Brassinosteroids and Other Signaling Molecules
A pollen grain lands on a stigma. A leaf gets bitten by a caterpillar. A root senses phosphate starvation in the soil. A stem responds to a fungal attack on the far side of the plant. These are four completely different problems. Yet in each case, the plant reaches for a small, non-classical signaling molecule to solve it. For decades, plant hormone physiology was built almost entirely around five "classical" hormones. These are auxin, cytokinin, gibberellin, ethylene, and abscisic acid, introduced in Plant Hormones: An Overview. But those five cannot explain everything. They cannot explain how a plant recognizes a pathogen as self versus non-self. They cannot explain how it decides to sacrifice a damaged leaf's growth to make defensive chemicals. Nor can they explain how symbiotic fungi are invited into the root. They also cannot explain how steroid-like molecules embedded in the plasma membrane relay a growth signal without entering the nucleus directly. This article covers four such molecules: brassinosteroids, jasmonates, salicylic acid, and strigolactones. Each has its own dedicated receptor, its own transduction logic, and its own well-documented experimental history. None of it is hand-waving. All of it is built on mutants, biochemistry, and grafting experiments. Any student can trace these back to a primary paper. Brassinosteroids alone justify close study. Their receptor, BRI1, was the first plant hormone receptor shown to be a cell-surface receptor kinase, rather than a nuclear or cytoplasmic protein. This discovery reshaped how biologists think about plant signal perception in general. Why does this matter beyond taxonomy of hormone names? Because these molecules are the plant's chemical vocabulary for context-dependent decisions. They govern choices like: grow now or defend now, branch or don't branch, tolerate this microbe or reject it. Agriculture already exploits this vocabulary. Brassinosteroid mimics increase yield and stress tolerance. Jasmonate and salicylic acid pathways underlie induced resistance sprays. Strigolactone biology explains both a devastating parasitic weed and the semi-dwarf "Green Revolution"-adjacent branching architecture of modern crops. This article walks through the molecular mechanism of each pathway. It covers the classic experiments that established each of them. These range from a Chinese cabbage extract that inspired the entire brassinosteroid field, to coi1 and npr1 mutant screens, to the Striga-germination bioassay that unmasked strigolactones. The article also includes two worked quantitative examples. Finally, it surveys translational uses in medicine-adjacent and agricultural biotechnology.
- Brassinosteroids are steroid hormones perceived at the cell surface by the receptor kinase BRI1, not by a nuclear receptor.
- BRI1 signaling works by relieving inhibition: active BRI1 ultimately inactivates the kinase BIN2, which allows the transcription factors BZR1/BES1 to accumulate and reprogram gene expression.
- Jasmonates (jasmonic acid, JA-Ile) signal wounding and herbivory; perception by the COI1–JAZ co-receptor triggers JAZ repressor degradation via the ubiquitin-proteasome pathway.
- Salicylic acid (SA) mediates defense against biotrophic pathogens and drives systemic acquired resistance (SAR), acting partly through the redox-sensitive coactivator NPR1.
- Strigolactones are carotenoid-derived hormones that suppress shoot branching, shape root architecture, and — secreted into soil — recruit mycorrhizal fungi while inadvertently germinating the parasite Striga.
- Across all four pathways, the same logic recurs: hormone perception often works by degrading a repressor protein, not by directly activating an activator.
Plants often "turn on" a response not by switching on an activator directly, but by using a hormone-bound receptor to mark a repressor protein for destruction — de-repression, not direct activation, is the dominant logic of brassinosteroid, jasmonate, and (partly) salicylic acid signaling.
The core idea
Brassinosteroids (BRs) are polyhydroxylated steroid molecules — the most biologically active being brassinolide — structurally similar to animal steroid hormones and insect ecdysteroids, but perceived in a fundamentally different way. Whereas animal steroids diffuse through the plasma membrane and bind soluble nuclear receptors, brassinosteroids are perceived by a leucine-rich-repeat receptor kinase (LRR-RK) called BRI1 (BRASSINOSTEROID INSENSITIVE 1) sitting in the plasma membrane, with its LRR domain facing the cell exterior/apoplast. This was a landmark realization: it showed that plants evolved an entirely independent solution to steroid signaling, using receptor kinase machinery that plants share (in modified form) with innate immune receptors, rather than nuclear hormone receptors as in animals.
