Figure 2.

A schematic diagram of chloroplast-located ion transporters and retrograde signal molecule 3′-phosphoadnenosine 5′-phosphate (PAP), and their roles in stomatal regulation. PHOT1 and PHOT2 sense blue light, which activates plasma membrane proton pump AHAs, and this leads to the efflux of H⁺ from cytosol [119]. The accumulated electrons on the cytosolic side lead to activation of plasma membrane-located potassium inward-rectifying channels [120], leading to K⁺ influx. However, these potassium inward-rectifying channels can be inhibited by cytosolic Ca²⁺ accumulation [121]. CNGCs and CAXs are responsible for cytosolic Ca²⁺ accumulation [36], and CAX can be inactivated by ABA, which increases cytosolic Ca²⁺ accumulation [122]. ABA also inhibits blue light-induced H⁺-ATPase activation, which leads to stomatal closure [123]. Sulphate can be transported into chloroplasts by SULTR for the biosynthesis of PAP [117,118]. PAP is degraded by SAL1/ALX8 to AMP [25]. Under drought stress, ROS production in chloroplasts reduces SAL1 activity, which leads to PAP accumulation in the protoplast [25]. PAP is then transported into the cytosol by PAP transporter, PAPST1 [79], from where it moves to the nucleus to bind to the stress response genes XRNs, which potentially leads to CDPKs expression [26]. CDPKs activate SLAC1 channels, which leads to anion efflux [26]. Cytosolic Ca²⁺ also has a role in regulating CDPKs [124]. Besides, CDPKs and protein 14-3-3 have a role in regulating vacuole potassium channels activity [125,126]. ABA-induced stomatal closure depends on OST1 activity. OST1 has a role in activating anion efflux and inhibits water aquaporin channel PIP2;1 activity [127,128], which leads to stomatal closure. Abbreviations: PHOT, phototropins; AHA, Plasma membrane H⁺-ATPase; ATP, adenosine triphosphate; ADP, Adenosine diphosphate; KAT1, K⁺ channel 1 in Arabidopsis; KAT2, K⁺ channel in Arabidopsis 2; AKT, Arabidopsis Thaliana Rectifying channel ; ACA, Ca²⁺-ATPase; CNGC, Arabidopsis Cyclic nucleotide-gated ion channels; NRT1.1, Nitrate Transporter 1.1; STP1, Sugar Transporter 1; ABA, Abscisic acid; ALMT, Aluminium-activated malate transporter; VHA, vacuolar H⁺-ATPase; AVP, vacuolar H⁺/K⁺-PPase; TIPs, Tonoplast Intrinsic Proteins; CAX, Cation Exchanger; CLCa, Chloride Channel a; NHX, Na⁺,K⁺/H⁺ antiporters; AMP, Adenosine Monophosphate; SAL1, Altered expression of APX2; PAP, 3′-phosphoadnenosine 5′-phosphate; SULTR, phloem-localized sulphate transporter; PAPST1, 3′-Phosphoadenosine 5′-Phosphosulfate Transporter 1;ABI, ABA Insensitive; OST1, Open Stomata 1; TPC, Two-pore Ca²⁺ channel; TPK, Two-pore K⁺ channel; CDPKs, Ca²⁺ dependent protein kinases; SLAC1, Slow Anion channel-associated 1; PIP2;1, Plasma Membrane Intrinsic Protein 2; GORK, Guard Cell Outwardly Rectifying K⁺ channel.