KIN10 promotes stomatal development through stabilization of the SPEECHLESS transcription factor

A natural variation in OsDSK2a modulates plant growth and salt tolerance through phosphorylation by SnRK1A in rice

Plants grow rapidly for maximal production under optimal conditions; however, they adopt a slower growth strategy to maintain survival when facing environmental stresses. As salt stress restricts crop architecture and grain yield, identifying genetic variations associated with growth and yield responses to salinity is critical for breeding optimal crop varieties. OsDSK2a is a pivotal modulator of plant growth and salt tolerance via the modulation of gibberellic acid (GA) metabolism; however, its regulation remains unclear. Here, we showed that OsDSK2a can be phosphorylated at the second amino acid (S2) to maintain its stability. The gene-edited mutant osdsk2aS2G showed decreased plant height and enhanced salt tolerance. SnRK1A modulated OsDSK2a-S2 phosphorylation and played a substantial role in GA metabolism. Genetic analysis indicated that SnRK1A functions upstream of OsDSK2a and affects plant growth and salt tolerance. Moreover, SnRK1A activity was suppressed under salt stress, resulting in decreased phosphorylation and abundance of OsDSK2a. Thus, SnRK1A preserves the stability of OsDSK2a to maintain plant growth under normal conditions, and reduces the abundance of OsDSK2a to limit growth under salt stress. Haplotype analysis using 3 K-RG data identified a natural variation in OsDSK2a-S2. The allele of OsDSK2a-G downregulates plant height and improves salt-inhibited grain yield. Thus, our findings revealed a new mechanism for OsDSK2a stability and provided a valuable target for crop breeding to overcome yield limitations under salinity stress.

Hydrogen peroxide is required for light-induced stomatal opening across different plant species

Stomatal movement is vital for plants to exchange gases and adaption to terrestrial habitats, which is regulated by environmental and phytohormonal signals. Here, we demonstrate that hydrogen peroxide (H2O2) is required for light-induced stomatal opening. H2O2 accumulates specifically in guard cells even when plants are under unstressed conditions. Reducing H2O2 content through chemical treatments or genetic manipulations results in impaired stomatal opening in response to light. This phenomenon is observed across different plant species, including lycopodium, fern, and monocotyledonous wheat. Additionally, we show that H2O2 induces the nuclear localization of KIN10 protein, the catalytic subunit of plant energy sensor SnRK1. The nuclear-localized KIN10 interacts with and phosphorylates the bZIP transcription factor bZIP30, leading to the formation of a heterodimer between bZIP30 and BRASSINAZOLE-RESISTANT1 (BZR1), the master regulator of brassinosteroid signaling. This heterodimer complex activates the expression of amylase, which enables guard cell starch degradation and promotes stomatal opening. Overall, these findings suggest that H2O2 plays a critical role in light-induced stomatal opening across different plant species.

Adaptive divergence, historical population dynamics, and simulation of suitable distributions for Picea Meyeri and P. Mongolica at the whole-genome level

The taxonomic classification of Picea meyeri and P. mongolica has long been controversial. To investigate the genetic relatedness, evolutionary history, and population history dynamics of these species, genotyping-by-sequencing (GBS) technology was utilized to acquire whole-genome single nucleotide polymorphism (SNP) markers, which were subsequently used to assess population structure, population dynamics, and adaptive differentiation. Phylogenetic and population structural analyses at the genomic level indicated that although the ancestor of P. mongolica was a hybrid of P. meyeri and P. koraiensis, P. mongolica is an independent Picea species. Additionally, P. mongolica is more closely related to P. meyeri than to P. koraiensis, which is consistent with its geographic distribution. There were up to eight instances of interspecific and intraspecific gene flow between P. meyeri and P. mongolica. The P. meyeri and P. mongolica effective population sizes generally decreased, and Maxent modeling revealed that from the Last Glacial Maximum (LGM) to the present, their habitat areas decreased initially and then increased. However, under future climate scenarios, the habitat areas of both species were projected to decrease, especially under high-emission scenarios, which would place P. mongolica at risk of extinction and in urgent need of protection. Local adaptation has promoted differentiation between P. meyeri and P. mongolica. Genotype‒environment association analysis revealed 96,543 SNPs associated with environmental factors, mainly related to plant adaptations to moisture and temperature. Selective sweeps revealed that the selected genes among P. meyeri, P. mongolica and P. koraiensis are primarily associated in vascular plants with flowering, fruit development, and stress resistance. This research enhances our understanding of Picea species classification and provides a basis for future genetic improvement and species conservation efforts.

KIN10-mediated HB16 protein phosphorylation and self-association improve cassava disease resistance by transcriptional activation of lignin biosynthesis genes

Cassava bacterial blight significantly affects cassava yield worldwide, while major cassava cultivars are susceptible to this disease. Therefore, it is crucial to identify cassava disease resistance gene networks and defence molecules for the genetic improvement of cassava cultivars. In this study, we found that MeHB16 transcription factor as a differentially expressed gene in cassava cultivars with contrasting disease resistance, positively modulated disease resistance by modulating defence molecule lignin accumulation. Further investigation showed that MeHB16 physically interacted with itself via the leucine-Zippe domain (L-Zip), which was necessary for the transcriptional activation of downstream lignin biosynthesis genes. In addition, protein kinase MeKIN10 directly interacted with MeHB16 to promote its phosphorylation at Ser6, which in turn enhanced MeHB16 self-association and downstream lignin biosynthesis. In summary, this study revealed the molecular network of MeKIN10-mediated MeHB16 protein phosphorylation improved cassava bacterial blight resistance by fine-tuning lignin biosynthesis and provides candidate genes and the defence molecule for improving cassava disease resistance.

GRIK phosphorylates and activates KIN10 which also promotes its degradation

The sensor kinase Sucrose Non-fermenting-1-Related Kinase 1 (SnRK1) plays a central role in energy and metabolic homeostasis. KIN10 is a major catalytic (α) kinase subunit of SnRK1 regulated by transcription, posttranslational modification, targeted protein degradation, and its subcellular localization. Geminivirus Rep Interacting Kinase 1 and 2 (GRIK1 and 2) are immediate upstream kinases of KIN10. In the transient protein expression assays carried out in Nicotiana benthamiana (N. benthamiana) leaves, GRIK1 not only phosphorylates KIN10 but also simultaneously initiates its degradation. Posttranslational GRIK-mediated KIN10 degradation is dependent on both GRIK kinase activity and phosphorylation of the KIN10 T-loop. KIN10 proteins are significantly enriched in the grik1-1 grik2-1 double mutant, consistent with the transient assays in N. benthamiana. Interestingly. Among the enriched KIN10 proteins from grik1-1 grik2-1, is a longer isoform, putatively derived by alternative splicing which is barely detectable in wild-type plants. The reduced stability of KIN10 upon phosphorylation and activation by GRIK represents a mechanism that enables the KIN10 activity to be rapidly reduced when the levels of intracellular sugar/energy are restored to their set point, representing an important homeostatic control that prevents a metabolic overreaction to low-sugar conditions. Since GRIKs are activating kinases of KIN10, KIN10s in the grik1 grik2 double null mutant background remain un-phosphorylated, with only their basal level of activity, are more stable, and therefore increase in abundance, which also explains the longer isoform KIN10L which is a minor isoform in wild type is clearly detected in the grik1 grik2 double mutant.

