Genetic strategies for improving crop yields

The current trajectory for crop yields is insufficient to nourish the world's population by 20501. Greater and more consistent crop production must be achieved against a backdrop of climatic stress that limits yields, owing to shifts in pests and pathogens, precipitation, heat-waves and other weather extremes. Here we consider the potential of plant sciences to address post-Green Revolution challenges in agriculture and explore emerging strategies for enhancing sustainable crop production and resilience in a changing climate. Accelerated crop improvement must leverage naturally evolved traits and transformative engineering driven by mechanistic understanding, to yield the resilient production systems that are needed to ensure future harvests.

Genetic strategies for improving crop yields

The current trajectory for crop yields is insufficient to nourish the world's population by 2050¹. Greater and more consistent crop production must be achieved against a backdrop of climatic stress that limits yields, owing to shifts in pests and pathogens, precipitation, heat-waves and other weather extremes. Here we consider the potential of plant sciences to address post-Green Revolution challenges in agriculture and explore emerging strategies for enhancing sustainable crop production and resilience in a changing climate. Accelerated crop improvement must leverage naturally evolved traits and transformative engineering driven by mechanistic understanding, to yield the resilient production systems that are needed to ensure future harvests.

Insights into the Molecular Mechanisms of CO2-Mediated Regulation of Stomatal Movements

Plants must continually balance the influx of CO₂ for photosynthesis against the loss of water vapor through stomatal pores in their leaves. This balance can be achieved by controlling the aperture of the stomatal pores in response to several environmental stimuli. Elevation in atmospheric [CO₂] induces stomatal closure and further impacts leaf temperatures, plant growth and water-use efficiency, and global crop productivity. Here, we review recent advances in understanding CO₂-perception mechanisms and CO₂-mediated signal transduction in the regulation of stomatal movements, and we explore how these mechanisms are integrated with other signaling pathways in guard cells.

Toward a better understanding of signaling networks in plants: yeast has the power!

In response to abiotic stresses, plants produce the hormone abscisic acid (ABA). The ABA signaling pathway is highly complex and relies on a large number of gene copies encoding homologous signaling components, theoretically enabling numerous permutations. In this issue, Ruschhaupt et al (2019) used yeast as a reconstitution system to examine the functionality, plasticity, and efficiency of this complex and highly multiplexed core signaling pathway.

Chemical genetic identification of a lectin receptor kinase that transduces immune responses and interferes with abscisic acid signaling

Insight into how plants simultaneously cope with multiple stresses, for example, when challenged with biotic stress from pathogen infection and abiotic stress from drought, is important both for understanding evolutionary trade-offs and optimizing crop responses to these stresses. Mechanisms by which initial plant immune signaling antagonizes abscisic acid (ABA) signal transduction require further investigation. Using a chemical genetics approach, the small molecule [5-(3,4-dichlorophenyl)furan-2-yl]-piperidine-1-ylmethanethione (DFPM) has previously been identified due to its ability to suppress ABA signaling via plant immune signaling components. Here, we have used forward chemical genetics screening to identify DFPM-insensitive loci by monitoring the activity of ABA-inducible pRAB18::GFP in the presence of DFPM and ABA. The ability of DFPM to attenuate ABA signaling was reduced in rda mutants (resistant to DFPM inhibition of ABA signaling). One of the mutants, rda2, was mapped and is defective in a gene encoding a lectin receptor kinase. RDA2 functions in DFPM-mediated inhibition of ABA-mediated reporter expression. RDA2 is required for DFPM-mediated activation of immune signaling, including phosphorylation of mitogen-activated protein kinase (MAPK) 3 (MPK3) and MPK6, and induction of immunity marker genes. Our study identifies a previously uncharacterized receptor kinase gene that is important for DFPM-mediated immune signaling and inhibition of ABA signaling. We demonstrate that the lectin receptor kinase RDA2 is essential for perceiving the DFPM signal and activating MAPKs, and that MKK4 and MKK5 are required for DFPM interference with ABA signal transduction.

