Abscisic acid signaling across land plants: PYR/PYL–PP2C–Raf–SnRK2 relay and transcriptional or stomatal outputs

Land plants share an abscisic acid (ABA) signaling toolkit comprising PYR/PYL/RCAR receptors, clade A PP2C phosphatases, Raf-like kinases, ABA-responsive SnRK2 kinases and ABRE-binding bZIP transcription factors. In the established angiosperm relay, ABA-stimulated receptors inhibit PP2Cs and relieve their inhibition of SnRK2; Raf-mediated phosphorylation initiates SnRK2 activity, followed by amplification through intermolecular SnRK2 phosphorylation. Constitutively active Arabidopsis B2 RAFs and stress-responsive B3 RAFs have distinct regulatory roles. Moss ARK also activates SnRK2, but moss evidence instead places PP2CAs mainly downstream of SnRK2 and do not impose the same upstream ABA-dependent release switch. Some receptors, including liverwort MpPYL1 and monomeric rice RCAR5/PYL5, retain basal phosphatase inhibition without hormone. Activated SnRK2s support bZIP-dependent protective transcription in both moss and angiosperm experiments. A guard-cell branch directly established in Arabidopsis and rice couples SnRK2 phosphorylation to SLAC1 anion efflux and stomatal closure. That branch is optional within this land-plant module; liverworts lack stomata. ABA biosynthesis, transport, catabolism, calcium-dependent branches and protein turnover lie outside the boundary.

The taxon is an envelope for the conserved receptor–phosphatase–kinase signaling architecture, not a claim that every lineage implements every output or uses identical paralogs. Representatives span a liverwort, a moss, a monocot and a eudicot; each is accompanied by a gene review. Family identifiers are PANTHER classifications with official labels, constrained by the experimentally established role; they do not transfer that role to every member of a broad family. The native receptor, phosphatase and kinase pairings differ among species. In particular, rice PP2C06 and SAPK9 exemplify the transcriptional relay; their inclusion does not assert that PP2C06 is the native guard-cell phosphatase for SAPK10. Published rice guard-cell work instead identifies PP2C50/53. No PAINT ancestral placement is inferred merely from the representative set. The 2009 four-component reconstitution is retained as evidence for its reduced assay, not as proof that Raf kinases are dispensable in vivo. Location descriptors describe the demonstrated signaling compartments, not obligatory localization of every family member in every tissue. Shared components do not imply conserved edge topology: the direct PP2C-to-SnRK2 inhibitory connection is explicitly angiosperm-scoped, while moss PP2CA has a downstream inhibitory role and an additional ABA-to-ARK/SnRK2 input. No unverified molecular PP2CA substrate edge is invented for moss. Current UniProt models at the experimentally tested moss ABI5A/B loci conflict with the reported cloned proteins, so moss transcriptional evidence is represented at the family/step level without a guessed protein accession.