Stomatal density suppressor PagSDD1 is a "generalist" gene that promotes plant growth and improves water use efficiency

The serine protease SDD1 regulates stomatal density, but its potential impact on plant vegetative growth is unclear. Our study reveals a substantial upregulation of SDD1 in triploid poplar apical buds and leaves, suggesting its possible role in their growth regulation. We cloned PagSDD1 from poplar 84 K (Populus alba × P. glandulosa) and found that overexpression in poplar, soybean, and lettuce led to decreased leaf stomatal density. Furthermore, PagSDD1 represses PagEPF1, PagEPF2, PagEPFL9, PagSPCH, PagMUTE, and PagFAMA expression. In contrast, PagSDD1 promotes the expression of its receptors, PagTMM and PagERECTA. PagSDD1-OE poplars showed stronger drought tolerance than wild-type poplars. Simultaneously, PagSDD1-OE poplar, soybean, and lettuce had vegetative growth advantages. RNA sequencing revealed a significant upregulation of genes PagLHCB2.1 and PagGRF5, correlating positively with photosynthetic rate, and PagCYCA3;4 and PagEXPA8 linked to cell division and differentiation in PagSDD1-OE poplars. This increase promoted leaf photosynthesis, boosted auxin and cytokinin accumulation, and enhanced vegetative growth. SDD1 overexpression can increase the biomass of poplar, soybean, and lettuce by approximately 70, 176, and 155 %, respectively, and increase the water use efficiency of poplar leaves by over 52 %, which is of great value for the molecular design and breeding of plants with growth and water-saving target traits.

Reactive oxygen species: Multidimensional regulators of plant adaptation to abiotic stress and development

Reactive oxygen species (ROS) are produced as undesirable by-products of metabolism in various cellular compartments, especially in response to unfavorable environmental conditions, throughout the life cycle of plants. Stress-induced ROS production disrupts normal cellular function and leads to oxidative damage. To cope with excessive ROS, plants are equipped with a sophisticated antioxidative defense system consisting of enzymatic and non-enzymatic components that scavenge ROS or inhibit their harmful effects on biomolecules. Nonetheless, when maintained at relatively low levels, ROS act as signaling molecules that regulate plant growth, development, and adaptation to adverse conditions. Here, we provide an overview of current approaches for detecting ROS. We also discuss recent advances in understanding ROS signaling, ROS metabolism, and the roles of ROS in plant growth and responses to various abiotic stresses.

XYLEM CYSTEINE PEPTIDASE 1 and its inhibitor CYSTATIN 6 regulate pattern-triggered immunity by modulating the stability of the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D

Plants produce a burst of reactive oxygen species (ROS) after pathogen infection to successfully activate immune responses. During pattern-triggered immunity (PTI), ROS are primarily generated by the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD). RBOHD is degraded in the resting state to avoid inappropriate ROS production; however, the enzyme mediating RBOHD degradation and how to prevent RBOHD degradation after pathogen infection is unclear. In this study, we identified an Arabidopsis (Arabidopsis thaliana) vacuole-localized papain-like cysteine protease, XYLEM CYSTEINE PEPTIDASE 1 (XCP1), and its inhibitor CYSTATIN 6 (CYS6). Pathogen-associated molecular pattern-induced ROS burst and resistance were enhanced in the xcp1 mutant but were compromised in the cys6 mutant, indicating that XCP1 and CYS6 oppositely regulate PTI responses. Genetic and biochemical analyses revealed that CYS6 interacts with XCP1 and depends on XCP1 to enhance PTI. Further experiments showed that XCP1 interacts with RBOHD and accelerates RBOHD degradation in a vacuole-mediated manner. CYS6 inhibited the protease activity of XCP1 toward RBOHD, which is critical for RBOHD accumulation upon pathogen infection. As CYS6, XCP1, and RBOHD are conserved in all plant species tested, our findings suggest the existence of a conserved strategy to precisely regulate ROS production under different conditions by modulating the stability of RBOHD.

Functions of sucrose and trehalose 6-phosphate in controlling plant development

Plants exhibit enormous plasticity in regulating their architecture to be able to adapt to a constantly changing environment and carry out vital functions such as photosynthesis, anchoring, and nutrient uptake. Phytohormones play a role in regulating these responses, but sugar signalling mechanisms are also crucial. Sucrose is not only an important source of carbon and energy fuelling plant growth, but it also functions as a signalling molecule that influences various developmental processes. Trehalose 6-phosphate (Tre6P), a sucrose-specific signalling metabolite, is emerging as an important regulator in plant metabolism and development. Key players involved in sucrose and Tre6P signalling pathways, including MAX2, SnRK1, bZIP11, and TOR, have been implicated in processes such as flowering, branching, and root growth. We will summarize our current knowledge of how these pathways shape shoot and root architecture and highlight how sucrose and Tre6P signalling are integrated with known signalling networks in shaping plant growth.

Update on stomata development and action under abiotic stress

Stomata, key gatekeepers of plant hydration, have long been known to play a pivotal role in mitigating the impacts of abiotic stressors. However, the complex molecular mechanisms underscoring this role remain unresolved fully and continue to be the subject of research. In the context of water-use efficiency (WUE), a key indicator of a plant's ability to conserve water, this aspect links intrinsically with stomatal behavior. Given the pivotal role of stomata in modulating water loss, it can be argued that the complex mechanisms governing stomatal development and function will significantly influence a plant's WUE under different abiotic stress conditions. Addressing these calls for a concerted effort to strengthen plant adaptability through advanced, targeted research. In this vein, recent studies have illuminated how specific stressors trigger alterations in gene expression, orchestrating changes in stomatal pattern, structure, and opening. This reveals a complex interplay between stress stimuli and regulatory sequences of essential genes implicated in stomatal development, such as MUTE, SPCH, and FAMA. This review synthesizes current discoveries on the molecular foundations of stomatal development and behavior in various stress conditions and their implications for WUE. It highlights the imperative for continued exploration, as understanding and leveraging these mechanisms guarantee enhanced plant resilience amid an ever-changing climatic landscape.

