Stomatal Development and Perspectives toward Agricultural Improvement

Fighting for Survival at the Stomatal Gate

Stomata serve as the battleground between plants and plant pathogens. Plants can perceive pathogens, inducing closure of the stomatal pore, while pathogens can overcome this immune response with their phytotoxins and elicitors. In this review, we summarize new discoveries in stomata-pathogen interactions. Recent studies have shown that stomatal movement continues to occur in a close-open-close-open pattern during bacterium infection, bringing a new understanding of stomatal immunity. Furthermore, the canonical pattern-triggered immunity pathway and ion channel activities seem to be common to plant-pathogen interactions outside of the well-studied Arabidopsis-Pseudomonas pathosystem. These developments can be useful to aid in the goal of crop improvement. New technologies to study intact leaves and advances in available omics data sets provide new methods for understanding the fight at the stomatal gate. Future studies should aim to further investigate the defense-growth trade-off in relation to stomatal immunity, as little is known at this time.

Stomatal improvement for crop stress resistance

The growth and yield of crop plants are threatened by environmental challenges such as water deficit, soil flooding, high salinity, and extreme temperatures, which are becoming increasingly severe under climate change. Stomata contribute greatly to plant adaptation to stressful environments by governing transpirational water loss and photosynthetic gas exchange. Increasing evidence has revealed that stomata formation is shaped by transcription factors, signaling peptides, and protein kinases, which could be exploited to improve crop stress resistance. The past decades have seen unprecedented progress in our understanding of stomata formation, but most of these advances have come from research on model plants. This review highlights recent research in stomata formation in crops and its multifaceted functions in abiotic stress tolerance. Current strategies, limitations, and future directions for harnessing stomatal development to improve crop stress resistance are discussed.

Thermal imaging: The digital eye facilitates high-throughput phenotyping traits of plant growth and stress responses

Plant phenotyping is important for plants to cope with environmental changes and ensure plant health. Imaging techniques are perceived as the most critical and reliable tools for studying plant phenotypes. Thermal imaging has opened up new opportunities for nondestructive imaging of plant phenotyping. However, a comprehensive summary of thermal imaging in plant phenotyping is still lacking. Here we discuss the progress and future prospects of thermal imaging for assessing plant growth and stress responses. First, we classify thermal imaging into ground-based and aerial platforms based on their adaptability to different experimental environments (including laboratory, greenhouse, and field). It is convenient to collect phenotypic information of different dimensions. Second, in order to enhance the efficiency of thermal image processing, automatic algorithms based on deep learning are employed instead of traditional manual methods, greatly reducing the time cost of experiments. Considering its ease of implementation, handling and instant response, thermal imaging has been widely used in research on environmental stress, crop yield, and seed vigor. We have found that thermal imaging can detect thermal energy dissipation caused by living organisms (e.g., pests, viruses, bacteria, fungi, and oomycetes), enabling early disease diagnosis. It also recognizes changes leaf surface temperatures resulting from reduced transpiration rates caused by nutrient deficiency, drought, salinity, or freezing. Furthermore, thermal imaging predicts crop yield under different water states and forecasts the viability of dormant seeds after water absorption by monitoring temperature changes in the seeds. This work will assist biologists and agronomists in studying plant phenotypes and serve a guide for breeders to develop high-yielding, stress-tolerant, and superior crops.

REGULATOR OF FLOWERING AND STRESS manipulates stomatal density and size in Brachypodium

Stomata are crucial for gas exchange and water evaporation, and environmental stimuli influence their density (SD) and size (SS). Although genes and mechanisms underlying stomatal development have been elucidated, stress-responsive regulators of SD and SS are less well-known. Previous studies have shown that the stress-inducible Brachypodium RFS (REGULATOR OF FLOWERING AND STRESS, BdRFS) gene affects heading time and enhances drought tolerance by reducing leaf water loss. Here, we report that overexpression lines (OXs) of BdRFS have reduced SD and increased SS, regardless of soil water status. Furthermore, biomass and plant water content of OXs were significantly increased compared to wild type. CRISPR/Cas9-mediated BdRFS knockout mutant (KO) exhibited the opposite stomatal characteristics and biomass changes. Reverse transcription-quantitative polymerase chain reaction analysis revealed that expression of BdICE1 was reversely altered in OXs and KO, pointing to a potential cause for the observed changes in stomatal phenotypes. Stomatal and transcriptional changes were not observed in the Arabidopsis rfs double mutant. Taken together, RFS is a novel regulator of SD and SS and is a promising candidate for genetic engineering of climate-resilient crops.

