Convergence and Divergence of Signaling Events in Guard Cells during Stomatal Closure by Plant Hormones or Microbial Elicitors

Abstract

Dynamic regulation of stomatal aperture is essential for plants to optimize water use and CO2 uptake. Stomatal opening or closure is accompanied by the modulation of guard cell turgor. Among the events leading to stomatal closure by plant hormones or microbial elicitors, three signaling components stand out as the major converging points. These are reactive oxygen species (ROS), cytosolic free Ca2+, and ion channels. Once formed, the ROS and free Ca2+ of guard cells regulate both downstream and upstream events. A major influence of ROS is to increase the levels of NO and cytosolic free Ca2+ in guard cells. Although the rise in NO is an important event during stomatal closure, the available evidences do not support the description of NO as the point of convergence. The rise in ROS and NO would cause an increase of free Ca2+ and modulate ion channels, through a network of events, in such a way that the guard cells lose K+/Cl−/anions. The efflux of these ions decreases the turgor of guard cells and leads to stomatal closure. Thus, ROS, NO, and cytosolic free Ca2+ act as points of divergence. The other guard cell components, which are modulated during stomatal closure are G-proteins, cytosolic pH, phospholipids, and sphingolipids. However, the current information on the role of these components is not convincing so as to assign them as the points of convergence or divergence. The interrelationships and interactions of ROS, NO, cytosolic pH, and free Ca2+ are quite complex and need further detailed examination. Our review is an attempt to critically assess the current status of information on guard cells, while emphasizing the convergence and divergence of signaling components during stomatal closure. The existing gaps in our knowledge are identified to stimulate further research.

Arabidopsis Calmodulin-Like Proteins, CML15 and CML16 Possess Biochemical Properties Distinct from Calmodulin and Show Non-overlapping Tissue Expression Patterns

“…As the gatekeepers of CO 2 entry and water vapor exit, guard cells play key roles in both water relations and carbon fixation by adjusting their turgidity and thereby controlling stomatal pore aperture. Ca 2+ is well-established as a key second messenger in guard cell signaling and the strong activity of CML16pro::GUS in these cells suggests that CML16 may be serving as a Ca 2+ sensor in these cells during stomatal pore opening and/or closing, both of which involve Ca 2+ signals (Murata et al, 2015 ; Agurla and Raghavendra, 2016 ). CML16pro::GUS activity was also strong in vascular tissues at all growth stages examined, indicating that CML16 may participate in vascular tissue development or function.…”

Nitrated Nucleotides: New Players in Signaling Pathways of Reactive Nitrogen and Oxygen Species in Plants

“…Recent reports described the identification of cGMPdependent protein kinase with a role in mediating gibberellin responses in rice (Shen et al, 2019) and a plant cGMPactivated phosphodiesterase involved in the UVA-induced cGMP degradation in Arabidopsis stomata (Isner et al, 2019). Signaling pathways involved in the regulation of stomatal movements by external stimuli and phytohormones comprise, beside cGMP, diverse components like NO, ROS, cytosolic pH, calcium ions and phospholipids (Daszkowska-Golec and Szarejko, 2013;Gayatri et al, 2013;Agurla et al, 2014;Agurla and Raghavendra, 2016). Moreover, protein PTMs by phosphorylation and redox modifications have key functions in the regulation of stomatal movement.…”

Element content and expression of genes of interest in guard cells are connected to spatiotemporal variations in stomatal conductance

“…In addition to the changes in element content, guard cells are also subjected to pH variations during stomatal movement. In general, stomatal closure induces cytosolic and vacuolar alkalinization (Irving, Gehring, & Parish, 1992;Zhang, Dong, Gao, & Song, 2001), whereas stomatal opening induces acidification (Huang, She, Zhang, & Zhao, 2013), linked with H 2 O 2 and Ca 2+ signalling during both ABA and dark-induced stomatal responses (Agurla & Raghavendra, 2016;Ma & Niu, 2017). Moreover, external pH changes strongly altered transcript levels, which was linked to ABA-responsive element motifs in the promoter region of pH-responsive genes (Lager et al, 2010).…”