Engineering ion flux of the stomatal complex for enhanced photosynthesis and water use efficiency

Stomata are pores that open and close to balance the requirement for CO2 entry to the leaf for photosynthesis against the need to reduce water loss via transpiration and prevent leaf drying. Stomata are at the centre of a crisis in water availability and crop production that is expected to unfold over the next 20-30 years: globally, agricultural water usage has increased 6-fold in the past 100 years, twice as fast as the human population, and is projected to double again before 2030. Thus stomata are an important target in efforts to improve crop performance, especially in the face of global climate change. Stomatal opening and closing are driven by solute and water transport of the guard cells which surround the stomatal pore. Our deep knowledge of these processes has made the guard cell one of the best-known plant cell models and gives real substance to prospects for engineering stomata to improve water use by crops.

By contrast, we know very little of the surrounding cells, sometimes called subsidiary cells, adjacent the guard cells in the epidermis. Changes in the ion contents of surrounding cells originally led to the idea of a 'shuttling' of solute between surrounding and guard cells. It has been argued that the surrounding cells store solute - notably K+ - for use by the guard cells during stomatal opening and, by releasing this solute, they also relieve the turgor that opposes the guard cell expansion to promote stomatal opening. Thus, in principle the stomatal complex may be considered a two-cell, two-stroke 'pump' for solute transfer between surrounding and guard cells, thereby accelerating stomatal kinetics. Until now, however, tools to probe cellular function within the stomatal complex have been lacking.

In the natural environment light fluctuates, for example as clouds pass over. The stomata of most plants respond to light by opening the stomatal pore to increase CO2 access for photosynthesis, and they reduce the pore aperture when the light intensity drops and the demand for CO2 by photosynthesis declines. Photosynthesis generally tracks light fluctuations, but stomata are much slower to respond. The slower response of stomata can limit gas exchange and reduce carbon assimilation by photosynthesis when light intensity rises, and it can lead to transpiration without corresponding assimilation when light intensity drops quickly. We and others have reasoned that assimilation, and consequently biomass generation, could be enhanced concurrent with an decrease in water use by the plant if the rates of stomatal movements could be better matched to variations in photosynthetic demand.

Recently, we found that accelerating ion flux in stomatal guard cells by introducing a light-activated K+ channel, BLINK1, was sufficient to increase the biomass and reduce the associated water use by 2-fold in the model plant Arabidopsis. These findings demonstrate the potential of accelerating stomata as a strategy to enhance crop gains while conserving water. The photocontrol offered by optogenetic tools such as BLINK1 also offers a means to probing the function of surrounding cells in the stomatal complex and, potentially, to further enhancing stomatal kinetics.

We propose here an interlinked effort to address this long-outstanding question of whether and, if so, how surrounding cells participate in stomatal movements and to translate the knowledge of stomatal kinetics in a practical demonstration with two model crops. We will build on the success with BLINK1 in Arabidopsis for these purposes. Our overarching aim is to extend the gains achieved to date in Arabidopsis, informed by new knowledge of surrounding cell function in the stomatal complex, as strategies for enhancing crop yields and reducing agricultural water consumption.

We propose a concerted assessment of stomatal kinetics and its relevance to crop yields and water use. Our aims are (1) to demonstrate the feasibility of manipulating stomatal kinetics by translating knowledge of BLINK1 optogenetics in Arabidopsis into two crop models, and (2) to test the hypothesis of a two-cell, two-stroke 'pump' for solute transfer as a target for further enhancements in accelerating stomatal kinetics. In the latter case, we propose (i) a quantitative analysis of the ion transport characteristics of the surrounding cells and their coordination with stimuli known to trigger guard cell transport for stomatal opening and closing, and (ii) optogenetic and related manipulations of K+ flux of the surrounding cells in order to understand the transport coordination between surrounding and guard cells.

We will build on our recent success with the light-activated K+ channel BLINK1 in Arabidopsis in each case. Both the practical and fundamental challenges will take advantage of targeted optogenetic expression within the leaf epidermis and, additionally, on the development of new optogenetic tools with improved light sensitivities and altered light regulation. Experiments will follow methodologies similar to those used successfully to date, including voltage clamp, gas exchange and biomass studies. We will use extant knowledge of BLINK1 variants in extending the optogenetic tools, and we will draw on heterologous expression to identify and quantify the most promising of these prior to use. Finally, we will target expression between cell types to assess, in response to light, the component contributions from the surrounding and guard cells and to extract stomatal kinetics and their coupling to photosynthesis. We expect these studies to expand our fundamental understanding of stomatal mechanics and to establish their kinetics as a bona fide target for future efforts in crop improvement.

This proposal is for a synergy in practical and fundamental research building on a core of ideas at the centre of the international plant photosynthesis and stomatal biology communities. The research will stimulate thinking around strategies for enhancing crop yields and reducing agricultural water consumption, and it should inform methodologies for approaching crop engineering. In the long term the research is expected to benefit fundamental researchers as well as agriculture and industry through conceptual developments as well as the introduction of new technologies relevant to plant productivity and water use efficiency. The research will feed into higher education training programmes through capacity building at the postgraduate and postdoctoral levels. Additional impact is proposed through public displays and the development of teaching resources building on the background work for this proposal. Finally the research will help guide future efforts in applications to agricultural/industrial systems. The applicants have established links with industrial/technology transfer partners and research institutes to take advantage of these developments. Further details of these, and additional impacts will be found in Part 1 of the Case for Support and in the attached Impact Pathways.