Enhancing climate change resilience in agricultural crops.

Climate change threatens global food and nutritional security through negative effects on crop growth and agricultural productivity. Many countries have adopted ambitious climate change mitigation and adaptation targets that will exacerbate the problem, as they require significant changes in current agri-food systems. In this review, we provide a roadmap for improved crop production that encompasses the effective transfer of current knowledge into plant breeding and crop management strategies that will underpin sustainable agriculture intensification and climate resilience. We identify the main problem areas and highlight outstanding questions and potential solutions that can be applied to mitigate the impacts of climate change on crop growth and productivity. Although translation of scientific advances into crop production lags far behind current scientific knowledge and technology, we consider that a holistic approach, combining disciplines in collaborative efforts, can drive better connections between research, policy, and the needs of society.

Leaf temperature responses to ABA and dead bacteria in wheat and Arabidopsis.

Stomatal densities, aperture openness and their responsiveness to environmental change determine plant water loss and regulate entry of pathogens. Stomatal responsiveness is usually assessed on restricted areas of leaves or isolated epidermal peels floated in solution. Analyzing these responses in the whole plant context could give valuable additional information, for example on the role of mesophyll in stomatal responses. We analyzed stomatal responses to the phytohormone abscisic acid (ABA) and pathogenic elicitors in intact plants by dynamic measurement of leaf temperature. We tested whether ABA-induced stomatal closure in wheat requires external nitrate and whether bacterial elicitor-induced stomatal closure can be detected by dynamic thermal imaging in intact Arabidopsis. We found that wheat was hypersensitive to all applied treatments, as even mock-treated leaves showed a strong increase in leaf temperature. Nevertheless, ABA activated stomatal closure in wheat independent of exogenous nitrate. Pathogenic elicitors triggered a fast and transient increase in leaf temperature in intact Arabidopsis, indicating short-term stomatal closure. The data suggest that the dynamics of pathogen-induced stomatal closure is different in whole plants compared to epidermal peels, where elicitor-induced stomatal closure persists longer. We propose that dynamic thermal imaging could be applied to address the effect of pathogenic elicitors on stomatal behavior in whole plants to complement detached sample assays and gain a better understanding of stomatal immunity.

Dynamic thermal imaging confirms local but not fast systemic ABA responses.

Abscisic acid (ABA) signals regulating stomatal aperture and water loss are usually studied in detached leaves or isolated epidermal peels and at infrequent timepoints. Measuring stomatal ABA responses in attached leaves across a time course enables the study of stomatal behaviour in the physiological context of the plant. Infrared thermal imaging is often used to characterize steady-state stomatal conductance via comparisons of leaf surface temperature but is rarely used to capture stomatal responses over time or across different leaf surfaces. We used dynamic thermal imaging as a robust, but sensitive, tool to observe stomatal ABA responses in a whole plant context. We detected stomatal responses to low levels of ABA in both monocots and dicots and identified differences between the responses of different leaves. Using whole plant thermal imaging, stomata did not always behave as described previously for detached samples: in Arabidopsis, we found no evidence for fast systemic ABA-induced stomatal closure, and in barley, we observed no requirement for exogenous nitrate during ABA-induced stomatal closure. Thus, we recommend dynamic thermal imaging as a useful approach to complement detached sample assays for the study of local and systemic stomatal responses and molecular mechanisms underlying stomatal responses to ABA in the whole plant context.

Reducing Stomatal Density in Barley Improves Drought Tolerance without Impacting on Yield.

The epidermal patterning factor (EPF) family of secreted signaling peptides regulate the frequency of stomatal development in model dicot and basal land plant species. Here, we identify and manipulate the expression of a barley (Hordeum vulgare) ortholog and demonstrate that when overexpressed HvEPF1 limits entry to, and progression through, the stomatal development pathway. Despite substantial reductions in leaf gas exchange, barley plants with significantly reduced stomatal density show no reductions in grain yield. In addition, HvEPF1OE barley lines exhibit significantly enhanced water use efficiency, drought tolerance, and soil water conservation properties. Our results demonstrate the potential of manipulating stomatal frequency for the protection and optimization of cereal crop yields under future drier environments.

Elevated CO2-Induced Responses in Stomata Require ABA and ABA Signaling.

An integral part of global environment change is an increase in the atmospheric concentration of CO2 ([CO2]) [1]. Increased [CO2] reduces leaf stomatal apertures and density of stomata that plays out as reductions in evapotranspiration [2-4]. Surprisingly, given the importance of transpiration to the control of terrestrial water fluxes [5] and plant nutrient acquisition [6], we know comparatively little about the molecular components involved in the intracellular signaling pathways by which [CO2] controls stomatal development and function [7]. Here, we report that elevated [CO2]-induced closure and reductions in stomatal density require the generation of reactive oxygen species (ROS), thereby adding a new common element to these signaling pathways. We also show that the PYR/RCAR family of ABA receptors [8, 9] and ABA itself are required in both responses. Using genetic approaches, we show that ABA in guard cells or their precursors is sufficient to mediate the [CO2]-induced stomatal density response. Taken together, our results suggest that stomatal responses to increased [CO2] operate through the intermediacy of ABA. In the case of [CO2]-induced reductions in stomatal aperture, this occurs by accessing the guard cell ABA signaling pathway. In both [CO2]-mediated responses, our data are consistent with a mechanism in which ABA increases the sensitivity of the system to [CO2] but could also be explained by requirement for a CO2-induced increase in ABA biosynthesis specifically in the guard cell lineage. Furthermore, the dependency of stomatal [CO2] signaling on ABA suggests that the ABA pathway is, in evolutionary terms, likely to be ancestral.