A cell size threshold triggers commitment to stomatal fate in Arabidopsis

How flexible developmental programs integrate information from internal and external factors to modulate stem cell behavior is a fundamental question in developmental biology. Cells of the Arabidopsis stomatal lineage modify the balance of stem cell proliferation and differentiation to adjust the size and cell type composition of mature leaves. Here, we report that meristemoids, one type of stomatal lineage stem cell, trigger the transition from asymmetric self-renewing divisions to commitment and terminal differentiation by crossing a critical cell size threshold. Through computational simulation, we demonstrate that this cell size-mediated transition allows robust, yet flexible termination of stem cell proliferation, and we observe adjustments in the number of divisions before the differentiation threshold under several genetic manipulations. We experimentally evaluate several mechanisms for cell size sensing, and our data suggest that this stomatal lineage transition is dependent on a nuclear factor that is sensitive to DNA content.

A Soybean Sucrose Non-Fermenting Protein Kinase 1 Gene, GmSNF1, Positively Regulates Plant Response to Salt and Salt-Alkali Stress in Transgenic Plants

Soybean is one of the most widely grown oilseed crops worldwide. Several unfavorable factors, including salt and salt-alkali stress caused by soil salinization, affect soybean yield and quality. Therefore, exploring the molecular basis of salt tolerance in plants and developing genetic resources for genetic breeding is important. Sucrose non-fermentable protein kinase 1 (SnRK1) belongs to a class of Ser/Thr protein kinases that are evolutionarily highly conserved direct homologs of yeast SNF1 and animal AMPKs and are involved in various abiotic stresses in plants. The GmPKS4 gene was experimentally shown to be involved with salinity tolerance. First, using the yeast two-hybrid technique and bimolecular fluorescence complementation (BiFC) technique, the GmSNF1 protein was shown to interact with the GmPKS4 protein. Second, the GmSNF1 gene responded positively to salt and salt-alkali stress according to qRT-PCR analysis, and the GmSNF1 protein was localized in the nucleus and cytoplasm using subcellular localization assay. The GmSNF1 gene was then heterologously expressed in yeast, and the GmSNF1 gene was tentatively identified as having salt and salt-alkali tolerance function. Finally, the salt-alkali tolerance function of the GmSNF1 gene was demonstrated by transgenic Arabidopsis thaliana, soybean hairy root complex plants overexpressing GmSNF1 and GmSNF1 gene-silenced soybean using VIGS. These results indicated that GmSNF1 might be useful in genetic engineering to improve plant salt and salt-alkali tolerance.

Sugar status in preexisting leaves determines systemic stomatal development within newly developing leaves

Stomata are pores found in the epidermis of stems or leaves that modulate both plant gas exchange and water/nutrient uptake. The development and function of plant stomata are regulated by a diverse range of environmental cues. However, how carbohydrate status in preexisting leaves might determine systemic stomatal formation within newly developing leaves has remained obscure. The glucose (Glc) sensor HEXOKINASE1 (HXK1) has been reported to decrease the stability of an ethylene/Glc signaling transcriptional regulator, EIN3 (ETHYLENE INSENSITIVE3). EIN3 in turn directly represses the expression of SUC2 (sucrose transporter 2), encoding a master transporter of sucrose (Suc). Further, KIN10, a nuclear regulator involved in energy homeostasis, has been reported to repress the transcription factor SPCH (SPEECHLESS), a master regulator of stomatal development. Here, we demonstrate that the Glc status of preexisting leaves determines systemic stomatal development within newly developing leaves by the HXK1-¦EIN3-¦SUC2 module. Further, increasing Glc levels in preexisting leaves results in a HXK1-dependent decrease of EIN3 and increase of SUC2, triggering the perception, amplification and relay of HXK1-dependent Glc signaling and thereby triggering Suc transport from mature to newly developing leaves. The HXK1-¦EIN3-¦SUC2 molecular module thereby drives systemic Suc transport from preexisting leaves to newly developing leaves. Subsequently, increasing Suc levels within newly developing leaves promotes stomatal formation through the established KIN10⟶ SPCH module. Our findings thus show how a carbohydrate signal in preexisting leaves is sensed, amplified and relayed to determine the extent of systemic stomatal development within newly developing leaves.

Asymmetric cell division in plant development

Asymmetric cell division (ACD) is a fundamental process that generates new cell types during development in eukaryotic species. In plant development, post-embryonic organogenesis driven by ACD is universal and more important than in animals, in which organ pattern is preset during embryogenesis. Thus, plant development provides a powerful system to study molecular mechanisms underlying ACD. During the past decade, tremendous progress has been made in our understanding of the key components and mechanisms involved in this important process in plants. Here, we present an overview of how ACD is determined and regulated in multiple biological processes in plant development and compare their conservation and specificity among different model cell systems. We also summarize the molecular roles and mechanisms of the phytohormones in the regulation of plant ACD. Finally, we conclude with the overarching paradigms and principles that govern plant ACD and consider how new technologies can be exploited to fill the knowledge gaps and make new advances in the field.

Emerging roles of protein phosphorylation in regulation of stomatal development

Stomata, tiny epidermal spores, control gas exchange between plants and their external environment, thereby playing essential roles in plant development and physiology. Stomatal development requires rapid regulation of components in signaling pathways to respond flexibly to numerous intrinsic and extrinsic signals. In support of this, reversible phosphorylation, which is particularly suitable for rapid signal transduction, has been implicated in this process. This review highlights the current understanding of the essential roles of reversible phosphorylation in the regulation of stomatal development, most of which comes from the dicot Arabidopsis thaliana. Protein phosphorylation tightly controls the activity of SPEECHLESS (SPCH)-SCREAM (SCRM), the stomatal lineage switch, and the activity of several mitogen-activated protein kinases and receptor kinases upstream of SPCH-SCRM, thereby regulating stomatal cell differentiation and patterning. In addition, protein phosphorylation is involved in the establishment of cell polarity during stomatal asymmetric cell division. Finally, cyclin-dependent kinase-mediated protein phosphorylation plays essential roles in cell cycle control during stomatal development.

Functional characterization of wild soybean (Glycine soja) GsSnRK1.1 protein kinase in plant resistance to abiotic stresses

Protein kinases play crucial roles in the regulation of plant resistance to various stresses. In this work, we determined that GsSnRK1.1 was actively responsive to saline-alkali, drought, and abscisic acid (ABA) stresses by histochemical staining and qRT-PCR analyses. The wild-type GsSnRK1.1 but not the kinase-dead mutant, GsSnRK1.1(K49M), demonstrated in vitro kinase activity by phosphorylating GsABF2. Intriguingly, we found that GsSnRK1.1 could complement the loss of SNF1 kinase in yeast Msy1193 (-snf1) mutant, rescue growth defects of yeast cells on medium with glycerol as a carbon resource, and promote yeast resistance to NaCl or NaHCO₃. To further elucidate GsSnRK1.1 function in planta, we knocked out SnRK1.1 gene from the Arabidopsis genome by the CRISPR/Cas9 approach, and then expressed GsSnRK1.1 and a series of mutants into snrk1.1-null lines. The transgenic Arabidopsis lines were subjected to various abiotic stress treatments. The results showed that GsSnRK1.1(T176E) mutant with enhanced protein kinase activity significantly promoted, but GsSnRK1.1(K49M) and GsSnRK1.1(T176A) mutants with disrupted protein kinase activity abrogated, plant stomatal closure and tolerance to abiotic stresses. In conclusion, this study provides the molecular clues to fully understand the physiological functions of plant SnRK1 protein kinases.