Paralog editing tunes rice stomatal density to maintain photosynthesis and improve drought tolerance

Rice (Oryza sativa) is of paramount importance for global nutrition, supplying at least 20% of global calories. However, water scarcity and increased drought severity are anticipated to reduce rice yields globally. We explored stomatal developmental genetics as a mechanism for improving drought resilience in rice while maintaining yield under climate stress. CRISPR/Cas9-mediated knockouts of the positive regulator of stomatal development STOMAGEN and its paralog EPIDERMAL PATTERNING FACTOR-LIKE10 (EPFL10) yielded lines with ∼25% and 80% of wild-type stomatal density, respectively. epfl10 lines with moderate reductions in stomatal density were able to conserve water to similar extents as stomagen lines but did not suffer from the concomitant reductions in stomatal conductance, carbon assimilation, or thermoregulation observed in stomagen knockouts. Moderate reductions in stomatal density achieved by editing EPFL10 present a climate-adaptive approach for safeguarding yield in rice. Editing the paralog of STOMAGEN in other species may provide a means for tuning stomatal density in agriculturally important crops beyond rice.

Advances in the study of the function and mechanism of the action of flavonoids in plants under environmental stresses

MAIN CONCLUSION

This review summarizes the anti-stress effects of flavonoids in plants and highlights its role in the regulation of polar auxin transport and free radical scavenging mechanism. As secondary metabolites widely present in plants, flavonoids play a vital function in plant growth, but also in resistance to stresses. This review introduces the classification, structure and synthetic pathways of flavonoids. The effects of flavonoids in plant stress resistance were enumerated, and the mechanism of flavonoids in plant stress resistance was discussed in detail. It is clarified that plants under stress accumulate flavonoids by regulating the expression of flavonoid synthase genes. It was also determined that the synthesized flavonoids are transported in plants through three pathways: membrane transport proteins, vesicles, and bound to glutathione S-transferase (GST). At the same time, the paper explores that flavonoids regulate polar auxin transport (PAT) by acting on the auxin export carrier PIN-FORMED (PIN) in the form of ATP-binding cassette subfamily B/P-glycoprotein (ABCB/PGP) transporter, which can help plants to respond in a more dominant form to stress. We have demonstrated that the number and location of hydroxyl groups in the structure of flavonoids can determine their free radical scavenging ability and also elucidated the mechanism by which flavonoids exert free radical removal in cells. We also identified flavonoids as signaling molecules to promote rhizobial nodulation and colonization of arbuscular mycorrhizal fungi (AMF) to enhance plant-microbial symbiosis in defense to stresses. Given all this knowledge, we can foresee that the in-depth study of flavonoids will be an essential way to reveal plant tolerance and enhance plant stress resistance.

Plants and Small Molecules: An Up-and-Coming Synergy

The emergence of Arabidopsis thaliana as a model system has led to a rapid and wide improvement in molecular genetics techniques for studying gene function and regulation. However, there are still several drawbacks that cannot be easily solved with molecular genetic approaches, such as the study of unfriendly species, which are of increasing agronomic interest but are not easily transformed, thus are not prone to many molecular techniques. Chemical genetics represents a methodology able to fill this gap. Chemical genetics lies between chemistry and biology and relies on small molecules to phenocopy genetic mutations addressing specific targets. Advances in recent decades have greatly improved both target specificity and activity, expanding the application of this approach to any biological process. As for classical genetics, chemical genetics also proceeds with a forward or reverse approach depending on the nature of the study. In this review, we addressed this topic in the study of plant photomorphogenesis, stress responses and epigenetic processes. We have dealt with some cases of repurposing compounds whose activity has been previously proven in human cells and, conversely, studies where plants have been a tool for the characterization of small molecules. In addition, we delved into the chemical synthesis and improvement of some of the compounds described.