A seed resource for screening functionally redundant genes and isolation of new mutants impaired in CO2 and ABA responses

The identification of homologous genes with functional overlap in forward genetic screens is severely limited. Here, we report the generation of over 14000 artificial microRNA (amiRNA)-expressing plants that enable screens of the functionally redundant gene space in Arabidopsis. A protocol was developed for isolating robust and reproducible amiRNA mutants. Examples of validation approaches and essential controls are presented for two new amiRNA mutants that exhibit genetically redundant phenotypes and circumvent double mutant lethality. In a forward genetic screen for abscisic acid (ABA)-mediated inhibition of seed germination, amiRNAs that target combinations of known redundant ABA receptor and SnRK2 kinase genes were rapidly isolated, providing a strong proof of principle for this approach. A new ABA-insensitive amiRNA line that targets three avirulence-induced gene 2(-like) genes was isolated . A thermal imaging screen for plants with impaired stomatal opening in response to low CO2 exposure led to the isolation of a new amiRNA targeting two essential proteasomal subunits, PAB1 and PAB2. The seed library of 11000 T2 amiRNA lines (with 3000 lines in progress) generated here provides a new platform for forward genetic screens and has been made available to the Arabidopsis Biological Resource Center (ABRC). Optimized procedures for amiRNA screening and controls are described.

Intact leaf gas exchange provides a robust method for measuring the kinetics of stomatal conductance responses to abscisic acid and other small molecules in Arabidopsis and grasses

BACKGROUND

Guard cells perceive external and internal stimuli and regulate stomatal conductance in plants. With the use of gas exchange analyzers, time-resolved stomatal conductance responses to light intensity, [CO₂] concentration and relative humidity changes can be measured. This is more difficult to achieve when measuring stomatal responses to small soluble molecules such as the plant hormone abscisic acid (ABA) or the bacterial peptide flagellin 22 (flg22), in particular when investigating mutants with response phenotypes.

RESULTS

A method to evaluate the dynamic effects of small molecules on stomatal conductance in a time-resolved fashion using gas exchange analyzers is presented here. ABA-induced stomatal closure was investigated by adding ABA to the transpiration stream of intact leaves placed in a microcentrifuge tube containing water. Strong ABA responses were resolved in time- and in a dose-dependent manner in wild-type Arabidopsis leaves, whereas the same response was not observed in leaves of the ABA-insensitive mutant open stomata 1-3 (ost1-3). Moreover, when leaves of the Plasma membrane Intrinsic Protein (PIP) aquaporin quadruple mutant pip1;1 pip1;2 pip2;1 pip2;2 were tested, robust wild-type-like responses to ABA were observed. When the bacterial peptide flg22 was added to the transpiration stream of intact wild-type leaves, a strong flg22-induced stomatal closure effect was observed. Finally, the proposed technique was further developed and optimized for evaluation of stomatal conductance responses to small molecules in leaves of grasses using the reference plant Brachypodium distachyon.

CONCLUSIONS

Due to the variable size of stomata in Arabidopsis and the limited dynamic response of stomata in isolated epidermal strips, evaluation of the effect of small molecules on stomatal physiology has been challenging and has led in some cases to inconsistent results. Moreover, potential signals from the mesophyll are missing when using epidermal peels to evaluate stomatal aperture responses. Here we propose a less invasive technique which allows for time-resolved measurements of stomatal conductance responses to small molecules optimized for both Arabidopsis and Brachypodium distachyon leaves.

Mitogen-activated protein kinases MPK4 and MPK12 are key components mediating CO2 -induced stomatal movements

Respiration in leaves and the continued elevation in the atmospheric CO2 concentration cause CO2 -mediated reduction in stomatal pore apertures. Several mutants have been isolated for which stomatal responses to both abscisic acid (ABA) and CO2 are simultaneously defective. However, there are only few mutations that impair the stomatal response to elevated CO2 , but not to ABA. Such mutants are invaluable in unraveling the molecular mechanisms of early CO2 signal transduction in guard cells. Recently, mutations in the mitogen-activated protein (MAP) kinase, MPK12, have been shown to partially impair CO2 -induced stomatal closure. Here, we show that mpk12 plants, in which MPK4 is stably silenced specifically in guard cells (mpk12 mpk4GC homozygous double-mutants), completely lack CO2 -induced stomatal responses and have impaired activation of guard cell S-type anion channels in response to elevated CO2 /bicarbonate. However, ABA-induced stomatal closure, S-type anion channel activation and ABA-induced marker gene expression remain intact in the mpk12 mpk4GC double-mutants. These findings suggest that MPK12 and MPK4 act very early in CO2 signaling, upstream of, or parallel to the convergence of CO2 and ABA signal transduction. The activities of MPK4 and MPK12 protein kinases were not directly modulated by CO2 /bicarbonate in vitro, suggesting that they are not direct CO2 /bicarbonate sensors. Further data indicate that MPK4 and MPK12 have distinguishable roles in Arabidopsis and that the previously suggested role of RHC1 in stomatal CO2 signaling is minor, whereas MPK4 and MPK12 act as key components of early stomatal CO2 signal transduction.