Compartmentalization, a key mechanism controlling the multitasking role of the SnRK1 complex

SNF1-related protein kinase 1 (SnRK1), the plant ortholog of mammalian AMP-activated protein kinase/fungal (yeast) Sucrose Non-Fermenting 1 (AMPK/SNF1), plays a central role in metabolic responses to reduced energy levels in response to nutritional and environmental stresses. SnRK1 functions as a heterotrimeric complex composed of a catalytic α- and regulatory β- and βγ-subunits. SnRK1 is a multitasking protein involved in regulating various cellular functions, including growth, autophagy, stress response, stomatal development, pollen maturation, hormone signaling, and gene expression. However, little is known about the mechanism whereby SnRK1 ensures differential execution of downstream functions. Compartmentalization has been recently proposed as a new key mechanism for regulating SnRK1 signaling in response to stimuli. In this review, we discuss the multitasking role of SnRK1 signaling associated with different subcellular compartments.

SnRK1A-mediated phosphorylation of a cytosolic ATPase positively regulates rice innate immunity and is inhibited by Ustilaginoidea virens effector SCRE1

Rice false smut caused by Ustilaginoidea virens is becoming one of the most recalcitrant rice diseases worldwide. However, the molecular mechanisms underlying rice immunity against U. virens remain unknown. Using genetic, biochemical and disease resistance assays, we demonstrated that the xb24 knockout lines generated in non-Xa21 rice background exhibit an enhanced susceptibility to the fungal pathogens U. virens and Magnaporthe oryzae. Consistently, flg22- and chitin-induced oxidative burst and expression of pathogenesis-related genes in the xb24 knockout lines were greatly attenuated. As a central mediator of energy signaling, SnRK1A interacts with and phosphorylates XB24 at Thr83 residue to promote ATPase activity. SnRK1A is activated by pathogen-associated molecular patterns and positively regulates plant immune responses and disease resistance. Furthermore, the virulence effector SCRE1 in U. virens targets host ATPase XB24. The interaction inhibits ATPase activity of XB24 by blocking ATP binding to XB24. Meanwhile, SCRE1 outcompetes SnRK1A for XB24 binding, and thereby suppresses SnRK1A-mediated phosphorylation and ATPase activity of XB24. Our results indicate that the conserved SnRK1A-XB24 module in multiple crop plants positively contributes to plant immunity and uncover an unidentified molecular strategy to promote infection in U. virens and a novel host target in fungal pathogenesis.

Identification and characteristics of SnRK genes and cold stress-induced expression profiles in Liriodendron chinense

Background

The sucrose non-fermenting 1 (SNF1)-related protein kinases (SnRKs) play a vivid role in regulating plant metabolism and stress response, providing a pathway for regulation between metabolism and stress signals. Conducting identification and stress response studies on SnRKs in plants contributes to the development of strategies for tree species that are more tolerant to stress conditions.

Results

In the present study, a total of 30 LcSnRKs were identified in Liriodendron chinense (L. chinense) genome, which was distributed across 15 chromosomes and 4 scaffolds. It could be divided into three subfamilies: SnRK1, SnRK2, and SnRK3 based on phylogenetic analysis and domain types. The LcSnRK of the three subfamilies shared the same Ser/Thr kinase structure in gene structure and motif composition, while the functional domains, except for the kinase domain, showed significant differences. A total of 13 collinear gene pairs were detected in L. chinense and Arabidopsis thaliana (A. thaliana), and 18 pairs were detected in L. chinense and rice, suggesting that the LcSnRK family genes may be evolutionarily more closely related to rice. Cis-regulation element analysis showed that LcSnRKs were LTR and TC-rich, which could respond to different environmental stresses. Furthermore, the expression patterns of LcSnRKs are different at different times under low-temperature stress. LcSnRK1s expression tended to be down-regulated under low-temperature stress. The expression of LcSnRK2s tended to be up-regulated under low-temperature stress. The expression trend of LcSnRK3s under low-temperature stress was mainly up-or down-regulated.

Conclusion

The results of this study will provide valuable information for the functional identification of the LcSnRK gene in the future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08902-0.

Abscisic acid regulates stomatal production by imprinting a SnRK2 kinase-mediated phosphocode on the master regulator SPEECHLESS

Stomata, the epidermal pores for gas exchange between plants and the atmosphere, are the major sites of water loss. During water shortage, plants limit the formation of new stoma via the phytohormone abscisic acid (ABA) to conserve water. However, how ABA suppresses stomatal production is largely unknown. Here, we demonstrate that three core SnRK2 kinases of ABA signaling inhibit the initiation and proliferation of the stomatal precursors in Arabidopsis. We show that the SnRK2s function within the precursors and directly phosphorylate SPEECHLESS (SPCH), the master transcription factor for stomatal initiation. We identify specific SPCH residues targeted by the SnRK2s, which mediate the ABA/drought-induced suppression of SPCH and stomatal production. This SnRK2-specific SPCH phosphocode connects stomatal development with ABA/drought signals and enables the independent control of this key water conservation response. Our work also highlights how distinct signaling activities can be specifically encoded on a master regulator to modulate developmental plasticity.

Overexpression of PagSTOMAGEN, a Positive Regulator of Stomatal Density, Promotes Vegetative Growth in Poplar

Poplar is an important fast-growing tree, and its photosynthetic capacity directly affects its vegetative growth. Stomatal density is closely related to photosynthetic capacity and growth characteristics in plants. Here, we isolated PagSTOMAGEN from the hybrid poplar (Populus alba × Populus glandulosa) clone 84K and investigated its biological function in vegetative growth. PagSTOMAGEN was expressed predominantly in young tissues and localized in the plasma membrane. Compared with wild-type 84K poplars, PagSTOMAGEN-overexpressing plants displayed an increased plant height, leaf area, internode number, basal diameter, biomass, IAA content, IPR content, and stomatal density. Higher stomatal density improved the net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, and transpiration rate in transgenic poplar. The differential expression of genes related to stomatal development showed a diverged influence of PagSTOMAGEN at different stages of stomatal development. Finally, transcriptomic analysis showed that PagSTOMAGEN affected vegetative growth by affecting the expression of photosynthesis and plant hormone-related genes (such as SAUR75, PQL2, PSBX, ERF1, GNC, GRF5, and ARF11). Taken together, our data indicate that PagSTOMAGEN could positively regulate stomatal density and increase the photosynthetic rate and plant hormone content, thereby promoting vegetative growth in poplar. Our study is of great significance for understanding the relationship between stoma, photosynthesis, and yield breeding in poplar.