Genetic mapping identifies genomic regions and candidate genes for seed weight and shelling percentage in groundnut

Seed size is not only a yield-related trait but also an important measure to determine the commercial value of groundnut in the international market. For instance, small size is preferred in oil production, whereas large-sized seeds are preferred in confectioneries. In order to identify the genomic regions associated with 100-seed weight (HSW) and shelling percentage (SHP), the recombinant inbred line (RIL) population (Chico × ICGV 02251) of 352 individuals was phenotyped for three seasons and genotyped with an Axiom_Arachis array containing 58K SNPs. A genetic map with 4199 SNP loci was constructed, spanning a map distance of 2708.36 cM. QTL analysis identified six QTLs for SHP, with three consistent QTLs on chromosomes A05, A08, and B10. Similarly, for HSW, seven QTLs located on chromosomes A01, A02, A04, A10, B05, B06, and B09 were identified. BIG SEED locus and spermidine synthase candidate genes associated with seed weight were identified in the QTL region on chromosome B09. Laccase, fibre protein, lipid transfer protein, senescence-associated protein, and disease-resistant NBS-LRR proteins were identified in the QTL regions associated with shelling percentage. The associated markers for major-effect QTLs for both traits successfully distinguished between the small- and large-seeded RILs. QTLs identified for HSW and SHP can be used for developing potential selectable markers to improve the cultivars with desired seed size and shelling percentage to meet the demands of confectionery industries.

An extremely low stomatal density mutant overcomes cooling limitations at supra-optimal temperature by adjusting stomatal size and leaf thickness

The impact of global warming on transpiration and photosynthesis would compromise plant fitness, impacting on crop yields and ecosystem functioning. In this frame, we explored the performance of a set of Arabidopsis mutants carrying partial or total loss-of-function alleles of stomatal development genes and displaying distinct stomatal abundances. Using microscopy and non-invasive imaging techniques on this genotype collection, we examined anatomical leaf and stomatal traits, plant growth and development, and physiological performance at optimal (22°C) and supra-optimal (30°C) temperatures. All genotypes showed thermomorphogenetic responses but no signs of heat stress. Data analysis singled out an extremely low stomatal abundance mutant, spch-5. At 22°C, spch-5 had lower transpiration and warmer leaves than the wild type. However, at 30°C, this mutant developed larger stomata and thinner leaves, paralleled by a notable cooling capacity, similar to that of the wild type. Despite their low stomatal density (SD), spch-5 plants grown at 30°C showed no photosynthesis or growth penalties. The behavior of spch-5 at supra-optimal temperature exemplifies how the effect of very low stomatal numbers can be counteracted by a combination of larger stomata and thinner leaves. Furthermore, it provides a novel strategy for coping with high growth temperatures.

ZmSPL10/14/26 are required for epidermal hair cell fate specification on maize leaf

The epidermal hair and stomata are two types of specialized structures on the surface of plant leaves. On mature maize leaves, stomatal complexes and three types of hairs are distributed in a stereotyped pattern on the adaxial epidermis. However, the spatiotemporal relationship between epidermal hair and stomata development and the regulatory mechanisms governing their formation in maize remain largely unknown. Here, we report that three homologous ZmSPL transcription factors, ZmSPL10, ZmSPL14 and ZmSPL26, act in concert to promote epidermal hair fate on maize leaf. Cytological analyses revealed that Zmspl10/14/26 triple mutants are completely glabrous, but possess ectopic stomatal files. Strikingly, the precursor cells for prickle and bicellular hairs are transdifferentiated into ectopic stomatal complexes in the Zmspl10/14/26 mutants. Molecular analyses demonstrated that ZmSPL10/14/26 bind directly to the promoter of a WUSCHEL-related homeobox gene, ZmWOX3A, and upregulate its expression in the hair precursor cells. Moreover, several auxin-related genes are downregulated in the Zmspl10/14/26 triple mutants. Our results suggest that ZmSPL10/14/26 play a key role in promoting epidermal hair fate on maize leaves, possibly through regulating ZmWOX3A and auxin-related gene expression, and that the fates of epidermal hairs and stomata are switchable.

