Modeling rapid stomatal closure with synchronous Boolean network approach

Abstract

The phytohormone Abscisic acid (ABA) is an endogenous messenger in plant abiotic stress responses. Drought stress increases the level of ABA triggering the fastest adaptive physiological response of plants- closure of stomata (guard cells). Understanding gene/protein expression involved in stomatal closure has great importance to genetic modification of plants to survive in severe climatic conditions. However, systems level information that defines the communication pattern of the related network of cellular molecules is not yet known. This study integrates fragmentary information collected from literature to define the dynamics of ABA signaling in rapid closure of stomata through a synchronous Boolean model. Stomatal closure in broad terms is a combined result of organic and inorganic ion regulation to release water from the guard cells through osmosis, and rearrangement of Actin to facilitate resulting stomatal movement. Our network consists of 57 nodes and their interaction dynamics defined in accordance with past experimental results to regulate stomatal closure. Perturbation analysis was conducted to identify the essential elements crucial for the above mentioned global functions of pumping out water and stomatal movement. It revealed that destruction of ABA receptor complex (PYR/PYL, PP2C and SnRK2 proteins) made stomata insensitive to closure as a result of disruption of signal transmission to downstream regulators. It was identified that plasma membrane outward ion channels GORK and SLAC1 are crucial for pumping out water by reducing the osmotic load inside the guard cell which facilitates osmosis. Inhibition of MAPK kinases and cytosolic alkalization, as being important regulators of SLAC1, and membrane depolarization, important for GORK, showed drastic effect on the stomatal closure. In contrast, overexpression of plasma membrane H⁺-pumping and potassium-in channels inhibit stomatal closure by enhancing the osmotic concentration and there by attracting water inside. Loss of function of Actin rearrangement resulted in a loss of stomatal closure as structural rearrangement of guard cell are necessary to facilitate the cell shrinkage. Disruption of Reactive Oxygen Species or their regulators (RBOH, PA, PLD or RCN1), SCAB1 protein and overexpression of AtRAC1 showed drastic effects on structural rearrangements. Perturbation analysis revealed that the number of elements crucial to stomatal closure comprises about 30% of the network; and thus stomatal closure is robust against perturbation in the other 70% of network elements. These results are in agreement with experimental findings and indicate potential redundancy with respect to stomatal closure.