Regulatory functions of cellular energy sensor SnRK1 for nitrate signalling through NLP7 repression

The coordinated metabolism of carbon and nitrogen is essential for optimal plant growth and development. Nitrate is an important molecular signal for plant adaptation to a changing environment, but how nitrate regulates plant growth under carbon deficiency conditions remains unclear. Here we show that the evolutionarily conserved energy sensor SnRK1 negatively regulates the nitrate signalling pathway. Nitrate promoted plant growth and downstream gene expression, but such effects were repressed when plants were grown under carbon deficiency conditions. Mutation of KIN10, the α-catalytic subunit of SnRK1, partially suppressed the inhibitory effects of carbon deficiency on nitrate-mediated plant growth. KIN10 phosphorylated NLP7, the master regulator of the nitrate signalling pathway, to promote its cytoplasmic localization and degradation. Furthermore, nitrate depletion induced KIN10 accumulation, whereas nitrate treatment promoted KIN10 degradation. Such KIN10-mediated NLP7 regulation allows carbon and nitrate availability to control optimal nitrate signalling and ensures the coordination of carbon and nitrogen metabolism in plants.

Spatially patterned hydrogen peroxide orchestrates stomatal development in Arabidopsis

Stomatal pores allow gas exchange between plant and atmosphere. Stomatal development is regulated by multiple intrinsic developmental and environmental signals. Here, we show that spatially patterned hydrogen peroxide (H₂O₂) plays an essential role in stomatal development. H₂O₂ is remarkably enriched in meristemoids, which is established by spatial expression patterns of H₂O₂-scavenging enzyme CAT2 and APX1. SPEECHLESS (SPCH), a master regulator of stomatal development, directly binds to the promoters of CAT2 and APX1 to repress their expression in meristemoid cells. Mutations in CAT2 or APX1 result in an increased stomatal index. Ectopic expression of CAT2 driven by SPCH promoter significantly inhibits the stomatal development. Furthermore, H₂O₂ activates the energy sensor SnRK1 by inducing the nuclear localization of the catalytic α-subunit KIN10, which stabilizes SPCH to promote stomatal development. Overall, these results demonstrate that the spatial pattern of H₂O₂ in epidermal leaves is critical for the optimal stomatal development in Arabidopsis.

Stomatal development is regulated by multiple intrinsic developmental and environmental signals. Here, the authors show that spatially patterned hydrogen peroxide activates the energy sensor SnRK1 to stabilize the SPCH transcription factor and optimize stomatal development in Arabidopsis.

Trehalose alleviates salt tolerance by improving photosynthetic performance and maintaining mineral ion homeostasis in tomato plants

Trehalose (Tre), which was an osmoprotective or stabilizing molecule, played a protective role against different abiotic stresses in plants and showed remarkable perspectives in salt stress. In this study, the potential role of Tre in improving the resistance to salt stress in tomato plants was investigated. Tomato plants (Micro Tom) were treated with Hoagland nutrient solution (CK), 10 mM Tre (T), 150 mM sodium chloride (NaCl, S), and 10 mM Tre+150 mM NaCl (S+T) for 5 days. Our results showed that foliar application of Tre alleviated the inhibition of tomato plant growth under salt stress. In addition, salt stress decreased the values of net photosynthetic rate (Pn, 85.99%), stomata conductance (gs, 57.3%), and transpiration rate (Tr, 47.97%), but increased that of intercellular carbon dioxide concentration (Ci, 26.25%). However, exogenous application of Tre significantly increased photosynthetic efficiency, increased the activity of Calvin cycle enzymes [ribulose diphosphate carboxylase/oxygenase (Rubisco), fructose-1,6-bisphosphate aldolase (FBA), fructose-1, 6-bisphosphatase (FBPase), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and transketolase (TK)], up-regulated the expression of genes encoding enzymes, induced stomatal opening, and alleviated salt-induced damage to the chloroplast membrane and structure. In the saline environment, photosynthetic electron transport was restricted, resulting the J-I-P phase to decrease. At the same time, the absorption, capture, and transport energies per excited cross-section and per active reaction center decreased, and the dissipation energy increased. Conversely, Tre reversed these values and enhanced the photosystem response to salt stress by protecting the photosynthetic electron transport system. In addition, foliage application with Tre significantly increased the potassium to sodium transport selectivity ratio (S _K-Na ) by 16.08%, and increased the levels of other ions to varying degrees. Principal component analysis (PCA) analysis showed that exogenous Tre could change the distribution of elements in different organs and affect the expressions of SlSOS1, SlNHX, SlHKT1.1, SlVHA, and SlHA-A at the transcriptional level under salt stress, thereby maintaining ion homeostasis. This study demonstrated that Tre was involved in the process of mitigating salt stress toxicity in tomato plants and provided specific insights into the effectiveness of Tre in mediating salt tolerance.

Genome-wide association study revealed TaHXK3-2A as a candidate gene controlling stomatal index in wheat seedlings

Stomata are important channels for the control of gas exchange between plants and the atmosphere. To examine the genetic architecture of wheat stomatal index, we performed a genome-wide association study (GWAS) using a panel of 539 wheat accessions and 450 678 polymorphic single nucleotide polymorphisms (SNPs) that were detected using wheat-specific 660K SNP array. A total of 130 SNPs were detected to be significantly associated with stomatal index in both leaf surfaces of wheat seedlings. These significant SNPs were distributed across 16 chromosomes and involved 2625 candidate genes which participate in stress response, metabolism and cell/organ development. Subsequent bulk segregant analysis (BSA), combined with GWAS identified one major haplotype on chromosome 2A, that is responsible for stomatal index on the abaxial leaf surface. Candidate gene association analysis revealed that genetic variation in the promoter region of the hexokinase gene TaHXK3-2A was significantly associated with the stomatal index. Moreover, transgenic analysis confirmed that TaHXK3-2A overexpression in wheat decreased the size of leaf pavement cells but increased stomatal density through the glucose metabolic pathway, resulting in drought sensitivity among TaHXK3-2A transgenic lines due to an increased transpiration rate. Taken together, these results provide valuable insights into the genetic control of the stomatal index in wheat seedlings.