Jasmonates (jasmonic acid and its bioactive conjugate jasmonoyl-isoleucine, JA-Ile) are oxylipins — molecules derived from oxidation of membrane fatty acids (the octadecanoid pathway, starting from linolenic acid) — that function as the plant's primary "wound and herbivory" alarm hormone, also active in some pathogen defense (especially against necrotrophs) and in flower development. Salicylic acid (SA) is a simple phenolic compound, biosynthesized mainly via the isochorismate pathway in chloroplasts, that governs defense against biotrophic and hemibiotrophic pathogens and is central to the whole-plant immune memory phenomenon called systemic acquired resistance. Strigolactones (SLs) are carotenoid-derived (apocarotenoid) molecules that act both as an internal hormone — chiefly a repressor of axillary bud outgrowth — and as an externally secreted rhizosphere signal that attracts beneficial arbuscular mycorrhizal fungi, a signal that the root parasite Striga has evolutionarily hijacked as its own germination cue. All four systems share deep structural logic even though their chemistry differs wildly: a specific receptor (a receptor kinase, an F-box/SCF co-receptor, or a soluble sensor), a small number of central hub proteins whose stability or conformation the hormone controls, and downstream transcriptional reprogramming that reshapes growth, defense, or symbiosis. Recognizing this shared logic — receptor perception → post-translational control of a hub protein → transcription factor activity change — lets you understand each new hormone pathway as a variation on a small set of themes rather than as isolated trivia.
Mechanism
Brassinosteroid perception by BRI1 — step by step
1. Ligand binding at the cell surface. Brassinolide binds directly to the extracellular LRR domain of BRI1, specifically to an "island domain" inserted between two LRR repeats near the C-terminal end of the ectodomain. This is unusual for LRR receptors, which typically bind ligands across a continuous concave LRR surface; BR instead nestles into this island pocket like a plug docking into a socket.
2. Co-receptor recruitment. Hormone binding induces BRI1 to heterodimerize with a smaller LRR receptor kinase partner, BAK1/SERK3 (a member of the SOMATIC EMBRYOGENESIS RECEPTOR KINASE family). BAK1 itself does not bind brassinosteroid; it is recruited only after BRI1 is already occupied, acting as a co-receptor that stabilizes the active complex and enables full kinase activation. (BAK1 moonlights in several other receptor complexes, notably plant immune receptors — a nice illustration of how plants reuse signaling hardware across pathways.)
3. Trans-phosphorylation and activation. BRI1 and BAK1 trans-phosphorylate each other's cytoplasmic kinase domains on multiple serine/threonine residues, boosting kinase activity. Before hormone binding, BRI1's own C-terminal tail and an inhibitory protein called BKI1 (BRI1 KINASE INHIBITOR 1) keep the receptor auto-inhibited; ligand-induced conformational change causes BKI1 to be phosphorylated and released from the membrane, permitting BAK1 recruitment.
4. Signal relay through BSK/CDG1 to BSU1. Activated BRI1 phosphorylates cytoplasmic receptor-like kinases called BSKs (BR-SIGNALING KINASES) and CDG1, which in turn activate a phosphatase, BSU1 (BRI1 SUPPRESSOR 1).
5. Inactivation of the central brake, BIN2. BSU1 dephosphorylates and thereby inactivates a GSK3-like kinase called BIN2 (BRASSINOSTEROID INSENSITIVE 2). This is the crux of the whole pathway: in the absence of brassinosteroid, BIN2 is constitutively active and phosphorylates the transcription factors BZR1 and BES1/BZR2, which forces them to be retained in the cytoplasm (bound to 14-3-3 proteins) and targeted for degradation. Brassinosteroid signaling therefore works by removing an active brake, not by stepping on an accelerator.
