected, higher expression levels of these two genes are associated with higher chlorophyll contents. These results provide a certain experimental basis for discovering the candidate genes of chlorophyll content and finally provide new insights for cultivating high-yield and excellent maize suitable for planting environment.

Arabidopsis guard cell chloroplasts import cytosolic ATP for starch turnover and stomatal opening

Stomatal opening requires the provision of energy in the form of ATP for proton pumping across the guard cell (GC) plasma membrane and for associated metabolic rearrangements. The source of ATP for GCs is a matter of ongoing debate that is mainly fuelled by controversies around the ability of GC chloroplasts (GCCs) to perform photosynthesis. By imaging compartment-specific fluorescent ATP and NADPH sensor proteins in Arabidopsis, we show that GC photosynthesis is limited and mitochondria are the main source of ATP. Unlike mature mesophyll cell (MC) chloroplasts, which are impermeable to cytosolic ATP, GCCs import cytosolic ATP through NUCLEOTIDE TRANSPORTER (NTT) proteins. GCs from ntt mutants exhibit impaired abilities for starch biosynthesis and stomatal opening. Our work shows that GCs obtain ATP and carbohydrates via different routes from MCs, likely to compensate for the lower chlorophyll contents and limited photosynthesis of GCCs.

Stomatal guard cells require ATP in order to fuel stomatal movements. Here the authors show that guard cell photosynthesis is limited, mitochondria are the main source of ATP and that guard cell chloroplasts import ATP via nucleotide transporters.

The Potential of the Synthetic Strigolactone Analogue GR24 for the Maintenance of Photosynthesis and Yield in Winter Wheat under Drought: Investigations on the Mechanisms of Action and Delivery Modes

Strigolactones (SLs) have been implicated in many plant biological and physiological processes, including the responses to abiotic stresses such as drought, in concert with other phytohormones. While it is now clear that exogenous SLs may help plants to survive in harsh environmental condition, the best, most effective protocols for treatment have not been defined yet, and the mechanisms of action are far from being fully understood. In the set of experiments reported here, we contrasted two application methods for treatment with a synthetic analog of SL, GR24. A number of morphometric, physiological and biochemical parameters were measured following foliar application of GR24 or application in the residual irrigation water in winter wheat plants under irrigated and drought stress conditions. Depending on the concentration and the method of GR24 application, differentiated photosynthesis and transpiration rate, stomatal conductance, leaf water potential, antioxidant enzyme activities and yield in drought conditions were observed. We present evidence that different methods of GR24 application led to increased photosynthesis and yield under stress by a combination of drought tolerance and escape factors, which should be considered for future research exploring the potential of this new family of bioactive molecules for practical applications.

A role for calcium-dependent protein kinases in differential CO2 - and ABA-controlled stomatal closing and low CO2 -induced stomatal opening in Arabidopsis

Low concentrations of CO2 cause stomatal opening, whereas [CO2 ] elevation leads to stomatal closure. Classical studies have suggested a role for Ca2+ and protein phosphorylation in CO2 -induced stomatal closing. Calcium-dependent protein kinases (CPKs) and calcineurin-B-like proteins (CBLs) can sense and translate cytosolic elevation of the second messenger Ca2+ into specific phosphorylation events. However, Ca2+ -binding proteins that function in the stomatal CO2 response remain unknown. Time-resolved stomatal conductance measurements using intact plants, and guard cell patch-clamp experiments were performed. We isolated cpk quintuple mutants and analyzed stomatal movements in response to CO2 , light and abscisic acid (ABA). Interestingly, we found that cpk3/5/6/11/23 quintuple mutant plants, but not other analyzed cpk quadruple/quintuple mutants, were defective in high CO2 -induced stomatal closure and, unexpectedly, also in low CO2 -induced stomatal opening. Furthermore, K+ -uptake-channel activities were reduced in cpk3/5/6/11/23 quintuple mutants, in correlation with the stomatal opening phenotype. However, light-mediated stomatal opening remained unaffected, and ABA responses showed slowing in some experiments. By contrast, CO2 -regulated stomatal movement kinetics were not clearly affected in plasma membrane-targeted cbl1/4/5/8/9 quintuple mutant plants. Our findings describe combinatorial cpk mutants that function in CO2 control of stomatal movements and support the results of classical studies showing a role for Ca2+ in this response.

SAUR proteins and PP2C.D phosphatases regulate H+-ATPases and K+ channels to control stomatal movements

Activation of plasma membrane (PM) H+-ATPase activity is crucial in guard cells to promote light-stimulated stomatal opening, and in growing organs to promote cell expansion. In growing organs, SMALL AUXIN UP RNA (SAUR) proteins inhibit the PP2C.D2, PP2C.D5, and PP2C.D6 (PP2C.D2/5/6) phosphatases, thereby preventing dephosphorylation of the penultimate phosphothreonine of PM H+-ATPases and trapping them in the activated state to promote cell expansion. To elucidate whether SAUR-PP2C.D regulatory modules also affect reversible cell expansion, we examined stomatal apertures and conductances of Arabidopsis thaliana plants with altered SAUR or PP2C.D activity. Here, we report that the pp2c.d2/5/6 triple knockout mutant plants and plant lines overexpressing SAUR fusion proteins exhibit enhanced stomatal apertures and conductances. Reciprocally, saur56 saur60 double mutants, lacking two SAUR genes normally expressed in guard cells, displayed reduced apertures and conductances, as did plants overexpressing PP2C.D5. Although altered PM H+-ATPase activity contributes to these stomatal phenotypes, voltage clamp analysis showed significant changes also in K+ channel gating in lines with altered SAUR and PP2C.D function. Together, our findings demonstrate that SAUR and PP2C.D proteins act antagonistically to facilitate stomatal movements through a concerted targeting of both ATP-dependent H+ pumping and channel-mediated K+ transport.