Overexpression of PpSnRK1α in Tomato Increased Autophagy Activity under Low Nutrient Stress

Plants suffer from a variety of environmental stresses during their growth and development. The evolutionarily conserved sucrose nonfermenting kinase 1-related protein kinase 1 (SnRK1) plays a central role in the regulation of energy homeostasis in response to stresses. In plant cells, autophagy is a degradation process occurring during development or under stress, such as nutrient starvation. In recent years, SnRK1 signaling has been reported to be an upstream activator of autophagy. However, these studies all focused on the regulatory effect of SnRK1 on TOR signaling and the autophagy-related gene 1 (ATG1) complex. In this study, overexpression of the gene encoding the Prunus persica SnRK1 α subunit (PpSnRK1α) in tomato improved the photosynthetic rates and enhanced the resistance to low nutrient stress (LNS). Overexpression of PpSnRK1α increased autophagy activity and upregulated the expression of seven autophagy-related genes (ATGs). The transcriptional levels of SlSnRK2 family genes were altered significantly by PpSnRK1α, signifying that PpSnRK1α may be involved in the ABA signaling pathway. Further analysis showed that PpSnRK1α not only activated autophagy by inhibiting target of rapamycin (TOR) signaling but also enhanced ABA-induced autophagy. This indicates that PpSnRK1α regulates the photosynthetic rate and induces autophagy, and then responds to low nutrient stress.

TOR and SnRK1 fine tune SPEECHLESS transcription and protein stability to optimize stomatal development in response to exogenously supplied sugar

In Arabidopsis, the differentiation of epidermal cells into stomata is regulated by endogenous and environmental signals. Sugar is required for plant epidermal cell proliferation and differentiation. However, it is unclear how epidermal cells maintain division and differentiation to generate proper amounts of stomata in response to different sugar availability. Here, we show that two evolutionarily conserved kinase Snf1-related protein kinase 1 (SnRK1) and Target of rapamycin (TOR) play critical roles in the regulation of stomatal development under different sugar availability. When plants are grown on a medium containing 1% sucrose, sucrose-activated TOR promotes the stomatal development by inducing the expression of SPEECHLESS (SPCH), a master regulator of stomatal development. SnRK1 promotes stomatal development through phosphorylating and stabilizing SPCH. However, under the high sucrose conditions, the highly accumulated trehalose-6-phosphate (Tre6P) represses the activity of KIN10, the catalytic α-subunit of SnRK1, by reducing the interaction between KIN10 and its upstream kinase, consequently promoting SPCH degradation and inhibiting stomatal development. Our findings revealed that TOR and SnRK1 finely regulate SPCH expression and protein stability to optimize the stomatal development in response to exogenously supplied sugar.

Dynamic epigenetic modifications in plant sugar signal transduction

In eukaryotes, dynamic chromatin states are closely related to changes in gene expression. Epigenetic modifications help plants adapt to their ever-changing environment by modulating gene expression via covalent modification at specific sites on DNA or histones. Sugars provide energy, but also function as signaling molecules to control plant growth and development. Various epigenetic modifications participate in sensing and transmitting sugar signals. Here we summarize recent progress in uncovering the epigenetic mechanisms involved in sugar signal transduction, including histone acetylation and deacetylation, histone methylation and demethylation, and DNA methylation. We also highlight changes in chromatin marks when crosstalk occurs between sugar signaling and the light, temperature, and phytohormone signaling pathways, and describe potential questions and approaches for future research.

TOR promotes guard cell starch degradation by regulating the activity of β-AMYLASE1 in Arabidopsis

Starch is the main energy storage carbohydrate in plants and serves as an essential carbon storage molecule for plant metabolism and growth under changing environmental conditions. The TARGET of RAPAMYCIN (TOR) kinase is an evolutionarily conserved master regulator that integrates energy, nutrient, hormone, and stress signaling to regulate growth in all eukaryotes. Here, we demonstrate that TOR promotes guard cell starch degradation and induces stomatal opening in Arabidopsis thaliana. Starvation caused by plants growing under short photoperiod or low light photon irradiance, as well as inactivation of TOR, impaired guard cell starch degradation and stomatal opening. Sugar and TOR induce the accumulation of β-AMYLASE1 (BAM1), which is responsible for starch degradation in guard cells. The plant steroid hormone brassinosteroid and transcription factor BRASSINAZOLE-RESISTANT1 play crucial roles in sugar-promoted expression of BAM1. Furthermore, sugar supply induced BAM1 accumulation, but TOR inactivation led to BAM1 degradation, and the effects of TOR inactivation on BAM1 degradation were abolished by the inhibition of autophagy and proteasome pathways or by phospho-mimicking mutation of BAM1 at serine-31. Such regulation of BAM1 activity by sugar-TOR signaling allows carbon availability to regulate guard cell starch metabolism and stomatal movement, ensuring optimal photosynthesis efficiency of plants.

Phosphorylation of RAV1/2 by KIN10 is essential for transcriptional activation of CAT6/7, which underlies oxidative stress response in cassava

Related to ABI3/VP1 (RAV) transcription factors have important roles in plant stress responses; however, it is unclear whether RAVs regulates oxidative stress response in cassava (Manihot esculenta). In this study, we report that MeRAV1/2 positively regulate oxidative stress resistance and catalase (CAT) activity in cassava. Consistently, RNA sequencing (RNA-seq) identifies three MeCATs that are differentially expressed in MeRAV1/2-silenced cassava leaves. Interestingly, MeCAT6 and MeCAT7 are identified as direct transcriptional targets of MeRAV1/2 via binding to their promoters. In addition, protein kinase MeKIN10 directly interacts with MeRAV1/2 to phosphorylate them at Ser45 and Ser44 residues, respectively, to promote their direct transcriptional activation on MeCAT6 and MeCAT7. Site mutation of MeRAV1^S45A or MeRAV2^S44A has no significant effect on the activities of MeCAT6 and MeCAT7 promoters or on oxidative stress resistance. In summary, this study demonstrates that the phosphorylation of MeRAV1/2 by MeKIN10 is essential for its direct transcriptional activation of MeCAT6/7 in response to oxidative stress.

SnRK1 stimulates the histone H3K27me3 demethylase JMJ705 to regulate a transcriptional switch to control energy homeostasis

Plant SNF1-Related Kinase1 (SnRK1) is an evolutionarily conserved energy-sensing protein kinase that orchestrates transcriptional networks to maintain cellular energy homeostasis when energy supplies become limited. However, the mechanism by which SnRK1 regulates this gene expression switch to gauge cellular energy status remains largely unclear. In this work, we show that the rice histone H3K27me3 demethylase JMJ705 is required for low energy stress tolerance in rice plants. The genetic inactivation of JMJ705 resulted in similar effects as those of the rice snrk1 mutant on the transcriptome, which impairs not only the promotion of the low energy stress-triggered transcriptional program but also the repression of the program under an energy-sufficient state. We show that the α-subunit of OsSnRK1 interacts with and phosphorylates JMJ705 to stimulate its H3K27me3 demethylase activity. Further analysis revealed that JMJ705 directly targets a set of low energy stress-responsive transcription factor genes. These results uncover the chromatin mechanism of SnRK1-regulated gene expression in both energy-sufficient and -limited states in plants and suggest that JMJ705 functions as an upstream regulator of the SnRK1α-controlled transcriptional network.