Shouting out loud: signaling modules in the regulation of stomatal development

Stomata are small pores on the surface of land plants that facilitate gas exchange for photosynthesis while minimizing water loss. The function of stomata is pivotal for plant growth and survival. Intensive research on the model plant Arabidopsis (Arabidopsis thaliana) has discovered key peptide signaling pathways, transcription factors, and polarity components that together drive proper stomatal development and patterning. In this review, we focus on recent findings that have revealed co-option of peptide-receptor kinase signaling modules-utilized for diverse developmental processes and immune response. We further discuss an emerging connection between extrinsic signaling and intrinsic polarity modules. These findings have further enlightened our understanding of this fascinating developmental process.

Chemical control of stomatal function and development

Stomata control trade-offs for plants: carbon dioxide uptake for photosynthetic growth and water loss via transpiration. While agrochemical control of transpiration is an old concept, recent discoveries of the core signaling components controlling stomatal function and numbers opened the door to develop chemical compounds with high potency and specificity. ABA agonists with potent anti-transpiration activity have been developed via in silico virtual screens and structure guided design and synthesis. Library-based chemical screens identified new compounds that influence stomatal movement in ABA-independent manners as well as those affecting stomatal numbers and division polarity. Subsequent hit compound derivatization can be employed to separate adverse side effects. Ultimately, such chemicals might help in optimizing plant productivity and water use in agriculture and florist industries.

N-glycosylation is involved in stomatal development by modulating the release of active abscisic acid and auxin in Arabidopsis

Asparagine-linked glycosylation (N-glycosylation) is one of the most important protein modifications in eukaryotes, affecting the folding, transport, and function of a wide range of proteins. However, little is known about the roles of N-glycosylation in the development of stomata in plants. In the present study, we provide evidence that the Arabidopsis stt3a-2 mutant, defective in oligosaccharyltransferase catalytic subunit STT3, has a greater transpirational water loss and weaker drought avoidance, accompanied by aberrant stomatal distribution. Through physiological, biochemical, and genetic analyses, we found that the abnormal stomatal density of stt3a-2 was partially attributed to low endogenous abscisic acid (ABA) and auxin (IAA) content. Exogenous application of ABA or IAA could partially rescue the mutant's salt-sensitive and abnormal stomatal phenotype. Further analyses revealed that the decrease of IAA or ABA in stt3a-2 seedlings was associated with the underglycosylation of β-glucosidase (AtBG1), catalysing the conversion of conjugated ABA/IAA to active hormone. Our results provide strong evidence that N-glycosylation is involved in stomatal development and participates in abiotic stress tolerance by modulating the release of active plant hormones.

YODA-HSP90 Module Regulates Phosphorylation-Dependent Inactivation of SPEECHLESS to Control Stomatal Development under Acute Heat Stress in Arabidopsis

Stomatal ontogenesis, patterning, and function are hallmarks of environmental plant adaptation, especially to conditions limiting plant growth, such as elevated temperatures and reduced water availability. The specification and distribution of a stomatal cell lineage and its terminal differentiation into guard cells require a master regulatory protein phosphorylation cascade involving the YODA mitogen-activated protein kinase kinase kinase. YODA signaling results in the activation of MITOGEN-ACTIVATED PROTEIN KINASEs (MPK3 and MPK6), which regulate transcription factors, including SPEECHLESS (SPCH). Here, we report that acute heat stress affects the phosphorylation and deactivation of SPCH and modulates stomatal density. By using complementary molecular, genetic, biochemical, and cell biology approaches, we provide solid evidence that HEAT SHOCK PROTEINS 90 (HSP90s) play a crucial role in transducing heat-stress response through the YODA cascade. Genetic studies revealed that YODA and HSP90.1 are epistatic, and they likely function linearly in the same developmental pathway regulating stomata formation. HSP90s interact with YODA, affect its cellular polarization, and modulate the phosphorylation of downstream targets, such as MPK6 and SPCH, under both normal and heat-stress conditions. Thus, HSP90-mediated specification and differentiation of the stomatal cell lineage couples stomatal development to environmental cues, providing an adaptive heat stress response mechanism in plants.