Salinity Effects on Guard Cell Proteome in Chenopodium quinoa

Epidermal fragments enriched in guard cells (GCs) were isolated from the halophyte quinoa (Chenopodium quinoa Wild.) species, and the response at the proteome level was studied after salinity treatment of 300 mM NaCl for 3 weeks. In total, 2147 proteins were identified, of which 36% were differentially expressed in response to salinity stress in GCs. Up and downregulated proteins included signaling molecules, enzyme modulators, transcription factors and oxidoreductases. The most abundant proteins induced by salt treatment were desiccation-responsive protein 29B (50-fold), osmotin-like protein OSML13 (13-fold), polycystin-1, lipoxygenase, alpha-toxin, and triacylglycerol lipase (PLAT) domain-containing protein 3-like (eight-fold), and dehydrin early responsive to dehydration (ERD14) (eight-fold). Ten proteins related to the gene ontology term “response to ABA” were upregulated in quinoa GC; this included aspartic protease, phospholipase D and plastid-lipid-associated protein. Additionally, seven proteins in the sucrose–starch pathway were upregulated in the GC in response to salinity stress, and accumulation of tryptophan synthase and L-methionine synthase (enzymes involved in the amino acid biosynthesis) was observed. Exogenous application of sucrose and tryptophan, L-methionine resulted in reduction in stomatal aperture and conductance, which could be advantageous for plants under salt stress. Eight aspartic proteinase proteins were highly upregulated in GCs of quinoa, and exogenous application of pepstatin A (an inhibitor of aspartic proteinase) was accompanied by higher oxidative stress and extremely low stomatal aperture and conductance, suggesting a possible role of aspartic proteinase in mitigating oxidative stress induced by saline conditions.

Light-Mediated Signaling and Metabolic Changes Coordinate Stomatal Opening and Closure

Stomata are valves on the leaf surface controlling carbon dioxide (CO2) influx for photosynthesis and water loss by transpiration. Thus, plants have to evolve elaborate mechanisms controlling stomatal aperture to allow efficient photosynthesis while avoid excessive water loss. Light is not only the energy source for photosynthesis but also an important signal regulating stomatal movement during dark-to-light transition. Our knowledge concerning blue and red light signaling and light-induced metabolite changes that contribute to stomatal opening are accumulating. This review summarizes recent advances on the signaling components that lie between the perception of blue/red light and activation of the PM H+-ATPases, and on the negative regulation of stomatal opening by red light-activated phyB signaling and ultraviolet (UV-B and UV-A) irradiation. Besides, light-regulated guard cell (GC)-specific metabolic levels, mesophyll-derived sucrose, and CO2 concentration within GCs also play dual roles in stomatal opening. Thus, light-induced stomatal opening is tightly accompanied by brake mechanisms, allowing plants to coordinate carbon gain and water loss. Knowledge on the mechanisms regulating the trade-off between stomatal opening and closure may have potential applications toward generating superior crops with improved water use efficiency (CO2 gain vs. water loss).

ABA-mediated modulation of elevated CO2 on stomatal response to drought

Elevated atmospheric CO₂ concentration (e[CO₂]) and soil water deficits have substantial effect on stomatal morphology and movement that regulate plant water relations and plant growth. e[CO₂] could alleviate the impact of drought stress, thus contributing to crop yield. Xylem-borne abscisic acid (ABA) plays a crucial role in regulating stomatal aperture serving as first line of defence against drought; whereas e[CO₂] may disrupt this fundamental drought adaptation mechanism by delaying the stomatal response to soil drying. We review the state-of-the-art knowledge on stomatal response to drought stress at e[CO₂] and discuss the role of ABA in mediating these responses.

Guard Cell Metabolism and Stomatal Function

The control of gaseous exchange between the leaf and external atmosphere is governed by stomatal conductance (g_s); therefore, stomata play a critical role in photosynthesis and transpiration and overall plant productivity. Stomatal conductance is determined by both anatomical features and behavioral characteristics. Here we review some of the osmoregulatory pathways in guard cell metabolism, genes and signals that determine stomatal function and patterning, and the recent work that explores coordination between g_s and carbon assimilation (A) and the influence of spatial distribution of functional stomata on underlying mesophyll anatomy. We also evaluate the current literature on mesophyll-driven signals that may coordinate stomatal behavior with mesophyll carbon assimilation and explore stomatal kinetics as a possible target to improve A and water use efficiency. By understanding these processes, we can start to provide insight into manipulation of these regulatory pathways to improve stomatal behavior and identify novel unexploited targets for altering stomatal behavior and improving crop plant productivity.

Role of blue and red light in stomatal dynamic behaviour

Plants experience changes in light intensity and quality due to variations in solar angle and shading from clouds and overlapping leaves. Stomatal opening to increasing irradiance is often an order of magnitude slower than photosynthetic responses, which can result in CO2 diffusional limitations on leaf photosynthesis, as well as unnecessary water loss when stomata continue to open after photosynthesis has reached saturation. Stomatal opening to light is driven by two distinct pathways; the 'red' or photosynthetic response that occurs at high fluence rates and saturates with photosynthesis, and is thought to be the main mechanism that coordinates stomatal behaviour with photosynthesis; and the guard cell-specific 'blue' light response that saturates at low fluence rates, and is often considered independent of photosynthesis, and important for early morning stomatal opening. Here we review the literature on these complicated signal transduction pathways and osmoregulatory processes in guard cells that are influenced by the light environment. We discuss the possibility of tuning the sensitivity and magnitude of stomatal response to blue light which potentially represents a novel target to develop ideotypes with the 'ideal' balance between carbon gain, evaporative cooling, and maintenance of hydraulic status that is crucial for maximizing crop performance and productivity.