6. Transcription factor accumulation and DNA binding. With BIN2 inactivated, protein phosphatase PP2A dephosphorylates BZR1/BES1, unmasking a nuclear localization signal. Dephosphorylated BZR1 and BES1 accumulate in the nucleus, bind BRRE and E-box DNA elements in promoters, and switch on or off thousands of BR-responsive genes controlling cell elongation, vascular differentiation, stomatal patterning, and stress tolerance.
7. Physiological output. The net effects include cell elongation (via loosening of the cell wall and modulation of expansins and cellulose synthase activity), promotion of xylem differentiation, pollen tube growth, and — notably — a dose-dependent interaction with light signaling: BR-deficient mutants show a "de-etiolated" phenotype even in darkness (short hypocotyl, open cotyledons), because BZR1/BES1 normally cooperate with the transcription factor PIF4 to promote skotomorphogenic elongation growth.
Jasmonate perception — COI1–JAZ, another de-repression module
Wounding or herbivore feeding triggers release of linolenic acid from chloroplast membranes; the octadecanoid pathway (lipoxygenase → allene oxide synthase → allene oxide cyclase, then peroxisomal β-oxidation) converts it to jasmonic acid (JA). JA is conjugated to isoleucine by the enzyme JAR1 to form the truly bioactive hormone, JA-Ile. JA-Ile is perceived not by a classical single receptor but by a co-receptor pair: the F-box protein COI1 (CORONATINE INSENSITIVE 1), part of an SCF (Skp1–Cullin–F-box) E3 ubiquitin ligase complex, and a JAZ (JASMONATE ZIM-DOMAIN) repressor protein. JA-Ile acts as molecular glue, binding in a pocket between COI1 and JAZ and increasing their affinity for one another — structurally analogous to how auxin glues TIR1 to Aux/IAA repressors, a mechanism this pathway shares by convergent logic. Once JAZ is bound by JA-Ile-loaded COI1, it is poly-ubiquitinated and degraded by the 26S proteasome. JAZ proteins normally repress transcription factors such as MYC2 by recruiting the co-repressor TOPLESS; their destruction liberates MYC2 to activate transcription of defense genes (proteinase inhibitors, terpene synthases, alkaloid biosynthetic enzymes) and of genes governing anther/pollen development.
Salicylic acid — a partly redox-based, partly de-repression logic
SA is synthesized mainly in the chloroplast via isochorismate synthase (ICS1), converting chorismate to isochorismate, which is then converted to SA. Pathogen recognition — either basal PAMP-triggered immunity or gene-for-gene effector-triggered immunity via NLR resistance proteins — drives a burst of SA accumulation at and around the infection site. SA changes the cellular redox state, and this reduces disulfide bonds holding an SA-binding protein called NPR1 (NONEXPRESSER OF PR GENES 1) in an inactive oligomeric form in the cytoplasm. Reduced, monomeric NPR1 translocates into the nucleus, where it acts as a transcriptional coactivator, partnering with TGA-family bZIP transcription factors to switch on PR (pathogenesis-related) genes such as PR-1, a classical SAR marker. SA additionally moves — together with other mobile signals such as methyl salicylate, azelaic acid, and pipecolic acid — from the infected leaf through the phloem to prime distal, uninfected tissue, establishing systemic acquired resistance: a plant-wide, long-lasting (weeks), broad-spectrum enhanced defensive readiness.
