STS3 (Vitis vinifera) — research notes

UniProt: P51071 (THS3_VITVI). Stilbene synthase 3 / Resveratrol synthase 3 /
Trihydroxystilbene synthase 3 (StSy 3). EC 2.3.1.95. AltName PSV368.
OrderedLocusNames VIT_16s0100g01030 (chromosome 16). 392 aa, ~42.8 kDa.

Summary of gene function

STS3 is a type III polyketide synthase (PKS III) of the chalcone/stilbene synthase
family. It catalyzes the iterative condensation of one molecule of p-coumaroyl-CoA
(4-coumaroyl-CoA) with three molecules of malonyl-CoA, followed by an intramolecular
aldol cyclization and decarboxylation, to produce trans-resveratrol
(3,4',5-trihydroxystilbene), the major stilbenoid phytoalexin of grapevine.

UniProt catalytic activity (Rhea:RHEA:11936):
4-coumaroyl-CoA + 3 malonyl-CoA + 3 H+ = trans-resveratrol + 4 CO2 + 4 CoA (EC 2.3.1.95).
UniProt PATHWAY: "Phytoalexin biosynthesis; 3,4',5-trihydroxystilbene biosynthesis;
3,4',5-trihydroxystilbene from trans-4-coumarate: step 2/2."
UniProt SUBUNIT: homodimer. SUBCELLULAR LOCATION: Cytoplasm. INDUCTION: by stress.
UniProt FUNCTION line: "Mediates resistance to pathogens which are sensitive to
stilbenes" (ECO:0000250, i.e. inferred by similarity, not direct evidence for STS3).

Type III PKS mechanism — relationship to chalcone synthase (CHS)

STS and CHS are paralogous type III PKS enzymes. They use the same substrates
(p-coumaroyl-CoA + 3 malonyl-CoA) and build the same linear tetraketide
intermediate via a single active-site Cys-His-Asn catalytic triad (the Cys is the
acyl carrier nucleophile; in STS3 the conserved active-site residue is annotated at
position 164, ECO:0000255 PROSITE-ProRule:PRU10023).

The enzymes diverge only at the cyclization step:
- CHS performs a C6→C1 Claisen condensation giving naringenin chalcone (a flavonoid
precursor).
- STS performs an alternative C2→C7 aldol condensation plus decarboxylation giving
the stilbene backbone of resveratrol.

PMID:10426957

PMID:15380179 — this is the "aldol switch" study; an "aldol switch" hydrogen-bonding
network (the "thioesterase-like" pocket) in STS reorients the terminal carbonyl to
favor aldol over Claisen cyclization.

PMID:10926848
Note: STS3 contains the FGPG loop (sequence ...LFGFGPGLTIE... near residues 367–377).

STS arose from CHS by convergent evolution multiple independent times across
stilbene-producing lineages PMID:22961129.

The STS gene family in grapevine

Unlike most stilbene-producing plants where STS forms small families of 2–5 paralogs,
grapevine has an unusually large STS gene family. Reannotation of the PN40024
reference genome yielded 48 STS genes, ≥32 potentially functional, all encoding
proteins with bona fide STS activity PMID:22961129. STS3 (VIT_16s0100g01030, chromosome 16) is one member of this family;
the family is dominated by purifying selection with no strong evidence of neofunctional
divergence PMID:22961129.

The historically best-characterized grapevine STS gene is Vst1 (also called StSy);
"STS3" / PSV368 / pSV21–pSV25 are early-isolated grape STS cDNAs. The original
P51071 sequence comes from a stress-/elicitor-induced grape (cv. Optima) cDNA library
PMID:1898048.

Phytoalexin biology and "defense response"

Resveratrol and its derivatives (piceids, viniferins, pterostilbene) are stilbenic
phytoalexins — antimicrobial secondary metabolites synthesized de novo by the plant
in response to biotic and abiotic stress.