Improved reproductive growth of euhalophyte Suaeda salsa under salinity is correlated with altered phytohormone biosynthesis and signal transduction

Phytohormones are essential for plant reproductive growth. Salinity limits crop reproductive growth and yield, but improves reproductive growth of euhalophytes. However, little is known about the mechanisms underlying salinity's effects on plant reproductive growth. To elucidate the role of plant hormones in flower development of the euhalophyte Suaeda salsa under saline conditions, we analysed endogenous gibberellic acid (GA3,4), indoleacetic acid (IAA), zeatin riboside (ZR), abscisic acid (ABA), and brassinosteroids (BRs) during flowering in control (0 mM) and NaCl-treated (200 mM) plants. At the end of vegetative growth, endogenous GA3, GA4, ABA and BR contents in stems of NaCl-treated plants were significantly higher than those in controls. During flowering, GA3, GA4, IAA and ZR contents showed the most significant enhancement in flower organs of plants treated with NaCl when compared with controls. Additionally, genes related to ZR, IAA, GA, BR and ABA biosynthesis and plant hormone signal transduction, such as those encoding CYP735A, CYP85A, GID1, NCED, PIF4, AHP, TCH4, SnRK2 and ABF, were upregulated in S. salsa flowers from NaCl-treated plants. These results suggest that coordinated upregulation of genes involved in phytohormone biosynthesis and signal transduction contributes to the enhanced reproductive growth of S. salsa under salinity.

Agave angustifolia albino plantlets lose stomatal physiology function by changing the development of the stomatal complex due to a molecular disruption

Stomatal development is regulated by signaling pathways that function in multiple cellular programs, including cell fate and cell division. However, recent studies suggest that molecular signals are affected by CO₂ concentration, light intensity, and water pressure deficit, thereby modifying distribution patterns and stomatic density and likely other foliar features as well. Here, we show that in addition to lacking chloroplasts, the albino somaclonal variants of Agave angustifolia Haw present an irregular epidermal development and morphological abnormalities of the stomatal complex, affecting the link between the stomatal conductance, transpiration and photosynthesis, as well as the development of the stoma in the upper part of the leaves. In addition, we show that changes in the transcriptional levels of SPEECHLESS (SPCH), TOO MANY MOUTHS (TMM), MITOGEN-ACTIVATED PROTEIN KINASE 4 and 6 (MAPK4 and MAPK6) and FOUR LIPS (FLP), all from the meristematic tissue and leaf, differentially modulate the stomatal function between the green, variegated and albino in vitro plantlets of A. angustifolia. Likewise, we highlight the conservation of microRNAs miR166 and miR824 as part of the regulation of AGAMOUS-LIKE16 (AGL16), recently associated with the control of cell divisions that regulate the development of the stomatal complex. We propose that molecular alterations happening in albino cells formed from the meristematic base can lead to different anomalies during the transition and specification of the stomatal cell state in leaf development of albino plantlets. We conclude that the molecular alterations in the meristematic cells in albino plants might be the main variable associated with stoma distribution in this phenotype.

Strengthened antioxidant capacity improves photosynthesis by regulating stomatal aperture and ribulose-1,5-bisphosphate carboxylase/oxygenase activity

ABA is important for plant growth and development; however, it also inhibits photosynthesis by regulating the stomatal aperture and ribulose-1,5-bisphosphate carboxylase/oxygenase activity. Noteworthy, this negative effect can be alleviated by antioxidants including ascorbic acid (AsA) and catalase (CAT), but the underlying mechanism remains unclear. Two rice cultivars, Zhefu802 (recurrent parent) and its near-isogenic line, fgl were selected and planted in a greenhouse with 30/24 °C (day/night) under natural sunlight conditions. Compared to fgl, Zhefu802 had significantly lower net photosynthetic rate (PN) and stomatal conductance (Cond) as well as significantly higher ABA and H₂O₂ contents. However, AsA and CAT increased PN, Cond, and stomatal aperture, which decreased H₂O₂ and malondialdehyde (MDA) levels. In this process, AsA and CAT significantly increased the ribulose-1,5-bisphosphate carboxylase activity, while they strongly decreased the ribulose-1,5-bisphosphate oxygenase activity, and finally caused an obvious decrease in the ratio of photorespiration (Pr) to PN. Additionally, AsA and CAT significantly increased the expression levels of RbcS and RbcL genes of leaves, while H₂O₂ significantly decreased them, especially the RbcS gene. In summary, the removal of H₂O₂ by AsA and CAT can improve the leaf photosynthesis by alleviating the inhibition on the stomatal conductance and ribulose-1,5-bisphosphate carboxylase capacity caused by ABA.

Molecular cloning and functional analysis of 4-Coumarate:CoA ligase 4(4CL-like 1)from Fraxinus mandshurica and its role in abiotic stress tolerance and cell wall synthesis

BACKGROUND

Four-Coumarate:CoA ligase gene (4CL) plays multiple important roles in plant growth and development by catalyzing the formation of CoA ester. 4CL belongs to the plant phenylpropane derivative, which is related to the synthesis of flavonoids and lignin and is a key enzyme in the biosynthetic pathway.

RESULTS

In this study, 12 4CL genes of Fraxinus mandschurica were identified and named Fm4CL1-Fm4CL12, respectively. The analysis of the expression pattern of Fm4CL genes indicate that Fm4CL-like 1 gene may play a role in the lignin synthesis pathway. Our study indicate that overexpression of Fm4CL-like 1 increases the lignin content of transgenic tobacco by 39.5% compared to WT, and the S/G ratio of transgenic tobacco increased by 19.7% compared with WT. The xylem cell layer of transgenic line is increased by 40% compared to WT, the xylem cell wall thickness increased by 21.6% compared to the WT. Under mannitol-simulated drought stress, the root length of transgenic tobacco is 64% longer than WT, and the seed germination rate of the transgenic lines is 47% higher than that of WT. In addition, the H2O2 content in the transgenic tobacco was 22% lower than that of WT, while the POD and SOD content was higher than WT by 30 and 24% respectively, which showed Fm4CL-like 1 affect the accumulation of reactive oxygen species (ROS). The MDA content and relative conductivity was 25 and 15% lower than WT, respectively. The water loss rate is 16.7% lower than that of WT. The relative expression levels of stress-related genes NtHAK, NtAPX, NtCAT, NtABF2, and NtZFP were higher than those of WT under stress treatment. The stomatal apertures of OE (Overexpression) were 30% smaller than those of WT, and the photosynthetic rate of OE was 48% higher than that of WT. These results showed that the overexpression line exhibited stronger adaptability to osmotic stress than WT.