Strigolactones — carotenoid-derived branching hormone and rhizosphere signal
SLs are synthesized from β-carotene through a conserved core pathway: the carotenoid isomerase D27, then two carotenoid cleavage dioxygenases CCD7 and CCD8, producing the intermediate carlactone, which cytochrome P450 MAX1 (or its orthologs) further oxidizes into diverse strigolactone species. SLs are perceived by an unusual receptor, D14 (DWARF14), an α/β-hydrolase that is simultaneously an enzyme and a receptor: D14 hydrolyzes the strigolactone molecule, and a covalently modified reaction intermediate remains bound, inducing a conformational change that lets D14 dock with the F-box protein MAX2 (an SCF component) and the target repressor D53 (or its homologs, the SMXL6/7/8 family in Arabidopsis). D53/SMXL proteins are then ubiquitinated and degraded — a third instance of hormone-triggered repressor destruction. D53 normally represses branching-promotion genes and stabilizes the transcription factor BRC1 (BRANCHED1), which itself suppresses axillary bud outgrowth; loss of D53 shifts the balance toward increased BRC1 activity and suppressed branching (the details of exactly how D53 loss reduces branching, rather than increases it, reflect a layered repressor network still being refined — indicated here as a genuine area of ongoing research). Independently of axillary bud control, SLs shape root architecture (inhibiting lateral root and adventitious root formation under phosphate-replete conditions, promoting root hair elongation under phosphate starvation) and are exuded from roots into the rhizosphere, where they stimulate hyphal branching in arbuscular mycorrhizal fungi — a beneficial signal that the parasitic plant Striga (witchweed) has co-opted as a seed-germination trigger, since Striga seeds will not germinate without detecting host-root strigolactones.
The evidence
Discovering brassinosteroids — the rape pollen bioassay. In 1970, USDA scientist John W. Mitchell and colleagues extracted pollen from Brassica napus (rapeseed) and found that the lipophilic fraction, applied to bean second internodes, caused dramatic stem-swelling and bending ("brassin" bioassay). The active compound was purified and its structure solved in 1979 by Grove and colleagues as a novel polyhydroxysteroid lactone, named brassinolide — establishing steroids as a new class of plant hormone distinct from anything then known.
BRI1 identified through a dwarf-mutant screen and biochemical rescue. In the 1990s, Joanne Chory's laboratory characterized Arabidopsis dwarf mutants that superficially resembled gibberellin-deficient dwarfs but, critically, did not respond to exogenous gibberellin or brassinosteroid application — pointing to a perception defect rather than a biosynthetic one. The causal gene, bri1, was cloned in 1997 (Li and Chory) and shown to encode an LRR receptor kinase. The definitive proof that BRI1 is the actual hormone receptor came from direct ligand-binding assays: radiolabeled brassinolide bound specifically to microsomal membrane fractions and to the purified extracellular domain of wild-type BRI1, but binding was lost or reduced in bri1 loss-of-function alleles carrying mutations in the island domain — a clean structure-function demonstration that this single receptor protein is both necessary and sufficient for hormone binding at the relevant affinity (dissociation constants in the low nanomolar range).
BIN2 and the epistasis argument for de-repression. Genetic epistasis was decisive in establishing the pathway's logic. Loss-of-function bin2 mutants suppress the dwarf phenotype of bri1 mutants — i.e., removing the brake rescues growth even without a functional receptor — showing BIN2 acts downstream of BRI1 and normally restrains growth. Conversely, gain-of-function (constitutively active) bin2 alleles produce dwarf, dark-green plants phenocopying brassinosteroid-deficient mutants, confirming BIN2 activity opposes BR signaling output. This kind of "does mutant A rescue mutant B" epistasis analysis is the standard genetic tool used to order any signaling pathway, and it is exactly how the BRI1→BIN2→BZR1 hierarchy was built without needing to guess at biochemistry first.
Jasmonate: the coi1 screen and coronatine mimicry. The COI1 gene was identified in a screen for Arabidopsis mutants resistant to coronatine, a phytotoxin produced by the bacterium Pseudomonas syringae that structurally mimics JA-Ile. Plants carrying loss-of-function coi1 mutations were insensitive to coronatine, male-sterile, and defective in wound and herbivore defense responses (Feys, Benedetti, Penfold, and Turner, 1994, and subsequent cloning by Xie et al., 1998, identifying COI1 as an F-box protein) — directly implicating a ubiquitin-ligase component in jasmonate perception, years before the co-receptor structure with JAZ was solved crystallographically (Sheard et al., 2010), which showed JA-Ile sitting in a pocket at the COI1–JAZ interface exactly as predicted by the "molecular glue" model already established for auxin–TIR1.
