A new discrete dynamic model of ABA-induced stomatal closure predicts key feedback loops

Abstract

Stomata, microscopic pores in leaf surfaces through which water loss and carbon dioxide uptake occur, are closed in response to drought by the phytohormone abscisic acid (ABA). This process is vital for drought tolerance and has been the topic of extensive experimental investigation in the last decades. Although a core signaling chain has been elucidated consisting of ABA binding to receptors, which alleviates negative regulation by protein phosphatases 2C (PP2Cs) of the protein kinase OPEN STOMATA 1 (OST1) and ultimately results in activation of anion channels, osmotic water loss, and stomatal closure, over 70 additional components have been identified, yet their relationships with each other and the core components are poorly elucidated. We integrated and processed hundreds of disparate observations regarding ABA signal transduction responses underlying stomatal closure into a network of 84 nodes and 156 edges and, as a result, established those relationships, including identification of a 36-node, strongly connected (feedback-rich) component as well as its in- and out-components. The network’s domination by a feedback-rich component may reflect a general feature of rapid signaling events. We developed a discrete dynamic model of this network and elucidated the effects of ABA plus knockout or constitutive activity of 79 nodes on both the outcome of the system (closure) and the status of all internal nodes. The model, with more than 1024 system states, is far from fully determined by the available data, yet model results agree with existing experiments in 82 cases and disagree in only 17 cases, a validation rate of 75%. Our results reveal nodes that could be engineered to impact stomatal closure in a controlled fashion and also provide over 140 novel predictions for which experimental data are currently lacking. Noting the paucity of wet-bench data regarding combinatorial effects of ABA and internal node activation, we experimentally confirmed several predictions of the model with regard to reactive oxygen species, cytosolic Ca2+ (Ca2+c), and heterotrimeric G-protein signaling. We analyzed dynamics-determining positive and negative feedback loops, thereby elucidating the attractor (dynamic behavior) repertoire of the system and the groups of nodes that determine each attractor. Based on this analysis, we predict the likely presence of a previously unrecognized feedback mechanism dependent on Ca2+c. This mechanism would provide model agreement with 10 additional experimental observations, for a validation rate of 85%. Our research underscores the importance of feedback regulation in generating robust and adaptable biological responses. The high validation rate of our model illustrates the advantages of discrete dynamic modeling for complex, nonlinear systems common in biology.

A Role for Plant KASH Proteins in Regulating Stomatal Dynamics

“…While there is a large degree of functional overlap between the two proteins, there are also critical differences, and their further analysis might shed light on the role of their intriguingly different expression patterns. This study reveals an unanticipated connection between stomatal regulation and a class of NE proteins known to be involved in nuclear anchoring and positioning, and adds two new players to the ever more complex world of guard cell biology (Albert et al, 2017). Addressing the connection between the phenotypes described here and the cellular role of SINE1 in guard cell nuclear positioning will likely be one of the more groundbreaking avenues of further study.…”

The Mode of Cytokinin Functions Assisting Plant Adaptations to Osmotic Stresses

“…Reduced CK levels inhibit cell division and cell growth. On the other hand, enhanced concentrations of ABA and ethylene promote the closure of stomatal apertures, thereby reducing water loss through transpiration, and activate the senescence-related Cys protease (phcp1) that promotes senescence of old leaves [16][17][18]. Characteristically, plants survive with minimum resources under osmotic stress.…”

“…Reactive oxygen species (ROS) are also generated in chloroplasts and mitochondrial electron transport chains in response to stress [88]. Ethylene promotes premature senescence of leaves through a series of coordinated events and enhances the activity of senescence-related Cys protease (PHCP1) [17][18]. Increased biosynthesis and signaling of ABA leads to reduced stomatal conductance and decreases the photosynthetic rate [26,89].…”

The PP2A‐interactor TIP41 modulates ABA responses in Arabidopsis thaliana

“…Several of these adaptive mechanisms are triggered by an increase in endogenous abscisic acid (ABA) levels. In addition to stress responses, ABA mediates numerous processes in growth and development, including seed maturation and germination (Finkelstein, ), stomatal movements (Albert et al ., ), root and seedling growth (Xiong et al ., ; Duan et al ., ) and leaf senescence (Song et al ., ). ABA is perceived by the PYR/PYL/RCAR (PYRABACTIN RESISTANCE; PYRABACTIN RESISTANCE‐LIKE; REGULATORY COMPONENT OF ABA RECEPTOR) receptors, and induces a conformational change that enables these receptors to interact with and negatively regulate the type 2C protein phosphatases (PP2C; Fujii et al ., ; Ma et al ., ; Park et al ., ).…”