3. Introduction
- Strigolactones are carotenoid-derived hormones
- First discovered in root exudates as potent germination stimulants (for a weed)
- Main functions:
- Signalling component for plant development (branching inhibition)
- Root exudates to promote symbiotic interaction
- Structure includes a tricyclic lactone linked to a hydroxymethyl butenolide
4. Signaling Mechanism
● Based upon hormone-activated
proteolysis
● In presence of SL, TCP-domain
transcription factor BRANCHED1 (BR1)
is transcribed.
● Hydrolysis of SL causes conformational
changes of receptor protein.
5. Strigolactone Signaling in Regulation of Shoot
Branching
- No consensus after the primary signalling mechanism
- Two models:
- SLs regulate protein levels of auxin efflux carrier (PIN) in plasma
membrane (Canalization model)
- SLs regulate transcription of TCP domain tf BRANCHED1 (BRC1)
(Direct action model)
6. Canalization
- Basis: Buds are also auxin sources; Buds that form the
strongest canalized link to main stem tend to be the
buds that grow out
- SLs repress PIN1 accumulation in plasma membrane
- Makes it harder for buds to canalize into the main stem
which results in reduced branching
- Controversial: Since it depends upon relative levels of
auxin transport (from buds and main stem); hard to
interpret data.
7. Direct Action Model
● SL and cytokinin affect branching directly in the
bud.
● Auxin inhibits outgrowth of dormant buds
indirectly.
● SL might be second messenger of auxin
● BRC1 might be direct target of SL signaling
8. Outstanding questions
- Functions of SMXL proteins
- Exact understanding of the receptor (D14) and repressor (SMXL) interactions
- How does SL regulate branching?
-F-box protein component of a Skp 1-Cullin-F-box (SCF) E3 ubiquitin ligase complex targets repressor protein (D53/SMXL) for polyubiquitination and degradation by the 26S proteasome.
-receptor is destabilized
-TPR is co-repressor
Auxin transport down the primary shoot dampens the flow of auxin from buds.
Further reading:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6433298/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4528729/
-SL and CK are primarily synthesized in root and transported to shoot through xylem.
-Cytokinin inhibit the expression of TF (BRC1) that regulate bud outgrowth; whereas, SL promote BRC1
-Both SL and CK synthesis are regulated by auxin, so they were thought to be plausible second messenger of auxin
-unless given a strigolactone supply by grafting, strigolactone-deficient mutants respond very poorly to auxin added to the stump after decapitation indicating that auxin requires strigolactones to elicit its inhibitory effect
-the constitutively low levels of BRC1 expression in SL-deficient mutants, and the constitutively high levels in smxl678, suggest that BRC1 is indeed a transcriptional target of SL signalling; but, in monocots, there is very little evidence that SLs act via transcriptional regulation of BRC1 family members (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5450845/)