Salicylic acid and systemic acquired resistance — classical grafting and NahG evidence. SAR was first rigorously documented by Ross (1961) in tobacco infected with tobacco mosaic virus, showing that a localized necrotic lesion conferred resistance to subsequent infection in distant, untreated leaves. The causal role of SA specifically was nailed down using transgenic tobacco expressing the bacterial gene NahG, encoding salicylate hydroxylase, which degrades SA to catechol. NahG plants cannot accumulate SA and fail to mount SAR, even though the local hypersensitive response can still occur — showing SA is required for the systemic, not merely the local, defense signal. NPR1's role was established by isolating Arabidopsis mutants (npr1, also called nim1) that failed to induce PR genes or SAR despite normal SA accumulation, again an epistasis-style argument placing NPR1 downstream of the SA signal itself.
Strigolactones — the Striga germination bioassay and the branching-mutant convergence. Strigolactones were first isolated in 1966 (Cook, Whichard, Turner, Wall, and Egley) from cotton root exudate as the chemical that stimulates germination of the parasitic weed Striga lutea — decades before anyone suspected these molecules were also endogenous plant hormones. The hormonal role emerged only in 2008, when two independent groups (Gomez-Roldan et al. and Umehara et al.) showed that classic highly-branched dwarf mutants in pea (ramosus), Arabidopsis (max), petunia (decreased apical dominance), and rice (dwarf/*high-tillering dwarf) were all defective in strigolactone biosynthesis or signaling — and, decisively, that <strong>applying synthetic strigolactone to the mutant roots restored the wild-type, low-branching phenotype</strong>, while application to biosynthetic mutants (but not to signaling mutants like max2*) rescued branching. This chemical-complementation logic — rescue by the hormone itself distinguishing biosynthesis mutants from perception mutants — is the same style of argument used throughout classical hormone physiology (compare rescue-by-GA experiments in gibberellin biology).
Worked examples
Example 1 — reading a BR-pathway epistasis cross. Suppose you cross a bri1 loss-of-function mutant (severely dwarfed, dark green, insensitive to applied brassinolide) with a gain-of-function, constitutively active bin2 mutant (also dwarfed, independently of BR status). You then examine an F2 population and also independently test a triple combination: a loss-of-function allele in BIN2 crossed into the bri1 background. Reasoning through it:
- bri1 is dwarf because, lacking a functional receptor, BIN2 remains fully active and constantly phosphorylates BZR1/BES1, keeping the growth-promoting transcription factors out of the nucleus. The defect is at the top of the pathway (perception).
- Gain-of-function bin2 is dwarf because BIN2 is now uncontrollable — even if BRI1 signaling occurs normally, BIN2 activity cannot be shut off. The defect is downstream, at the level of the pathway's own brake.
- If you cross bri1 (loss-of-function receptor) with loss-of-function bin2</strong> (not gain-of-function), the double mutant is predicted, and observed in the literature, to have a wild-type or near-wild-type phenotype — because removing the downstream brake entirely bypasses the need for the receptor to inactivate it. This is the epistasis relationship: bin2 loss-of-function is epistatic to bri1, proving BIN2 acts downstream of BRI1.
- By contrast, crossing bri1 with the gain-of-function bin2 allele gives a double mutant that is dwarf, similar to or more severe than either single mutant, because gain-of-function BIN2 cannot be inactivated regardless of receptor status — consistent with BIN2 acting downstream and dominantly.
This single thought experiment recapitulates exactly how the field ordered BRI1 → BIN2 → BZR1/BES1 without yet knowing the biochemical steps in between.