STS expression is elicitor- and stress-inducible: STS and PAL (phenylalanine
ammonia-lyase) transcripts are co-induced within 1 h of fungal cell-wall elicitor
treatment of grape cell cultures PMID:1898048.
STS transcripts and stilbenoid content increase upon powdery mildew (Erysiphe /
Uncinula necator) infection [PMID:23116673 "transcripts of selected STS genes
increased significantly in Cabernet Sauvignon leaves at 24 and 48 h post inoculation
with PM spores"; PMID:31531782 "the levels of STS expression and stilbenoids
increased in response to powdery mildew infection"].
Resveratrol is directly antimicrobial: 1.0 mM resveratrol inhibits mycelial growth of
Phytophthora palmivora in vitro PMID:15309535.
Heterologous transfer confers disease resistance: expressing grapevine STS genes
(Vst1) in non-stilbene plants (papaya, tobacco, etc.) raises stilbene content and
increases pathogen resistance PMID:15309535. But this is not universal — transgenic white
poplar accumulated resveratrol glucosides yet showed no increased rust resistance
PMID:15359598, showing that the STS enzyme itself is one biosynthetic input, not the
whole defense program.
STS expression is governed by defense-associated transcription factors, notably
MYB14/MYB15, downstream of oxidative burst, Ca2+ influx, MAPK and jasmonate
signaling PMID:26842984.

Key curation point — "defense response" (GO:0006952) is process-conflation

STS3's molecular function is a biosynthetic synthase (trihydroxystilbene synthase).
Its product (resveratrol) is antimicrobial, so the gene is defense-associated, but
"defense response" (GO:0006952) as a biological-process annotation conflates the
downstream protective role of the metabolite with the catalytic activity of the enzyme.
The enzyme does not itself sense pathogens or execute a defense response; it
biosynthesizes a stilbene that happens to be a phytoalexin. The accurate biological
process is stilbene biosynthetic process (GO:0009811) and/or phytoalexin
biosynthetic process (GO:0052315). GO:0052315 "phytoalexin biosynthetic process" is a
child of both secondary metabolite biosynthetic process (GO:0044550) and toxin
biosynthetic process (GO:0009403), and it captures the defense context far more
precisely than the generic GO:0006952 without misrepresenting the enzyme as a defense
effector. This is the classic "biosynthetic-enzyme-labelled-with-the-downstream-process"
over-annotation pattern.

GO term reference (verified via OLS)

GO:0050350 trihydroxystilbene synthase activity — "Catalysis of the reaction: 3
malonyl-CoA + 4-coumaroyl-CoA = 4 CoA + 3,4',5-trihydroxy-stilbene + 4 CO2." Direct
parent: GO:0016747. This is the precise MF for STS3 and matches EC 2.3.1.95 and
Rhea:11936 exactly.
GO:0016747 acyltransferase activity, transferring groups other than amino-acyl groups
— parent of GO:0050350.
GO:0016746 acyltransferase activity — grandparent of GO:0050350.
GO:0009811 stilbene biosynthetic process — "formation of stilbenes, a class of
polyketide compounds formed from cinnamic acid and three molecules of malonyl CoA."
Precise BP for STS3.
GO:0052315 phytoalexin biosynthetic process — child of GO:0044550 (secondary
metabolite biosynthetic process) and GO:0009403 (toxin biosynthetic process).
GO:0044550 secondary metabolite biosynthetic process — broader BP.
GO:0006952 defense response — the retired SPKW annotation; over-broad / process-
conflation for a biosynthetic enzyme.
GO:0005737 cytoplasm — consistent with UniProt SUBCELLULAR LOCATION.

Provenance notes

All four directly cited primary PMIDs (1898048, 22961129, 15380179, 10426957) are
cached abstract-only (no PMC full text); supporting quotes are from the abstracts.
PMID:23116673, PMID:31531782, PMID:15309535, PMID:15359598, PMID:26842984 used for
context (phytoalexin/defense biology); abstracts retrieved via PubMed.
Bibliographic data retrieved from PubMed.