CONCLUSIONS

Our results indicate that Fm4CL-like 1 is involved in secondary cell wall development and lignin synthesis. Fm4CL-like 1 play an important role in osmotic stress by affecting cell wall and stomatal development.

Differential effects of environmental stressors on physiological processes and methane emissions in pea (Pisum sativum) plants at various growth stages

Many studies have investigated the effects of one or two environmental factors on methane (CH4) emissions from plants at a single growth stage, but the impact that multiple co-occurring stress factors may have on emissions at different growth stages has rarely been studied. The objective of this study was to examine the effects of temperature, ultraviolet-B (UVB) radiation, and watering regime on CH4 emissions and some relevant physiological characteristics of pea (Pisum sativum L. cv. 237 J Sundance) plants at three growth stages. We grew plants under two temperature regimes (22/18 °C and 28/24 °C; 16 h light/8 h dark), two UVB levels [0 and 5 kJ m-2 d-1] and two watering regimes (well-watered, watering plants to field capacity, and water-stressed, watering plants at wilting point). Measurements were then taken after 10, 20, and 30 days of growth under experimental conditions, following seven days of initial growth under 22/18 °C. Higher temperatures, UVB5, and water stress adversely affected photosynthesis and chlorophyll fluorescence, but increased CH4 emissions, which decreased with increased plant age. Also, interaction of higher temperatures and UVB5 reversed the pattern of CH4 emissions at growth stages, compared to that of other treatments. We conclude that CH4 emission decreases with plant age, and it is affected by stress factors through changes in physiological activities of plants.

Guard cell photosynthesis is crucial in abscisic acid-induced stomatal closure

Reactive oxygen species (ROS) are ubiquitous signaling molecules involved in diverse physiological processes, including stomatal closure. Photosynthetic electron transport (PET) is the main source of ROS generation in plants, but whether it functions in guard cell signaling remains unclear. Here, we assessed whether PET functions in abscisic acid (ABA) signaling in guard cells. ABA-elicited ROS were localized to guard cell chloroplasts in Arabidopsis thaliana, Commelina benghalensis, and Vicia faba in the light and abolished by the PET inhibitors 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea and 2, 5-dibromo-3-methyl-6-isopropyl-p-benzoquinone. These inhibitors reduced ABA-induced stomatal closure in all three species, as well as in the NADPH oxidase-lacking mutant atrboh D/F. However, an NADPH oxidase inhibitor did not fully eliminate ABA-induced ROS in the chloroplasts, and ABA-induced ROS were still observed in the guard cell chloroplasts of atrboh D/F. This study demonstrates that ROS generated through PET act as signaling molecules in ABA-induced stomatal closure and that this occurs in concert with ROS derived through NADPH oxidase.

Reactive Oxygen Species, Photosynthesis, and Environment in the Regulation of Stomata

SIGNIFICANCE

Stomata sense the intercellular carbon dioxide (CO₂) concentration (C_i) and water availability under changing environmental conditions and adjust their apertures to maintain optimal cellular conditions for photosynthesis. Stomatal movements are regulated by a complex network of signaling cascades where reactive oxygen species (ROS) play a key role as signaling molecules. Recent Advances: Recent research has uncovered several new signaling components involved in CO₂- and abscisic acid-triggered guard cell signaling pathways. In addition, we are beginning to understand the complex interactions between different signaling pathways.

CRITICAL ISSUES

Plants close their stomata in reaction to stress conditions, such as drought, and the subsequent decrease in C_i leads to ROS production through photorespiration and over-reduction of the chloroplast electron transport chain. This reduces plant growth and thus drought may cause severe yield losses for agriculture especially in arid areas.

FUTURE DIRECTIONS

The focus of future research should be drawn toward understanding the interplay between various signaling pathways and how ROS, redox, and hormonal balance changes in space and time. Translating this knowledge from model species to crop plants will help in the development of new drought-resistant crop species with high yields.

Blue and red light effects on stomatal oscillations

The response of stomata to red and blue light was investigated using small fibre optics (66µm diameter) to control light levels on a single pair of guard cells without affecting the surrounding tissue. Low intensity red light (50µmolm-2s-1) applied to the entire leaf caused stomata to oscillate continuously for several hours with no apparent decrease in amplitude with time. Adding low intensity blue light (50µmolm-2s-1) caused stomata to stop oscillating, but oscillations resumed when the blue light was removed. Adding the same intensity of red light to an oscillating leaf changed the amplitude of the oscillations but did not stop them. When blue light was added to a single guard cell pair (using a fibre optic) in a red-light-illuminated leaf, the stoma formed by that pair stopped oscillating, but adjacent stomata did not. Red light added to a single guard cell pair did not stop oscillations. Finally, blue light applied through a fibre optic to areas of leaf without stomata caused proximal stomata to stop oscillating, but distal stomata continued to oscillate. The data suggest that blue light affects stomata via direct effects on guard cells as well as by indirect effects on other cells in the leaf.

