Plasma membrane H+-ATPases promote TORC1 activation in plant suspension cells

Target of Rapamycin (TOR): A Master Regulator in Plant Growth, Development, and Stress Responses

The target of rapamycin (TOR) is a central regulator of growth, development, and stress adaptation in plants. This review delves into the molecular intricacies of TOR signaling, highlighting its conservation and specificity across eukaryotic lineages. We explore the molecular architecture of TOR complexes, their regulation by a myriad of upstream signals, and their consequential impacts on plant physiology. The roles of TOR in orchestrating nutrient sensing, hormonal cues, and environmental signals are highlighted, illustrating its pivotal function in modulating plant growth and development. Furthermore, we examine the impact of TOR on plant responses to various biotic and abiotic stresses, underscoring its potential as a target for agricultural improvements. This synthesis of current knowledge on plant TOR signaling sheds light on the complex interplay between growth promotion and stress adaptation, offering a foundation for future research and applications in plant biology.

Identification of a Novel Stomatal Opening Chemical, PP242, That Inhibits Early Abscisic Acid Signal Transduction in Guard Cells

Plants control their stomatal apertures to optimize carbon dioxide uptake and water loss. Stomata open in response to light through the phosphorylation of the penultimate residue, Thr, of plasma membrane (PM) H+-ATPase in guard cells. Stomata close in response to drought and the phytohormone abscisic acid (ABA), and ABA suppresses the light-induced activation of PM H+-ATPase. However, the signaling pathways that regulate the stomatal aperture remain unclear. Previously, we identified a target of rapamycin (TOR) inhibitor, temsirolimus, to induce stomatal opening through chemical screening. In the present study, we further investigated other TOR inhibitors and identified PP242 as a novel stomatal opening chemical. PP242 induced stomatal opening even in the dark, as well as phosphorylation of the penultimate Thr of PM H+-ATPase in guard cells. Interestingly, PP242 completely suppressed ABA-induced stomatal closure, and inhibited ABA-induced activation of SNF1-related protein kinase 2s (SnRK2s), which are essential kinases for ABA signal transduction in guard cells. In vitro biochemical analysis revealed that PP242 did not directly inhibit SnRK2 but rather inhibited upstream ABA signaling components, specifically B3 clade Raf-like kinases. A quadruple mutant of B3 clade Raf-like kinases exhibited an open stoma phenotype that resembled the effect of PP242. However, PP242 still induced stomatal opening in this mutant, suggesting that PP242 also targets other guard cell components. Together, these results reveal that PP242 induces stomatal opening partly by inhibiting steady-state ABA signal transduction.

The fusicoccin story revisited

Fusicoccin (FC) is one of the most studied fungal metabolites to date. The finding that the plasma membrane H+-ATPase in combination with 14-3-3 proteins acts as a high-affinity receptor for FC was a breakthrough in the field. Ever since, the binding of FC to the ATPase-14-3-3 receptor complex has taken center stage in explaining all FC-induced physiological effects. However, a more critical review shows that this is not evident for a number of FC-induced effects. This review challenges the notion that all FC-affected processes start with the binding to and activation of the plasma membrane ATPase, and raises the question of whether other proteins with a key role in the respective processes are directly targeted by FC. A second unresolved question is whether FC may be another example of a fungal molecule turning out to be a 'copy' of an as yet unknown plant molecule. In view of the evidence, albeit not conclusive, that plants indeed produce 'FC-like ligands', it is worthwhile making a renewed attempt with modern improved technology to answer this question; the answer might upgrade FC or its structural analogue(s) to the classification of plant hormone.

Phosphoregulation of the yeast Pma1 H+-ATPase autoinhibitory domain involves the Ptk1/2 kinases and the Glc7 PP1 phosphatase and is under TORC1 control

Plasma membrane (PM) H+-ATPases of the P-type family are highly conserved in yeast, other fungi, and plants. Their main role is to establish an H+ gradient driving active transport of small ions and metabolites across the PM and providing the main component of the PM potential. Furthermore, in both yeast and plant cells, conditions have been described under which active H+-ATPases promote activation of TORC1, the rapamycin-sensitive kinase complex controlling cell growth. Fungal and plant PM H+-ATPases are self-inhibited by their respective cytosolic carboxyterminal tails unless this domain is phosphorylated at specific residues. In the yeast H+-ATPase Pma1, neutralization of this autoinhibitory domain depends mostly on phosphorylation of the adjacent Ser911 and Thr912 residues, but the kinase(s) and phosphatase(s) controlling this tandem phosphorylation remain unknown. In this study, we show that S911-T912 phosphorylation in Pma1 is mediated by the largely redundant Ptk1 and Ptk2 kinase paralogs. Dephosphorylation of S911-T912, as occurs under glucose starvation, is dependent on the Glc7 PP1 phosphatase. Furthermore, proper S911-T912 phosphorylation in Pma1 is required for optimal TORC1 activation upon H+ influx coupled amino-acid uptake. We finally show that TORC1 controls S911-T912 phosphorylation in a manner suggesting that activated TORC1 promotes feedback inhibition of Pma1. Our results shed important new light on phosphoregulation of the yeast Pma1 H+-ATPase and on its interconnections with TORC1.