Identification of SLAC1 anion channel residues required for CO2/bicarbonate sensing and regulation of stomatal movements

Increases in CO2 concentration in plant leaves due to respiration in the dark and the continuing atmospheric [CO2] rise cause closing of stomatal pores, thus affecting plant-water relations globally. However, the underlying CO2/bicarbonate (CO2/HCO3-) sensing mechanisms remain unknown. [CO2] elevation in leaves triggers stomatal closure by anion efflux mediated via the SLAC1 anion channel localized in the plasma membrane of guard cells. Previous reconstitution analysis has suggested that intracellular bicarbonate ions might directly up-regulate SLAC1 channel activity. However, whether such a CO2/HCO3- regulation of SLAC1 is relevant for CO2 control of stomatal movements in planta remains unknown. Here, we computationally probe for candidate bicarbonate-interacting sites within the SLAC1 anion channel via long-timescale Gaussian accelerated molecular dynamics (GaMD) simulations. Mutations of two putative bicarbonate-interacting residues, R256 and R321, impaired the enhancement of the SLAC1 anion channel activity by CO2/HCO3- in Xenopus oocytes. Mutations of the neighboring charged amino acid K255 and residue R432 and the predicted gate residue F450 did not affect HCO3- regulation of SLAC1. Notably, gas-exchange experiments with slac1-transformed plants expressing mutated SLAC1 proteins revealed that the SLAC1 residue R256 is required for CO2 regulation of stomatal movements in planta, but not for abscisic acid (ABA)-induced stomatal closing. Patch clamp analyses of guard cells show that activation of S-type anion channels by CO2/HCO3-, but not by ABA, was impaired, indicating the relevance of R256 for CO2 signal transduction. Together, these analyses suggest that the SLAC1 anion channel is one of the physiologically relevant CO2/HCO3- sensors in guard cells.

Cytosolic malate and oxaloacetate activate S-type anion channels in Arabidopsis guard cells

Intracellular malate-starch interconversion plays an important role in stomatal movements. We investigated whether malate or oxaloacetate from the cytosolic membrane side regulate anion channels in the plasma membrane of Arabidopsis thaliana guard cells. Physiological concentrations of cytosolic malate have been reported in the range of 0.4-3 mM in leaf cells. Guard cell patch clamp and two-electrode oocyte voltage-clamp experiments were pursued. We show that a concentration of 1 mM cytosolic malate greatly activates S-type anion channels in Arabidopsis thaliana guard cells. Interestingly, 1 mM cytosolic oxaloacetate also activates S-type anion channels. Malate activation was abrogated at 10 mM malate and in SLAC1 anion channel mutant alleles. Interestingly, malate activation of S-type anion currents was disrupted at below resting cytosolic-free calcium concentrations ([Ca2+ ]cyt ), suggesting a key role for basal [Ca2+ ]cyt signaling. Cytosolic malate was not able to directly activate or enhance SLAC1-mediated anion currents in Xenopus oocytes, whereas in positive controls, cytosolic NaHCO3 enhanced SLAC1 activity, suggesting that malate may not directly modulate SLAC1. Cytosolic malate activation of S-type anion currents was impaired in ost1 and in cpk5/6/11/23 quadruple mutant guard cells. Together these findings show that these cytosolic organic anions function in guard cell 'plasma membrane' ion channel regulation.

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.