Example 2 — quantifying a strigolactone dose-response and Striga seed germination bioassay. A researcher tests root exudate from a wild-type sorghum plant and from a ccd8 strigolactone-biosynthesis mutant sorghum line, applying serial dilutions to pre-conditioned Striga hermonthica seeds and scoring percent germination after 24 hours.
| Exudate dilution | Wild-type germination (%) | ccd8 mutant germination (%) |
|---|---|---|
| Undiluted | 78 | 4 |
| 1:10 | 61 | 2 |
| 1:100 | 34 | 0 |
| 1:1000 | 9 | 0 |
| Water control | 2 | 2 |
Interpretation:
- Wild-type exudate shows a classic dose-dependent decline in germination as it is diluted — consistent with a diffusible, concentration-dependent chemical signal (strigolactone) rather than an all-or-nothing cue.
- The ccd8 mutant exudate germinates Striga seeds at essentially the water-control background rate (2–4%) at every dilution, showing that CCD8-dependent strigolactone biosynthesis is necessary for the germination-stimulant activity of the exudate; residual low germination reflects assay background, not a second active compound.
- This kind of bioassay — comparing wild-type versus biosynthetic-mutant exudate potency across a dilution series — is precisely the logic used historically to purify strigolactones in the first place, and it is still the standard method plant breeders use today to screen low-strigolactone sorghum and cowpea lines for Striga resistance, since low-SL root systems attract fewer parasite seeds to germinate near the root.
Common misconceptions & traps
| Misconception | Why it's wrong |
|---|---|
| "Brassinosteroids work like animal steroid hormones, diffusing to a nuclear receptor." | Plant BRs are perceived at the cell surface by the receptor kinase BRI1; the transcription factors they regulate (BZR1/BES1) are controlled by a cytoplasmic kinase relay, not direct steroid–nuclear-receptor binding. |
| "Jasmonate and salicylic acid always cooperate in defense." | They are frequently antagonistic: SA signaling (biotroph defense) often suppresses JA signaling (herbivore/necrotroph defense), and vice versa, so pathogens and herbivores can each exploit the antagonism to blunt the other pathway. |
| "Strigolactones simply promote branching, like a typical growth hormone." | Strigolactones suppress axillary bud outgrowth; low-SL mutants are bushier, not less branched. |
| "Hormone perception always activates a transcription factor directly." | In BR, JA, and SL signaling the receptor's job is to trigger degradation or inactivation of a repressor, unmasking a pre-existing transcription factor rather than switching one on from scratch. |
| "SAR means the plant becomes immune everywhere, permanently." | SAR is a primed, faster/stronger response to subsequent infection across the whole plant, broad-spectrum but not absolute, and it fades over weeks unless reinforced. |
The scientist behind it
Joanne Chory built much of the genetic and molecular foundation of brassinosteroid signaling described here, working at the Salk Institute for Biological Studies. Trained originally as a microbiologist studying Rhodobacter photosynthetic gene regulation, she shifted to plant developmental genetics in the late 1980s and became fascinated by a peculiar class of Arabidopsis seedlings that developed as if grown in light even when raised in complete darkness — short, thick hypocotyls, open and expanded cotyledons, chloroplast-like plastids. Rather than assuming these were light-signaling mutants outright, her lab systematically tested each candidate against exogenous hormone treatments, and the bri1 mutant's stubborn refusal to respond to applied brassinolide was the clue that reframed the entire phenotype as a hormone-receptor problem. The subsequent cloning of BRI1 as a receptor kinase, and the direct radiolabeled-hormone binding assays confirming brassinolide binds the BRI1 ectodomain, established the first rigorously demonstrated plant steroid receptor and opened the modern era of plant receptor-kinase signaling research, a framework since extended to immune receptors, peptide hormone receptors, and symbiosis signaling across the field.
Origins · 1 minJoanne Chory and how this was discovered
Where this matters today
Agriculture and yield. Synthetic brassinosteroid analogs (e.g., 24-epibrassinolide) are used as foliar sprays in several countries to increase yield, improve stress tolerance (cold, drought, salinity), and boost seed set in crops including rice, tomato, and grape, by amplifying the same BZR1/BES1-driven growth and stress-response transcriptional programs described above.