Comparative Proteomic Analysis Reveals Elevated Capacity for Photosynthesis in Polyphenol Oxidase Expression-Silenced Clematis terniflora DC. Leaves

Polyphenol oxidase (PPO) catalyzes the o-hydroxylation of monophenols and oxidation of o-diphenols to quinones. Although the effects of PPO on plant physiology were recently proposed, little has been done to explore the inherent molecular mechanisms. To explore the in vivo physiological functions of PPO, a model with decreased PPO expression and enzymatic activity was constructed on Clematis terniflora DC. using virus-induced gene silencing (VIGS) technology. Proteomics was performed to identify the differentially expressed proteins (DEPs) in the model (VC) and empty vector-carrying plants (VV) untreated or exposed to high levels of UV-B and dark (HUV-B+D). Following integration, it was concluded that the DEPs mainly functioned in photosynthesis, glycolysis, and redox in the PPO silence plants. Mapman analysis showed that the DEPs were mainly involved in light reaction and Calvin cycle in photosynthesis. Further analysis illustrated that the expression level of adenosine triphosphate (ATP) synthase, the content of chlorophyll, and the photosynthesis rate were increased in VC plants compared to VV plants pre- and post HUV-B+D. These results indicate that the silence of PPO elevated the plant photosynthesis by activating the glycolysis process, regulating Calvin cycle and providing ATP for energy metabolism. This study provides a prospective approach for increasing crop yield in agricultural production.

Modulation of Guard Cell Turgor and Drought Tolerance by a Peroxisomal Acetate-Malate Shunt

In plants, stomatal movements are tightly controlled by changes in cellular turgor pressure. Carbohydrates produced by glycolysis and the tricarboxylic acid cycle play an important role in regulating turgor pressure. Here, we describe an Arabidopsis mutant, bzu1, isolated in a screen for elevated leaf temperature in response to drought stress, which displays smaller stomatal pores and higher drought resistance than wild-type plants. BZU1 encodes a known acetyl-coenzyme A synthetase, ACN1, which acts in the first step of a metabolic pathway converting acetate to malate in peroxisomes. We showed that BZU1/ACN1-mediated acetate-to-malate conversion provides a shunt that plays an important role in osmoregulation of stomatal turgor. We found that the smaller stomatal pores in the bzu1 mutant are a consequence of reduced accumulation of malate, which acts as an osmoticum and/or a signaling molecule in the control of turgor pressure within guard cells, and these results provided new genetic evidence for malate-regulated stomatal movement. Collectively, our results indicate that a peroxisomal BZU1/ACN1-mediated acetate-malate shunt regulates drought resistance by controlling the turgor pressure of guard cells in Arabidopsis.

Eukaryotic lipid metabolic pathway is essential for functional chloroplasts and CO2 and light responses in Arabidopsis guard cells

Stomatal guard cells develop unique chloroplasts in land plant species. However, the developmental mechanisms and function of chloroplasts in guard cells remain unclear. In seed plants, chloroplast membrane lipids are synthesized via two pathways: the prokaryotic and eukaryotic pathways. Here we report the central contribution of endoplasmic reticulum (ER)-derived chloroplast lipids, which are synthesized through the eukaryotic lipid metabolic pathway, in the development of functional guard cell chloroplasts. We gained insight into this pathway by isolating and examining an Arabidopsis mutant, gles1 (green less stomata 1), which had achlorophyllous stomatal guard cells and impaired stomatal responses to CO2 and light. The GLES1 gene encodes a small glycine-rich protein, which is a putative regulatory component of the trigalactosyldiacylglycerol (TGD) protein complex that mediates ER-to-chloroplast lipid transport via the eukaryotic pathway. Lipidomic analysis revealed that in the wild type, the prokaryotic pathway is dysfunctional, specifically in guard cells, whereas in gles1 guard cells, the eukaryotic pathway is also abrogated. CO2-induced stomatal closing and activation of guard cell S-type anion channels that drive stomatal closure were disrupted in gles1 guard cells. In conclusion, the eukaryotic lipid pathway plays an essential role in the development of a sensing/signaling machinery for CO2 and light in guard cell chloroplasts.

Sucrose breakdown within guard cells provides substrates for glycolysis and glutamine biosynthesis during light-induced stomatal opening

Sucrose has long been thought to play an osmolytic role in stomatal opening. However, recent evidence supports the idea that the role of sucrose in this process is primarily energetic. Here we used a combination of stomatal aperture assays and kinetic [U-¹³ C]-sucrose isotope labelling experiments to confirm that sucrose is degraded during light-induced stomatal opening and to define the fate of the C released from sucrose breakdown. We additionally show that addition of sucrose to the medium did not enhance light-induced stomatal opening. The isotope experiment showed a consistent ¹³ C enrichment in fructose and glucose, indicating that during light-induced stomatal opening sucrose is indeed degraded. We also observed a clear ¹³ C enrichment in glutamate and glutamine (Gln), suggesting a concerted activation of sucrose degradation, glycolysis and the tricarboxylic acid cycle. This is in contrast to the situation for Gln biosynthesis in leaves under light, which has been demonstrated to rely on previously stored C. Our results thus collectively allow us to redraw current models concerning the influence of sucrose during light-induced stomatal opening, in which, instead of being accumulated, sucrose is degraded providing C skeletons for Gln biosynthesis.

Cortical microtubules and fusicoccin response in clustered stomatal guard cells induced by sucrose solution immersion

We previously found that sucrose solution immersion treatment permitted ectopic guard cell differentiation, resulting in clustered stomatal guard cells. Using this system, we examined the effects of sucrose solution-induced stomatal clustering on guard cell cortical microtubules and the stomatal response to fusicoccin. Confocal observation revealed that the radial orientation of cortical microtubules was largely maintained in clustered guard cells. Outward movement of cortical microtubule plus-ends was also kept in the clustered guard cells. Fusicoccin treatment induced stomatal opening in both spaced and clustered stomata, although sucrose solution-treated guard cells had lower stomatal apertures. These results suggested that immersion treatment with sucrose solution perturbed the one-cell spacing of stomata but not the cortical microtubule organization required to open stomatal pores.