Control of seed dormancy and germination by DOG1-AHG1 PP2C phosphatase complex via binding to heme

Abscisic acid (ABA) regulates abiotic stress and developmental responses including regulation of seed dormancy to prevent seeds from germinating under unfavorable environmental conditions. ABA HYPERSENSITIVE GERMINATION1 (AHG1) encoding a type 2C protein phosphatase (PP2C) is a central negative regulator of ABA response in germination; however, the molecular function and regulation of AHG1 remain elusive. Here we report that AHG1 interacts with DELAY OF GERMINATION1 (DOG1), which is a pivotal positive regulator in seed dormancy. DOG1 acts upstream of AHG1 and impairs the PP2C activity of AHG1 in vitro. Furthermore, DOG1 has the ability to bind heme. Binding of DOG1 to AHG1 and heme are independent processes, but both are essential for DOG1 function in vivo. Our study demonstrates that AHG1 and DOG1 constitute an important regulatory system for seed dormancy and germination by integrating multiple environmental signals, in parallel with the PYL/RCAR ABA receptor-mediated regulatory system.

The hormone abscisic acid (ABA) prevents seeds from germination when conditions are not suitable. Here the authors show that DOG1, a positive regulator of germination, impairs ABA signaling via genetic and physical interactions with the AHG1 phosphatase and that DOG1 binding to heme is required for this activity.

The BIG protein distinguishes the process of CO2 -induced stomatal closure from the inhibition of stomatal opening by CO2

We conducted an infrared thermal imaging-based genetic screen to identify Arabidopsis mutants displaying aberrant stomatal behavior in response to elevated concentrations of CO₂ . This approach resulted in the isolation of a novel allele of the Arabidopsis BIG locus (At3g02260) that we have called CO₂ insensitive 1 (cis1). BIG mutants are compromised in elevated CO₂ -induced stomatal closure and bicarbonate activation of S-type anion channel currents. In contrast with the wild-type, they fail to exhibit reductions in stomatal density and index when grown in elevated CO₂ . However, like the wild-type, BIG mutants display inhibition of stomatal opening when exposed to elevated CO₂ . BIG mutants also display wild-type stomatal aperture responses to the closure-inducing stimulus abscisic acid (ABA). Our results indicate that BIG is a signaling component involved in the elevated CO₂ -mediated control of stomatal development. In the control of stomatal aperture by CO₂ , BIG is only required in elevated CO₂ -induced closure and not in the inhibition of stomatal opening by this environmental signal. These data show that, at the molecular level, the CO₂ -mediated inhibition of opening and promotion of stomatal closure signaling pathways are separable and BIG represents a distinguishing element in these two CO₂ -mediated responses.

SnapShot: Abscisic Acid Signaling

Abscisic acid is a key phytohormone produced in response to abiotic stress conditions and is an activator of abiotic stress resistance mechanisms and a regulator during diverse developmental stages in plants. This SnapShot explores how ABA signaling operates and coordinates resistance during stress responses and modulates plant development.

Seeing is believing

Gibberellins (GAs) control key growth and developmental processes in plants. Real-time monitoring of GA concentrations in living tissues is critical for understanding the actions of this hormone class. A first-generation optogenetic GA-nano-indicator now illuminates the effects of GA levels on cell length and light signalling.

Screening for Natural Variation in Water Use Efficiency Traits in a Diversity Set of Brassica napus L. Identifies Candidate Variants in Photosynthetic Assimilation

Seed yield and quality of crop species are significantly reduced by water deficit. Stable isotope screening (δ13C) of a diversity set of 147 accessions of Brassica napus grown in the field identified several accessions with extremes in water use efficiency (WUE). We next conducted an investigation of the physiological characteristics of selected natural variants with high and low WUE to understand how these characteristics translate to differences in WUE. We identified an interesting Spring accession, G302 (Mozart), that exhibited the highest WUE in the field and high CO2 assimilation rates coupled with an increased electron transport capacity (Jmax) under the imposed conditions. Differences in stomatal density and stomatal index did not translate to differences in stomatal conductance in the investigated accessions. Stomatal conductance response to exogenous ABA was analyzed in selected high and low WUE accessions. Spring lines showed little variation in response to exogenous ABA, while one Semi-Winter line (SW047) showed a significantly more rapid response to exogenous ABA, that corresponded to the high WUE indicated by δ13C measurements. This research illustrates the importance of examining natural variation at a physiological level for investigation of the underlying mechanisms influencing the diversity of carbon isotope discrimination values in the field and identifies natural variants in B. napus with improved WUE and potential relevant traits.

