Is IAA the first essential signal hormone for the stomatal opening?
The hypothesis which is still widely accepted to explain stomatal activities involves fluxes of inorganic cations and anions across the plasma membrane and tonoplast of guard cells associated with the synthesis and degradations of photosynthetic products. It was reported that previously darkened leaves expose to light showed quenching of fluorescence in the apoplast surrounding the guard cells up to 20 min. before stomatal opening (Edwards et al. 1988). They showed that proton efflux originating at the guard cells preceded stomatal opening, confirming earlier works which suggested that proton efflux was a necessary precursor of stomatal opening (Raschke & Humble 1973). Therefore, when stomata open, protons are first pumped out from the guard cell, resulting in hyperpolarization of plasma membrane potential difference. Consequently, K+ may then passively enter guard cell cytoplasm from the guard cell wall or subsidiary cells. Therefore, the first reaction that occurs when stomata are opened, is the activity of PM (plasma membrane) H+-ATPase present in the guard cell. How does PM H+-ATPase activate? It can be assumed that PM H+-ATPase can be activated by light simply because stomata open when plants receive light and stomata close in the dark. Very important points here are understandings of the structures and activities of PM H+-ATPase. PM H+-ATPase can be activated by phosphorylation of their penultimate residue (a Thr) and the subsequent binding of regulatory 14-3-3 proteins. The confusions are that PM H+-ATPase is activated by regulatory 14-3-3 proteins by IAA, fusicoccin and light (Olsson et al. 1998, Svennelid et al. 1999, Takahashiet al. 2012, Eigo & Kinoshita 2018). These results suggest that the first signal material required for stomata to open can be IAA or light. Therefore, it means that the initial signal transduction pathway of stomatal opening can be induced by light regardless of IAA. However, delicate reactions such as stomatal opening require light, but it must be speculated that IAA, which acts as a signal for cell wall growth, phototropism, thigmotropism, and many metabolisms of plants as a signal material, is highly likely to be the first signal material for stomatal opening (Fig. 1). Plant V (vacuolar)-H+-ATPase and H+-ppase maintain the vacuole acidity, so their activities are very high. IAA transports to guard cells through PGP/ABCB using ATP in the plasma membrane of the mesophyll cells (Fig. 1). The main synthesis sites for IAA are the cytoplasms of young leaf mesophyll cells, which are also synthesized in mature leaves (Fig. 1). IAA promotes the activity of PM H+-ATPase to acidify the cell wall. Plant V-H+-ATPase could be also activated by IAA. This is because PM H+-ATPase and V-H+-ATPase are monophyly, so they have high phylogenetic relationships, similar structures and functions. The high activities of many V-H+-ATPase and H+-ppase promote H+ transport to vacuole and increases sucrose transport to vacuole by H+-sucrose antiporters to balance H+charges (Fig. 1). It was also reported that the transcription of H+-PPase gene IbVP1 in sweet potato plants was strongly induced by auxin in hydroponics (Fan et al. 2017).