UniProt: Q700C7 (SPCH_ARATH), AT5G53210, 364 aa, bHLH transcription factor (bHLH098 / AtbHLH98).
Taxon: Arabidopsis thaliana (NCBITaxon:3702).
SPCH is the master bHLH transcription factor that initiates the stomatal cell
lineage. It is necessary and sufficient to trigger the asymmetric "entry"
divisions of meristemoid mother cells (MMCs) that establish the stomatal
lineage. It acts at the first step of a three-bHLH cascade (SPCH → MUTE →
FAMA): SPCH drives lineage initiation/amplifying asymmetric divisions, MUTE
drives the meristemoid-to-guard-mother-cell transition, and FAMA drives
terminal guard cell differentiation.
SPCH is a genuine sequence-specific DNA-binding transcription factor. It
functions as a heterodimer with SCRM/ICE1 (ICE1) and SCRM2; these heterodimers
are nuclear-localized DNA-binding transcription factors. The bHLH domain
(residues 99–150; basic motif 99–112) is required for DNA binding and target
gene activation.
The spch-5 allele carries a point mutation in the bHLH domain (R111W;
HVTVERNRR→PVTVPRNRP at 104-112) and shows decreased ability to initiate/amplify
lineages, defects in asymmetric cell fate allocation, and misorientation of
division planes — confirming the bHLH domain mediates SPCH's DNA-binding/
transactivation function in vivo.
GO:0001216 "DNA-binding transcription activator activity" definition (QuickGO):
"A DNA-binding transcription factor activity that activates or increases
transcription of specific gene sets." This is the IDA annotation from
PMID:28507175 (SPCH activates target gene transcription with SCRM/ICE1) — a
correct, specific MF for SPCH. It subsumes the more general GO:0003700
"DNA-binding transcription factor activity" (IEA/ISS).
SPCH is nuclear. UniProt SUBCELLULAR LOCATION: Nucleus (PROSITE-ProRule and
PubMed:19008449 IDA). The cytoplasm annotation (GO:0005737) is from a single
computational predictor (AtSubP / GO_REF:0000122, ISM) and is not supported by
experimental data; SPCH function is nuclear. Treat cytoplasm as
over-annotation (prediction-only, contradicted by IDA nuclear localization).
SPCH controls regulation of stomatal complex development and stomatal complex
development by promoting the first asymmetric (entry) divisions and the
amplifying asymmetric divisions of the stomatal lineage.
SPCH activity is negatively regulated by MPK3/MPK6 phosphorylation in a unique
MAPK target domain (the disordered region ~171-227, including Ser193, Ser211,
Thr214, Ser219). This couples the EPF–ERECTA/TMM–YODA(YDA) MAPK cascade to
stomatal patterning. SPCH is also phosphorylated by BIN2/ASK7 (GSK3-like;
inhibitory) and stabilized by CDKA;1 phosphorylation at Ser186 (promotes
stomatal development).
SPCH protein level is post-transcriptionally decreased under osmotic stress via
the MAPK cascade, providing a route by which environmental stress reduces
growth by reducing MMC number. This is environmental modulation of SPCH abundance
(SPCH is the developmental effector), not a dedicated stress-response function
of SPCH itself.
Low relative humidity (LRH) represses SPCH transcription via RNA-directed de
novo DNA methylation (RdDM) of the SPCH locus. Here SPCH is the target/effector
of the epigenetic response; the "response to low humidity" IEP captures
SPCH transcriptional repression under LRH, a downstream/environmental modulation
rather than a core SPCH molecular function.
Three IPI "protein binding" annotations exist:
- PMID:18641265 → SCRM/ICE1 (Q9LPW3) and SCRM2 (Q9LSE2): functionally the core bHLH heterodimerization partners. Per curation guidelines, "protein binding" is uninformative; the informative term is protein dimerization activity / bHLH heterodimerization (captured by GO:0046983 and by core_functions). Keep as non-core (records a real, important interaction but with an uninformative MF term).
- PMID:25680231 → CDKA;1 (P24100): a regulatory kinase interaction (Ser186 phosphorylation). Real but uninformative MF term.
- PMID:32612234 → RACK1B (Q9C4Z6): from a large-scale phytohormone interactome (Y2H) screen. UniProt INTERACTION block records Q700C7–Q9C4Z6 (RACK1B), NbExp=3. High-throughput interactome; biological significance for SPCH unclear; uninformative MF term. Keep as non-core, not core.
GO:0046983 "protein dimerization activity" IEA InterPro: family-level but
biologically correct — SPCH forms bHLH dimers (with SCRM/SCRM2). This is the
informative MF version of the "protein binding" interactions and supports the
heterodimerization core function.