Two-electrode Voltage-clamp Recordings in Xenopus laevis Oocytes: Reconstitution of Abscisic Acid Activation of SLAC1 Anion Channel via PYL9 ABA Receptor

Two-Electrode Voltage-Clamp (TEVC) recording in Xenopus laevis oocytes provides a powerful method to investigate the functions and regulation of ion channel proteins. This approach provides a well-known tool to characterize ion channels or transporters expressed in Xenopus laevis oocytes. The plasma membrane of the oocyte is impaled by two microelectrodes, one for voltage sensing and the other one for current injection. Here we list a protocol that allows robust reconstitution of multi-component signaling pathways. This protocol has been used to study plant ion channels, including the SLAC1 channel (SLOW ANION CHANNEL-ASSOCIATED 1), in particular SLAC1 activation by either the protein kinase OST1 (OPEN STOMATA 1), Ca²⁺-dependent protein kinases (CPKs) or the GHR1 (GUARD CELL HYDROGEN PEROXIDE-RESISTANT 1) transmembrane receptor-like protein. Data are presented showing reconstitution of abscisic acid activation of the SLAC1 anion channel by the 'monomeric' ABA (abscisic acid) receptor RCAR1/PYL9 (PYRABACT INRESISTANCE1 [PYR1]/PYR1-LIKE [PYL]/REGULATORYCOMPONENTS OF ABA RECEPTORS [RCAR]) by co-expressing four components of the abscisic acid signaling core. This protocol is also suitable for studying other ion channel functions and regulation mechanisms, as well as transporter proteins.

Natural Variation in Arabidopsis Cvi-0 Accession Reveals an Important Role of MPK12 in Guard Cell CO2 Signaling

Plant gas exchange is regulated by guard cells that form stomatal pores. Stomatal adjustments are crucial for plant survival; they regulate uptake of CO2 for photosynthesis, loss of water, and entrance of air pollutants such as ozone. We mapped ozone hypersensitivity, more open stomata, and stomatal CO2-insensitivity phenotypes of the Arabidopsis thaliana accession Cvi-0 to a single amino acid substitution in MITOGEN-ACTIVATED PROTEIN (MAP) KINASE 12 (MPK12). In parallel, we showed that stomatal CO2-insensitivity phenotypes of a mutant cis (CO2-insensitive) were caused by a deletion of MPK12. Lack of MPK12 impaired bicarbonate-induced activation of S-type anion channels. We demonstrated that MPK12 interacted with the protein kinase HIGH LEAF TEMPERATURE 1 (HT1)-a central node in guard cell CO2 signaling-and that MPK12 functions as an inhibitor of HT1. These data provide a new function for plant MPKs as protein kinase inhibitors and suggest a mechanism through which guard cell CO2 signaling controls plant water management.

Abscisic acid and other plant hormones: Methods to visualize distribution and signaling

The exploration of plant behavior on a cellular scale in a minimal invasive manner is key to understanding plant adaptations to their environment. Plant hormones regulate multiple aspects of growth and development and mediate environmental responses to ensure a successful life cycle. To monitor the dynamics of plant hormone actions in intact tissue, we need qualitative and quantitative tools with high temporal and spatial resolution. Here, we describe a set of biological instruments (reporters) for the analysis of the distribution and signaling of various plant hormones. Furthermore, we provide examples of their utility for gaining novel insights into plant hormone action with a deeper focus on the drought hormone abscisic acid.

A Dominant Mutation in the HT1 Kinase Uncovers Roles of MAP Kinases and GHR1 in CO2-Induced Stomatal Closure

Activation of the guard cell S-type anion channel SLAC1 is important for stomatal closure in response to diverse stimuli, including elevated CO2 The majority of known SLAC1 activation mechanisms depend on abscisic acid (ABA) signaling. Several lines of evidence point to a parallel ABA-independent mechanism of CO2-induced stomatal regulation; however, molecular details of this pathway remain scarce. Here, we isolated a dominant mutation in the protein kinase HIGH LEAF TEMPERATURE1 (HT1), an essential regulator of stomatal CO2 responses, in an ozone sensitivity screen of Arabidopsis thaliana The mutation caused constitutively open stomata and impaired stomatal CO2 responses. We show that the mitogen-activated protein kinases (MPKs) MPK4 and MPK12 can inhibit HT1 activity in vitro and this inhibition is decreased for the dominant allele of HT1. We also show that HT1 inhibits the activation of the SLAC1 anion channel by the protein kinases OPEN STOMATA1 and GUARD CELL HYDROGEN PEROXIDE-RESISTANT1 (GHR1) in Xenopus laevis oocytes. Notably, MPK12 can restore SLAC1 activation in the presence of HT1, but not in the presence of the dominant allele of HT1. Based on these data, we propose a model for sequential roles of MPK12, HT1, and GHR1 in the ABA-independent regulation of SLAC1 during CO2-induced stomatal closure.

