Systems analysis of guard cell oscillatory mechanics in stomatal dynamics

植物叶片表皮中的气孔在调节光合作用的CO2交换，同时最大限度地减少叶片内部空气和大气之间的蒸腾水分损失方面起着至关重要的作用。气孔周围的保卫细胞吸收无机溶质和水，当叶片中的二氧化碳耗尽时，保卫细胞的体积增加以打开气孔孔；当二氧化碳含量较高时，保卫细胞失去溶质和水，体积减小以关闭气孔孔并保存叶片水分。我们知道很多关于驱动气孔在气孔开度两端之间运动的机制。相比之下，我们对产生通常在野外观察到的动态连续光圈的机制知之甚少，更不用说这种微调是如何调节的了。例如，这种知识上的差距可以从过去定量建模工作的重点中看出。气孔特征是蒸腾作用和植物水分利用效率模型的基础，这些模型已被证明在植物和社区一级成功地再现和预测了蒸腾行为。然而，所有这些模型都反映了一种“自上而下”的方法，并将保卫细胞机制视为一个“黑匣子”，将这些过程归结为几个经验参数、水力路径和电导。尽管我们对保卫细胞的运输和信号传递有丰富的知识，但从保卫细胞本身的特性发展出来的自下而上的模型很少，也没有一个足够普遍的模型可以广泛应用于预测气孔行为。更复杂的是，我们在细胞水平上的许多知识都是基于对表皮中的保卫细胞或作为原生质体分离出来的体外研究。我们需要弥合我们知识中的这些差距，并了解气孔如何在面对真正的环境挑战时动态补偿。过去15年的研究已经为气孔动力学背后的机制提供了几条重要线索。线索指向保卫细胞膜在控制和平衡渗透通量的两个准稳定状态之间的振荡。这一假设得到了有充分记录的观察的支持，即气孔也会振荡，并可以在确定的条件下通过实验来驱动。事实上，这种“时间平均”机制已经从保卫细胞离子传输的系统分析中预测到了，尽管使用的是具有显著参数限制的数学模型。现在需要将这几条证据汇集在一起，并接受严格的实验测试，以解决一些关键问题。我们需要知道更全面的保卫细胞传输/结合例如主要离子转运体的已知调节性质/是否能够返回在孔径和电压中观察到的全部行为，并预测新的行为。我们想知道保卫细胞离子流量和渗透含量的动态变化是如何支持这些行为的。最后，我们想要测试相关保卫细胞参数的实验操作是否可以证明在气孔行为中产生明确和可预测的变化。在这里，我们建议通过系统动力学模型以及通过实验分析和验证，共同开发这一探索线，以得出定量和可检验的预测。我们关于保卫细胞运输和动态平衡的知识现在已经足够发达，使这种方法成为一个容易实现的目标。我们完全期待对我们提出的问题的回答能够对气孔的行为产生新的和令人兴奋的见解，并为农业和作物发展的实际应用开辟全新的层面。

Stomata in the epidermis of plant leaves play a vital role in regulating CO2 exchange for photosynthesis while minimising transpirational water loss between the inner leaf air space and the atmosphere. Guard cells surrounding the stomata take up inorganic solutes and water, increasing in volume to open the stomatal pore when CO2 in the leaf is depleted; and they lose solutes and water, decreasing in volume to close the stomatal pore and conserve leaf water under stress, in the dark and when CO2 is high. We know a great deal about the mechanisms that drive stomatal movements between the extremes in pore aperture. By contrast, our knowledge is remarkably poor of the mechanisms that give rise to the dynamic continuum of apertures normally observed in the field, much less how such fine-tuning is regulated. This gap in knowledge can be seen, for example, in the focus of past efforts in quantitative modelling. Stomatal characteristics underpin models for transpiration and plant water use efficiency that have proven successful in reproducing and predicting transpirational behaviours at the plant and community levels. However all of these models reflect a 'top-down' approach and consider guard cell mechanics as a 'black box', subsuming these processes within a few empirical parameters, hydraulic pathways and conductances. There are very few models that have been developed 'bottom-up' from the properties of the guard cells themselves, despite the wealth of knowledge we have for guard cell transport and signalling, and none that are sufficiently generalised to be widely applicable in predicting stomatal behaviour. A further complication is that much of our knowledge at the cellular level is based on in vitro studies with guard cells in epidermal peels or isolated as protoplasts. We need to bridge these gaps in our knowledge and to understand how stomata compensate dynamically in the face of real environmental challenges. Studies over the past 15 years have yielded several important clues to the mechanisms behind stomatal dynamics. The clues point to oscillations of the guard cell membrane between two quasi-stable states that control and balance osmotic fluxes. This postulate finds support in well-documented observations that stomatal apertures also oscillate and can be driven experimentally under defined conditions. Indeed, such a 'time-averaging' mechanism has already been predicted from a systems analysis of guard cell ion transport, albeit using a mathematical model with significant parameter limitations. These several lines of evidence need now to be drawn together and subject to rigorous experimental testing in order to address a number of key issues. We need to know whether more comprehensive mathematical models for guard cell transport / incorporating, for example, known regulatory properties for the major ion transporters / are able to return the full range of observed behaviours in aperture and voltage, and to predict novel ones. We want to know how these behaviours are underpinned by the dynamics of guard cell ion fluxes and osmotic contents. Finally, we want to test whether experimental manipulations of the relevant guard cell parameters can be shown to yield well-defined and predictable changes in stomatal behaviour. We propose here to develop this line of enquiry jointly through systems kinetic modelling to derive quantitative and testable predictions and through experimental analysis and validation. Our knowledge of guard cell transport and homeostasis is now sufficiently well-developed to make an approach of this kind a readily achievable goal. We fully expect answers to the questions we pose to yield new and exciting insights into the behaviour of stomata and to open entirely new dimensions to practical applications in agriculture and crop development.