Metabolism within the specialized guard cells of plants

Contents 1018 I. 1018 II. 1019 III. 1022 IV. 1025 V. 1026 VI. 1029 1030 References 1030 SUMMARY: Stomata are leaf epidermal structures consisting of two guard cells surrounding a pore. Changes in the aperture of this pore regulate plant water-use efficiency, defined as gain of C by photosynthesis per leaf water transpired. Stomatal aperture is actively regulated by reversible changes in guard cell osmolyte content. Despite the fact that guard cells can photosynthesize on their own, the accumulation of mesophyll-derived metabolites can seemingly act as signals which contribute to the regulation of stomatal movement. It has been shown that malate can act as a signalling molecule and a counter-ion of potassium, a well-established osmolyte that accumulates in the vacuole of guard cells during stomatal opening. By contrast, their efflux from guard cells is an important mechanism during stomatal closure. It has been hypothesized that the breakdown of starch, sucrose and lipids is an important mechanism during stomatal opening, which may be related to ATP production through glycolysis and mitochondrial metabolism, and/or accumulation of osmolytes such as sugars and malate. However, experimental evidence supporting this theory is lacking. Here we highlight the particularities of guard cell metabolism and discuss this in the context of the guard cells themselves and their interaction with the mesophyll cells.

Impaired Malate and Fumarate Accumulation Due to the Mutation of the Tonoplast Dicarboxylate Transporter Has Little Effects on Stomatal Behavior

Malate is a central metabolite involved in a multiplicity of plant metabolic pathways, being associated with mitochondrial metabolism and playing significant roles in stomatal movements. Vacuolar malate transport has been characterized at the molecular level and is performed by at least one carrier protein and two channels in Arabidopsis (Arabidopsis thaliana) vacuoles. The absence of the Arabidopsis tonoplast Dicarboxylate Transporter (tDT) in the tdt knockout mutant was associated previously with an impaired accumulation of malate and fumarate in leaves. Here, we investigated the consequences of this lower accumulation on stomatal behavior and photosynthetic capacity as well as its putative metabolic impacts. Neither the stomatal conductance nor the kinetic responses to dark, light, or high CO2 were highly affected in tdt plants. In addition, we did not observe any impact on stomatal aperture following incubation with abscisic acid, malate, or citrate. Furthermore, an effect on photosynthetic capacity was not observed in the mutant lines. However, leaf mitochondrial metabolism was affected in the tdt plants. Levels of the intermediates of the tricarboxylic acid cycle were altered, and increases in both light and dark respiration were observed. We conclude that manipulation of the tonoplastic organic acid transporter impacted mitochondrial metabolism, while the overall stomatal and photosynthetic capacity were unaffected.

Metabolic Signatures in Response to Abscisic Acid (ABA) Treatment in Brassica napus Guard Cells Revealed by Metabolomics

Drought can severely damage crops, resulting in major yield losses. During drought, vascular land plants conserve water via stomatal closure. Each stomate is bordered by a pair of guard cells that shrink in response to drought and the associated hormone abscisic acid (ABA). The activation of complex intracellular signaling networks underlies these responses. Therefore, analysis of guard cell metabolites is fundamental for elucidation of guard cell signaling pathways. Brassica napus is an important oilseed crop for human consumption and biodiesel production. Here, non-targeted metabolomics utilizing gas chromatography mass spectrometry (GC-MS/MS) and liquid chromatography mass spectrometry (LC-MS/MS) were employed for the first time to identify metabolic signatures in response to ABA in B. napus guard cell protoplasts. Metabolome profiling identified 390 distinct metabolites in B. napus guard cells, falling into diverse classes. Of these, 77 metabolites, comprising both primary and secondary metabolites were found to be significantly ABA responsive, including carbohydrates, fatty acids, glucosinolates, and flavonoids. Selected secondary metabolites, sinigrin, quercetin, campesterol, and sitosterol, were confirmed to regulate stomatal closure in Arabidopsis thaliana, B. napus or both species. Information derived from metabolite datasets can provide a blueprint for improvement of water use efficiency and drought tolerance in crops.

Resolving the central metabolism of Arabidopsis guard cells

Photosynthesis and water use efficiency, key factors affecting plant growth, are directly controlled by microscopic and adjustable pores in the leaf-the stomata. The size of the pores is modulated by the guard cells, which rely on molecular mechanisms to sense and respond to environmental changes. It has been shown that the physiology of mesophyll and guard cells differs substantially. However, the implications of these differences to metabolism at a genome-scale level remain unclear. Here, we used constraint-based modeling to predict the differences in metabolic fluxes between the mesophyll and guard cells of Arabidopsis thaliana by exploring the space of fluxes that are most concordant to cell-type-specific transcript profiles. An independent 13C-labeling experiment using isolated mesophyll and guard cells was conducted and provided support for our predictions about the role of the Calvin-Benson cycle in sucrose synthesis in guard cells. The combination of in silico with in vivo analyses indicated that guard cells have higher anaplerotic CO2 fixation via phosphoenolpyruvate carboxylase, which was demonstrated to be an important source of malate. Beyond highlighting the metabolic differences between mesophyll and guard cells, our findings can be used in future integrated modeling of multi-cellular plant systems and their engineering towards improved growth.