An ABA-increased interaction of the PYL6 ABA receptor with MYC2 Transcription Factor: A putative link of ABA and JA signaling

Abscisic acid (ABA) is a plant hormone that mediates abiotic stress tolerance and regulates growth and development. ABA binds to members of the PYL/RCAR ABA receptor family that initiate signal transduction inhibiting type 2C protein phosphatases. Although crosstalk between ABA and the hormone Jasmonic Acid (JA) has been shown, the molecular entities that mediate this interaction have yet to be fully elucidated. We report a link between ABA and JA signaling through a direct interaction of the ABA receptor PYL6 (RCAR9) with the basic helix-loop-helix transcription factor MYC2. PYL6 and MYC2 interact in yeast two hybrid assays and the interaction is enhanced in the presence of ABA. PYL6 and MYC2 interact in planta based on bimolecular fluorescence complementation and co-immunoprecipitation of the proteins. Furthermore, PYL6 was able to modify transcription driven by MYC2 using JAZ6 and JAZ8 DNA promoter elements in yeast one hybrid assays. Finally, pyl6 T-DNA mutant plants show an increased sensitivity to the addition of JA along with ABA in cotyledon expansion experiments. Overall, the present study identifies a direct mechanism for transcriptional modulation mediated by an ABA receptor different from the core ABA signaling pathway, and a putative mechanistic link connecting ABA and JA signaling pathways.

An ABA-increased interaction of the PYL6 ABA receptor with MYC2 Transcription Factor: A putative link of ABA and JA signaling

Abscisic acid (ABA) is a plant hormone that mediates abiotic stress tolerance and regulates growth and development. ABA binds to members of the PYL/RCAR ABA receptor family that initiate signal transduction inhibiting type 2C protein phosphatases. Although crosstalk between ABA and the hormone Jasmonic Acid (JA) has been shown, the molecular entities that mediate this interaction have yet to be fully elucidated. We report a link between ABA and JA signaling through a direct interaction of the ABA receptor PYL6 (RCAR9) with the basic helix-loop-helix transcription factor MYC2. PYL6 and MYC2 interact in yeast two hybrid assays and the interaction is enhanced in the presence of ABA. PYL6 and MYC2 interact in planta based on bimolecular fluorescence complementation and co-immunoprecipitation of the proteins. Furthermore, PYL6 was able to modify transcription driven by MYC2 using JAZ6 and JAZ8 DNA promoter elements in yeast one hybrid assays. Finally, pyl6 T-DNA mutant plants show an increased sensitivity to the addition of JA along with ABA in cotyledon expansion experiments. Overall, the present study identifies a direct mechanism for transcriptional modulation mediated by an ABA receptor different from the core ABA signaling pathway, and a putative mechanistic link connecting ABA and JA signaling pathways.

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.

Molecular and systems approaches towards drought-tolerant canola crops

1169 I. 1170 II. 1170 III. 1172 IV. 1176 V. 1181 VI. 1182 1183 References 1183 SUMMARY: Modern agriculture is facing multiple challenges including the necessity for a substantial increase in production to meet the needs of a burgeoning human population. Water shortage is a deleterious consequence of both population growth and climate change and is one of the most severe factors limiting global crop productivity. Brassica species, particularly canola varieties, are cultivated worldwide for edible oil, animal feed, and biodiesel, and suffer dramatic yield loss upon drought stress. The recent release of the Brassica napus genome supplies essential genetic information to facilitate identification of drought-related genes and provides new information for agricultural improvement in this species. Here we summarize current knowledge regarding drought responses of canola, including physiological and -omics effects of drought. We further discuss knowledge gained through translational biology based on discoveries in the closely related reference species Arabidopsis thaliana and through genetic strategies such as genome-wide association studies and analysis of natural variation. Knowledge of drought tolerance/resistance responses in canola together with research outcomes arising from new technologies and methodologies will inform novel strategies for improvement of drought tolerance and yield in this and other important crop species.