Guard cell regulation: pulling the strings behind the scenes

The precise regulation of stomata is critical to plant growth and development, especially under drought conditions. Two recent studies (Dou et al., Xu et al.) shed new light on stomata physiology in response to dynamic environmental conditions, revealing novel key mechanisms related to microtubule dynamics and energy sensing within the guard cells.

Establishing asymmetry: stomatal division and differentiation in plants

In the leaf epidermis, stomatal pores allow gas exchange between plants and the environment. The production of stomatal guard cells requires the lineage cells to divide asymmetrically. In this Insight review, we describe an emerging picture of how intrinsic molecules drive stomatal asymmetric cell division in multidimensions, from transcriptional activities in the nucleus to the dynamic assembly of the polarity complex at the cell cortex. Given the significant roles of stomatal activity in plant responses to environmental changes, we incorporate recent advances in external cues feeding into the regulation of core molecular machinery required for stomatal development. The work we discuss here is mainly based on the dicot plant Arabidopsis thaliana with summaries of recent progress in the monocots.

The Arabidopsis stomatal polarity protein BASL mediates distinct processes before and after cell division to coordinate cell size and fate asymmetries

In many land plants, asymmetric cell divisions (ACDs) create and pattern differentiated cell types on the leaf surface. In the Arabidopsis stomatal lineage, BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) regulates division plane placement and cell fate enforcement. Polarized subcellular localization of BASL is initiated before ACD and persists for many hours after the division in one of the two daughters. Untangling the respective contributions of polarized BASL before and after division is essential to gain a better understanding of its roles in regulating stomatal lineage ACDs. Here, we combine quantitative imaging and lineage tracking with genetic tools that provide temporally restricted BASL expression. We find that pre-division BASL is required for division orientation, whereas BASL polarity post-division ensures proper cell fate commitment. These genetic manipulations allowed us to uncouple daughter-cell size asymmetry from polarity crescent inheritance, revealing independent effects of these two asymmetries on subsequent cell behavior. Finally, we show that there is coordination between the division frequencies of sister cells produced by ACDs, and this coupling requires BASL as an effector of peptide signaling.

Single-cell resolution of lineage trajectories in the Arabidopsis stomatal lineage and developing leaf

Dynamic cell identities underlie flexible developmental programs. The stomatal lineage in the Arabidopsis leaf epidermis features asynchronous and indeterminate divisions that can be modulated by environmental cues. The products of the lineage, stomatal guard cells and pavement cells, regulate plant-atmosphere exchanges, and the epidermis as a whole influences overall leaf growth. How flexibility is encoded in development of the stomatal lineage and how cell fates are coordinated in the leaf are open questions. Here, by leveraging single-cell transcriptomics and molecular genetics, we uncovered models of cell differentiation within Arabidopsis leaf tissue. Profiles across leaf tissues identified points of regulatory congruence. In the stomatal lineage, single-cell resolution resolved underlying cell heterogeneity within early stages and provided a fine-grained profile of guard cell differentiation. Through integration of genome-scale datasets and spatiotemporally precise functional manipulations, we also identified an extended role for the transcriptional regulator SPEECHLESS in reinforcing cell fate commitment.

Dynamic Nutrient Signaling Networks in Plants

Nutrients are vital to life through intertwined sensing, signaling, and metabolic processes. Emerging research focuses on how distinct nutrient signaling networks integrate and coordinate gene expression, metabolism, growth, and survival. We review the multifaceted roles of sugars, nitrate, and phosphate as essential plant nutrients in controlling complex molecular and cellular mechanisms of dynamic signaling networks. Key advances in central sugar and energy signaling mechanisms mediated by the evolutionarily conserved master regulators HEXOKINASE1 (HXK1), TARGET OF RAPAMYCIN (TOR), and SNF1-RELATED PROTEIN KINASE1 (SNRK1) are discussed. Significant progress in primary nitrate sensing, calcium signaling, transcriptome analysis, and root-shoot communication to shape plant biomass and architecture are elaborated. Discoveries on intracellular and extracellular phosphate signaling and the intimate connections with nitrate and sugar signaling are examined. This review highlights the dynamic nutrient, energy, growth, and stress signaling networks that orchestrate systemwide transcriptional, translational, and metabolic reprogramming, modulate growth and developmental programs, and respond to environmental cues. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Stomatal development and genetic expression in Arabidopsis thaliana L

Stomata are turgor-driven microscopic epidermal valves of land plants. The controlled opening and closing of the valves are essential for regulating the gas exchange and minimizing the water loss and eventually regulating the internal temperatures. Stomata are also a major site of pathogen/microbe entry and plant defense system. Maintaining proper stomatal density, distribution, and development are pivotal for plant survival. Arabidopsis is a model plant to study molecular basis including signaling pathways, transcription factors, and key components for the growth and development of specific organs as well as the whole plant. It has intensively been studied and found out the driver for the development and patterning of stomata. In this review, we have explained how the MAPK signaling cascade is controlled by TOO MANY MOUTHS (TMM) receptor-like protein and the Erecta (ER) receptor-like kinase family. We have also summarized how this MAPK cascade affects primary transcriptional regulators to finally activate the main three basic Helix-Loop-Helix (bHLH) principal transcription factors, which are required for the development and patterning of stomata. Moreover, regulatory activity and cellular connections of polar proteins and environmentally mediated ligand-receptor interactions in the stomatal developmental pathways have extensively been discussed in this review.

Light regulates stomatal development by modulating paracrine signaling from inner tissues

Developmental outcomes are shaped by the interplay between intrinsic and external factors. The production of stomata—essential pores for gas exchange in plants—is extremely plastic and offers an excellent system to study this interplay at the cell lineage level. For plants, light is a key external cue, and it promotes stomatal development and the accumulation of the master stomatal regulator SPEECHLESS (SPCH). However, how light signals are relayed to influence SPCH remains unknown. Here, we show that the light-regulated transcription factor ELONGATED HYPOCOTYL 5 (HY5), a critical regulator for photomorphogenic growth, is present in inner mesophyll cells and directly binds and activates STOMAGEN. STOMAGEN, the mesophyll-derived secreted peptide, in turn stabilizes SPCH in the epidermis, leading to enhanced stomatal production. Our work identifies a molecular link between light signaling and stomatal development that spans two tissue layers and highlights how an environmental signaling factor may coordinate growth across tissue types.

Light promotes stomatal development in plants. Here Wang et al. show that light stimulates stomatal development via the HY5 transcription factor which induces expression of STOMAGEN, a mesophyll-derived secreted peptide, that in turn leads to stabilization of a master regulator of stomatal development in the epidermis.