Metabolomics and Proteomics of Brassica napus Guard Cells in Response to Low CO2

Stomatal guard cell response to various stimuli is an important process that balances plant carbon dioxide (CO2) uptake and water transpiration. Elevated CO2 induces stomatal closure, while low CO2 promotes stomatal opening. The signaling process of elevated CO2 induced stomatal closure has been extensively studied in recent years. However, the mechanism of low CO2 induced stomatal opening is not fully understood. Here we report metabolomic and proteomic responses of Brassica napus guard cells to low CO2 using hyphenated mass spectrometry technologies. A total of 411 metabolites and 1397 proteins were quantified in a time-course study of low CO2 effects. Metabolites and proteins that exhibited significant changes are overrepresented in fatty acid metabolism, starch and sucrose metabolism, glycolysis and redox regulation. Concomitantly, multiple hormones that promote stomatal opening increased in response to low CO2. Interestingly, jasmonic acid precursors were diverted to a branch pathway of traumatic acid biosynthesis. These results indicate that the low CO2 response is mediated by a complex crosstalk between different phytohormones.

Stomatal Biology of CAM Plants

Recent advances in the stomatal biology of CAM plants are reviewed, and key opportunities for future progress are identified.

Constitutive Expression of Arabidopsis SMALL AUXIN UP RNA19 (SAUR19) in Tomato Confers Auxin-Independent Hypocotyl Elongation

The plant hormone indole-3-acetic acid (IAA or auxin) mediates the elongation growth of shoot tissues by promoting cell expansion. According to the acid growth theory proposed in the 1970s, auxin activates plasma membrane H⁺-ATPases (PM H⁺-ATPases) to facilitate cell expansion by both loosening the cell wall through acidification and promoting solute uptake. Mechanistically, however, this process is poorly understood. Recent findings in Arabidopsis (Arabidopsis thaliana) have demonstrated that auxin-induced SMALL AUXIN UP RNA (SAUR) genes promote elongation growth and play a key role in PM H⁺-ATPase activation by inhibiting PP2C.D family protein phosphatases. Here, we extend these findings by demonstrating that SAUR proteins also inhibit tomato PP2C.D family phosphatases and that AtSAUR19 overexpression in tomato (Solanum lycopersicum) confers the same suite of phenotypes as previously reported for Arabidopsis. Furthermore, we employ a custom image-based method for measuring hypocotyl segment elongation with high resolution and a method for measuring cell wall mechanical properties, to add mechanistic details to the emerging description of auxin-mediated cell expansion. We find that constitutive expression of GFP-AtSAUR19 bypasses the normal requirement of auxin for elongation growth by increasing the mechanical extensibility of excised hypocotyl segments. In contrast, hypocotyl segments overexpressing a PP2C.D phosphatase are specifically impaired in auxin-mediated elongation. The time courses of auxin-induced SAUR expression and auxin-dependent elongation growth were closely correlated. These findings indicate that induction of SAUR expression is sufficient to elicit auxin-mediated expansion growth by activating PM H⁺-ATPases to facilitate apoplast acidification and mechanical wall loosening.

Photorespiration Is Crucial for Dynamic Response of Photosynthetic Metabolism and Stomatal Movement to Altered CO2 Availability

The photorespiratory pathway or photorespiration is an essential process in oxygenic photosynthetic organisms, which can reduce the efficiency of photosynthetic carbon assimilation and is hence frequently considered as a wasteful process. By comparing the response of the wild-type plants and mutants impaired in photorespiration to a shift in ambient CO₂ concentrations, we demonstrate that photorespiration also plays a beneficial role during short-term acclimation to reduced CO₂ availability. The wild-type plants responded with few differentially expressed genes, mostly involved in drought stress, which is likely a consequence of enhanced opening of stomata and concomitant water loss upon a shift toward low CO₂. In contrast, mutants with impaired activity of photorespiratory enzymes were highly stressed and not able to adjust stomatal conductance to reduced external CO₂ availability. The transcriptional response of mutant plants was congruent, indicating a general reprogramming to deal with the consequences of reduced CO₂ availability, signaled by enhanced oxygenation of ribulose-1,5-bisphosphate and amplified by the artificially impaired photorespiratory metabolism. Central in this reprogramming was the pronounced reallocation of resources from growth processes to stress responses. Taken together, our results indicate that unrestricted photorespiratory metabolism is a prerequisite for rapid physiological acclimation to a reduction in CO₂ availability.

Transcript, protein and metabolite temporal dynamics in the CAM plant Agave

Already a proven mechanism for drought resilience, crassulacean acid metabolism (CAM) is a specialized type of photosynthesis that maximizes water-use efficiency by means of an inverse (compared to C₃ and C₄ photosynthesis) day/night pattern of stomatal closure/opening to shift CO₂ uptake to the night, when evapotranspiration rates are low. A systems-level understanding of temporal molecular and metabolic controls is needed to define the cellular behaviour underpinning CAM. Here, we report high-resolution temporal behaviours of transcript, protein and metabolite abundances across a CAM diel cycle and, where applicable, compare the observations to the well-established C₃ model plant Arabidopsis. A mechanistic finding that emerged is that CAM operates with a diel redox poise that is shifted relative to that in Arabidopsis. Moreover, we identify widespread rescheduled expression of genes associated with signal transduction mechanisms that regulate stomatal opening/closing. Controlled production and degradation of transcripts and proteins represents a timing mechanism by which to regulate cellular function, yet knowledge of how this molecular timekeeping regulates CAM is unknown. Here, we provide new insights into complex post-transcriptional and -translational hierarchies that govern CAM in Agave. These data sets provide a resource to inform efforts to engineer more efficient CAM traits into economically valuable C₃ crops.