Light Regulation of Stomatal Development and Patterning: Shifting the Paradigm from Arabidopsis to Grasses

The stomatal pores of plant leaves control gas exchange with the environment. Stomatal development is prevised regulated by both internal genetic programs and environmental cues. Among various environmental factors, light regulation of stomata formation has been extensively studied in Arabidopsis. In this review, we summarize recent advances in the genetic control of stomata development and its regulation by light. We also present a comparative analysis of the conserved and diverged stomatal regulatory networks between Arabidopsis and cereal grasses. Lastly, we provide our perspectives on manipulation of the stomata density on plant leaves for the purpose of breeding crops that are better adapted to the adverse environment and high-density planting conditions.

A Genetic Dissection of Natural Variation for Stomatal Abundance Traits in Arabidopsis

Stomatal abundance varies widely across natural populations of Arabidopsis thaliana, and presumably affects plant performance because it influences water and CO2 exchange with the atmosphere and thence photosynthesis and transpiration. In order to determine the genetic basis of this natural variation, we have analyzed a recombinant inbred line (RIL) population derived from the wild accession Ll-0 and the reference strain Landsberg erecta (Ler), which show low and high stomatal abundance, respectively. Quantitative trait locus (QTL) analyses of stomatal index, stomatal density, and pavement cell density measured in the adaxial cotyledon epidermis, identified five loci. Three of the genomic regions affect all traits and were named MID (Modulator of Cell Index and Density) 1 to 3. MID2 is a large-effect QTL overlapping with ERECTA (ER), the er-1 allele from Ler increasing all trait values. Additional analyses of natural and induced loss-of-function er mutations in different genetic backgrounds revealed that ER dysfunctions have differential and opposite effects on the stomatal index in adaxial and abaxial cotyledon epidermis and confirmed that ER is the gene underlying MID2. Ll-0 alleles at MID1 and MID3 displayed moderate and positive effects on the various traits. Furthermore, detailed developmental studies tracking primary and satellite stomatal lineages show that MID3-Ll-0 allele promotes the spacing divisions that initiate satellite lineages, while the ER allele limits them. Finally, expression analyses suggest that ER and MID3 modulate satellization through partly different regulatory pathways. Our characterization of MID3 indicates that genetic modulation of satellization contributes to the variation for stomatal abundance in natural populations, and subsequently that this trait might be involved in plant adaptation.

Pores for Thought: Can Genetic Manipulation of Stomatal Density Protect Future Rice Yields?

Rice (Oryza sativa L.) contributes to the diets of around 3.5 billion people every day and is consumed more than any other plant. Alarmingly, climate predictions suggest that the frequency of severe drought and high-temperature events will increase, and this is set to threaten the global rice supply. In this review, we consider whether water or heat stresses in crops - especially rice - could be mitigated through alterations to stomata; minute pores on the plant epidermis that permit carbon acquisition and regulate water loss. In the short-term, water loss is controlled via alterations to the degree of stomatal "openness", or, in the longer-term, by altering the number (or density) of stomata that form. A range of molecular components contribute to the regulation of stomatal density, including transcription factors, plasma membrane-associated proteins and intercellular and extracellular signaling molecules. Much of our existing knowledge relating to stomatal development comes from research conducted on the model plant, Arabidopsis thaliana. However, due to the importance of cereal crops to global food supply, studies on grass stomata have expanded in recent years, with molecular-level discoveries underscoring several divergent developmental and morphological features. Cultivation of rice is particularly water-intensive, and there is interest in developing varieties that require less water yet still maintain grain yields. This could be achieved by manipulating stomatal development; a crop with fewer stomata might be more conservative in its water use and therefore more capable of surviving periods of water stress. However, decreasing stomatal density might restrict the rate of CO₂ uptake and reduce the extent of evaporative cooling, potentially leading to detrimental effects on yields. Thus, the extent to which crop yields in the future climate will be affected by increasing or decreasing stomatal density should be determined. Here, our current understanding of the regulation of stomatal development is summarised, focusing particularly on the genetic mechanisms that have recently been described for rice and other grasses. Application of this knowledge toward the creation of "climate-ready" rice is discussed, with attention drawn to the lesser-studied molecular elements whose contributions to the complexity of grass stomatal development must be understood to advance efforts.