Single-cell resolution of lineage trajectories in the Arabidopsis stomatal lineage and developing leaf

Dynamic cell identities underlie flexible developmental programs. The stomatal lineage in the Arabidopsis leaf epidermis features asynchronous and indeterminate divisions that can be modulated by environmental cues. The products of the lineage, stomatal guard cells and pavement cells, regulate plant-atmosphere exchanges, and the epidermis as a whole influences overall leaf growth. How flexibility is encoded in development of the stomatal lineage and how cell fates are coordinated in the leaf are open questions. Here, by leveraging single-cell transcriptomics and molecular genetics, we uncovered models of cell differentiation within Arabidopsis leaf tissue. Profiles across leaf tissues identified points of regulatory congruence. In the stomatal lineage, single-cell resolution resolved underlying cell heterogeneity within early stages and provided a fine-grained profile of guard cell differentiation. Through integration of genome-scale datasets and spatiotemporally precise functional manipulations, we also identified an extended role for the transcriptional regulator SPEECHLESS in reinforcing cell fate commitment.

Genome-wide Identification and Characterization of FCS-Like Zinc Finger (FLZ) Family Genes in Maize (Zea mays) and Functional Analysis of ZmFLZ25 in Plant Abscisic Acid Response

FCS-like zinc finger family proteins (FLZs), a class of plant-specific scaffold of SnRK1 complex, are involved in the regulation of various aspects of plant growth and stress responses. Most information of FLZ family genes was obtained from the studies in Arabidopsis thaliana, whereas little is known about the potential functions of FLZs in crop plants. In this study, 37 maize FLZ (ZmFLZ) genes were identified to be asymmetrically distributed on 10 chromosomes and can be divided into three subfamilies. Protein interaction and subcellular localization assays demonstrated that eight typical ZmFLZs interacted and partially co-localized with ZmKIN10, the catalytic α-subunit of the SnRK1 complex in maize leaf mesophyll cells. Expression profile analysis revealed that several ZmFLZs were differentially expressed across various tissues and actively responded to diverse abiotic stresses. In addition, ectopic overexpression of ZmFLZ25 in Arabidopsis conferred hypersensitivity to exogenous abscisic acid (ABA) and triggered higher expression of ABA-induced genes, pointing to the positive regulatory role of ZmFLZ25 in plant ABA signaling, a scenario further evidenced by the interactions between ZmFLZ25 and ABA receptors. In summary, these data provide the most comprehensive information on FLZ family genes in maize, and shed light on the biological function of ZmFLZ25 in plant ABA signaling.

Tuning self-renewal in the Arabidopsis stomatal lineage by hormone and nutrient regulation of asymmetric cell division

Asymmetric and self-renewing divisions build and pattern tissues. In the Arabidopsis stomatal lineage, asymmetric cell divisions, guided by polarly localized cortical proteins, generate most cells on the leaf surface. Systemic and environmental signals modify tissue development, but the mechanisms by which plants incorporate such cues to regulate asymmetric divisions are elusive. In a screen for modulators of cell polarity, we identified CONSTITUTIVE TRIPLE RESPONSE1, a negative regulator of ethylene signaling. We subsequently revealed antagonistic impacts of ethylene and glucose signaling on the self-renewing capacity of stomatal lineage stem cells. Quantitative analysis of cell polarity and fate dynamics showed that developmental information may be encoded in both the spatial and temporal asymmetries of polarity proteins. These results provide a framework for a mechanistic understanding of how nutritional status and environmental factors tune stem-cell behavior in the stomatal lineage, ultimately enabling flexibility in leaf size and cell-type composition.

SPEECHLESS and MUTE Mediate Feedback Regulation of Signal Transduction during Stomatal Development

Stomatal density, spacing, and patterning greatly influence the efficiency of gas exchange, photosynthesis, and water economy. They are regulated by a complex of extracellular and intracellular factors through the signaling pathways. After binding the extracellular epidermal patterning factor 1 (EPF1) and 2 (EPF2) as ligands, the receptor-ligand complexes activate by phosphorylation through the MAP-kinase cascades, regulating basic helix-loop-helix (bHLH) transcription factors SPEECHLESS (SPCH), MUTE, and FAMA. In this review, we summarize the molecular mechanisms and signal transduction pathways running within the transition of the protodermal cell into a pair of guard cells with a space (aperture) between them, called a stoma, comprising asymmetric and symmetric cell divisions and draw several functional models. The feedback mechanisms involving the bHLH factors SPCH and MUTE are not fully recognized yet. We show the feedback mechanisms driven by SPCH and MUTE in the regulation of EPF2 and the ERECTA family. Intersections of the molecular mechanisms for fate determination of stomatal lineage cells with the role of core cell cycle-related genes and stabilization of SPCH and MUTE are also reported.

Stomatal Lineage Control by Developmental Program and Environmental Cues

Stomata are micropores that allow plants to breathe and play a critical role in photosynthesis and nutrient uptake by regulating gas exchange and transpiration. Stomatal development, therefore, is optimized for survival and growth of the plant despite variable environmental conditions. Signaling cascades and transcriptional networks that determine the birth, proliferation, and differentiation of a stomate have been identified. These networks ensure proper stomatal patterning, density, and polarity. Environmental cues also influence stomatal development. In this review, we highlight recent findings regarding the developmental program governing cell fate and dynamics of stomatal lineage cells at the cell state- or single-cell level. We also overview the control of stomatal development by environmental cues as well as developmental plasticity associated with stomatal function and physiology. Recent advances in our understanding of stomatal development will provide a route to improving photosynthesis and water-stress resilience of crop plants in the climate change we currently face.

Phospho-Mutant Activity Assays Provide Evidence for the Negative Regulation of Transcriptional Regulator PRE1 by Phosphorylation

The PACLOBUTRAZOL-RESISTANCE (PRE) gene family encodes a group of atypical helix-loop-helix (HLH) proteins that act as the major hub integrating a wide range of environmental and hormonal signals to regulate plant growth and development. PRE1, as a positive regulator of cell elongation, activates HBI1 DNA binding by sequestering its inhibitor IBH1. Furthermore, PRE1 can be phosphorylated at Ser-46 and Ser-67, but how this phosphorylation regulates the functions of PRE1 remains unclear. Here, we used a phospho-mutant activity assay to reveal that the phosphorylation at Ser-67 negatively regulates the functions of PRE1 on cell elongation. Both of mutations of serine 46, either to phospho-dead alanine or phospho-mimicking glutamic acid, had no significant effects on the functions of PRE1. However, the mutation of serine 67 to glutamic acid (PRE1^S67E-Ox), but not alanine (PRE1^S67A-Ox), significantly reduced the promoting effects of PRE1 on cell elongation. The mutation of Ser-67 to Glu-67 impaired the interaction of PRE1 with IBH1 and resulted in PRE1 failing to inhibit the interaction between IBH1 and HBI1, losing the ability to induce the expression of the subsequent cell elongation-related genes. Furthermore, we showed that PRE1-Ox and PRE1^S67A-Ox both suppressed but PRE1^S67E-Ox had no strong effects on the dwarf phenotypes of IBH1-Ox. Our study demonstrated that the PRE1 activity is negatively regulated by the phosphorylation at Ser-67.