Rethinking Guard Cell Metabolism

Stomata control gaseous fluxes between the internal leaf air spaces and the external atmosphere and, therefore, play a pivotal role in regulating CO2 uptake for photosynthesis as well as water loss through transpiration. Guard cells, which flank the stomata, undergo adjustments in volume, resulting in changes in pore aperture. Stomatal opening is mediated by the complex regulation of ion transport and solute biosynthesis. Ion transport is exceptionally well understood, whereas our knowledge of guard cell metabolism remains limited, despite several decades of research. In this review, we evaluate the current literature on metabolism in guard cells, particularly the roles of starch, sucrose, and malate. We explore the possible origins of sucrose, including guard cell photosynthesis, and discuss new evidence that points to multiple processes and plasticity in guard cell metabolism that enable these cells to function effectively to maintain optimal stomatal aperture. We also discuss the new tools, techniques, and approaches available for further exploring and potentially manipulating guard cell metabolism to improve plant water use and productivity.

Modelling water use efficiency in a dynamic environment: An example using Arabidopsis thaliana

Intrinsic water use efficiency (Wi), the ratio of net CO2 assimilation (A) over stomatal conductance to water vapour (gs), is a complex trait used to assess plant performance. Improving Wi could lead in theory to higher productivity or reduced water usage by the plant, but the physiological traits for improvement and their combined effects on Wi have not been clearly identified. Under fluctuating light intensity, the temporal response of gs is an order of magnitude slower than A, which results in rapid variations in Wi. Compared to traditional approaches, our new model scales stoma behaviour at the leaf level to predict gs and A during a diurnal period, reproducing natural fluctuations of light intensity, in order to dissect Wi into traits of interest. The results confirmed the importance of stomatal density and photosynthetic capacity on Wi but also revealed the importance of incomplete stomatal closure under dark conditions as well as stomatal sensitivity to light intensity. The observed continuous decrease of A and gs over the diurnal period was successfully described by negative feedback of the accumulation of photosynthetic products. Investigation into the impact of leaf anatomy on temporal responses of A, gs and Wi revealed that a high density of stomata produces the most rapid response of gs but may result in lower Wi.

A Tale of Two Sugars: Trehalose 6-Phosphate and Sucrose

Trehalose 6-phosphate (Tre6P), the intermediate of trehalose biosynthesis, is an essential signal metabolite in plants, linking growth and development to carbon status. The Suc-Tre6P nexus model postulates that Tre6P is both a signal and negative feedback regulator of Suc levels, forming part of a mechanism to maintain Suc levels within an optimal range and functionally comparable to the insulin-glucagon system for regulating blood Glc levels in animals. The target range and sensitivity of the Tre6P-Suc feedback control circuit can be adjusted according to the cell type, developmental stage, and environmental conditions. In source leaves, Tre6P modulates Suc levels by affecting Suc synthesis, whereas in sink organs it regulates Suc consumption. In illuminated leaves, Tre6P influences the partitioning of photoassimilates between Suc, organic acids, and amino acids via posttranslational regulation of phosphoenolpyruvate carboxylase and nitrate reductase. At night, Tre6P regulates the remobilization of leaf starch reserves to Suc, potentially linking starch turnover in source leaves to carbon demand from developing sink organs. Use of Suc for growth in developing tissues is strongly influenced by the antagonistic activities of two protein kinases: SUC-NON-FERMENTING-1-RELATED KINASE1 (SnRK1) and TARGET OF RAPAMYCIN (TOR). The relationship between Tre6P and SnRK1 in developing tissues is complex and not yet fully resolved, involving both direct and indirect mechanisms, and positive and negative effects. No direct connection between Tre6P and TOR has yet been described. The roles of Tre6P in abiotic stress tolerance and stomatal regulation are also discussed.

Roles of sucrose in guard cell regulation

The control of stomatal aperture involves reversible changes in the concentration of osmolytes in guard cells. Sucrose has long been proposed to have an osmolytic role in guard cells. However, direct evidence for such a role is lacking. Furthermore, recent evidence suggests that sucrose may perform additional roles in guard cells. Here, we provide an update covering the multiple roles of sucrose in guard cell regulation, highlighting the knowledge accumulated regarding spatiotemporal differences in the synthesis, accumulation, and degradation of sucrose as well as reviewing the role of sucrose as a metabolic connector between mesophyll and guard cells. Analysis of transcriptomic data from previous studies reveals that several genes encoding sucrose and hexose transporters and genes involved in gluconeogenesis, sucrose and trehalose metabolism are highly expressed in guard cells compared with mesophyll cells. Interestingly, this analysis also showed that guard cells have considerably higher expression of C4 -marker genes than mesophyll cells. We discuss the possible roles of these genes in guard cell function and the role of sucrose in stomatal opening and closure. Finally, we provide a perspective for future experiments which are required to fill gaps in our understanding of both guard cell metabolism and stomatal regulation.

Starch Biosynthesis in Guard Cells But Not in Mesophyll Cells Is Involved in CO2-Induced Stomatal Closing

Starch metabolism is involved in stomatal movement regulation. However, it remains unknown whether starch-deficient mutants affect CO2-induced stomatal closing and whether starch biosynthesis in guard cells and/or mesophyll cells is rate limiting for high CO2-induced stomatal closing. Stomatal responses to [CO2] shifts and CO2 assimilation rates were compared in Arabidopsis (Arabidopsis thaliana) mutants that were either starch deficient in all plant tissues (ADP-Glc-pyrophosphorylase [ADGase]) or retain starch accumulation in guard cells but are starch deficient in mesophyll cells (plastidial phosphoglucose isomerase [pPGI]). ADGase mutants exhibited impaired CO2-induced stomatal closure, but pPGI mutants did not, showing that starch biosynthesis in guard cells but not mesophyll functions in CO2-induced stomatal closing. Nevertheless, starch-deficient ADGase mutant alleles exhibited partial CO2 responses, pointing toward a starch biosynthesis-independent component of the response that is likely mediated by anion channels. Furthermore, whole-leaf CO2 assimilation rates of both ADGase and pPGI mutants were lower upon shifts to high [CO2], but only ADGase mutants caused impairments in CO2-induced stomatal closing. These genetic analyses determine the roles of starch biosynthesis for high CO2-induced stomatal closing.