Induced resistance products in crop protection. Commercial "plant activator" chemistries (such as benzothiadiazole/BTH-based products) work by mimicking salicylic acid's activation of the NPR1–TGA pathway, priming crops for SAR-like broad-spectrum disease resistance without directly killing pathogens — an alternative to conventional fungicide/bactericide chemistry that reduces resistance-evolution pressure on pathogens.
Herbivore-resistant crop breeding and biopesticides. Because jasmonate signaling through COI1–JAZ–MYC2 governs induced anti-herbivore defenses (protease inhibitors, terpenoids, glucosinolates), breeders select for lines with robust jasmonate responsiveness, and some biostimulant products contain methyl jasmonate to pre-prime crops against expected pest pressure.
Striga control in African agriculture. Because Striga germination depends on host strigolactone exudation, two complementary strategies exploit the mechanism directly: breeding low-strigolactone-exuding sorghum and cowpea varieties (reducing parasite germination near the root) and "suicide germination" approaches, applying synthetic strigolactone analogs to fields before a non-host or resistant crop is planted, tricking Striga seeds into germinating with no host present, depleting the soil seed bank.
Mycorrhizal inoculant optimization. Strigolactone biology underlies efforts in sustainable agriculture to select crop varieties or rootstocks with strigolactone exudation profiles that favor beneficial arbuscular mycorrhizal colonization, improving phosphate uptake efficiency and reducing fertilizer dependence.
Conservation and ecological signaling research. Jasmonate- and SA-mediated plant-plant "eavesdropping" (volatile organic compounds released from wounded plants priming neighboring plants' defenses) is studied for potential use in low-input, ecologically integrated pest management strategies in both agricultural and restoration contexts.
Ask the tutor: How does brassinosteroid signaling interact with light signaling (PIF4) and with auxin to jointly control stem elongation?
Why don't brassinosteroids use a nuclear receptor like animal steroid hormones do?
Plants and animals evolved steroid-hormone signaling independently after their lineages diverged, so there was no ancestral steroid receptor to inherit or modify. Instead, plants repurposed receptor-kinase architecture — the same general toolkit used for immune receptors and many peptide hormone receptors — placing the ligand-binding domain outside the cell and the signaling machinery in the cytoplasm. This is a clear case of convergent evolution: both lineages solved "detect a lipophilic steroid signal" but with completely different molecular hardware, which is why BRI1 looks nothing like an animal glucocorticoid receptor despite brassinolide's structural resemblance to animal steroids.
Is BIN2 brassinosteroid-specific, or does it do other things in the cell?
BIN2 is a GSK3/SHAGGY-like kinase, and this family is deeply conserved and multifunctional across plants (and, distantly, related to GSK3β in animal Wnt signaling). Beyond brassinosteroid signaling, BIN2 and its relatives also phosphorylate transcription factors involved in stomatal development, root growth, and cold and salt stress responses, and BIN2 physically interacts with components of other hormone pathways, including auxin and abscisic acid signaling. This makes BIN2 a genuine signaling hub rather than a BR-dedicated switch, which is part of why brassinosteroid responses are so context-dependent across tissues.
Why are jasmonic acid and salicylic acid often described as antagonistic?
Biotrophic pathogens (which feed on living tissue) are generally countered by SA-dependent defenses, while necrotrophic pathogens and chewing herbivores are countered by JA-dependent defenses. Because mounting both full-strength defense programs simultaneously is metabolically costly and sometimes mechanistically incompatible, plants have evolved cross-talk in which elevated SA signaling (via NPR1) can suppress components of the JA pathway, and vice versa. This trade-off is exploited by some pathogens: certain bacteria produce coronatine, a JA-Ile mimic, specifically to activate JA signaling and thereby suppress the SA-based defense that would otherwise stop them.
How exactly does strigolactone end up germinating a parasitic weed like Striga if the hormone's "real" job is inside the host plant?
Strigolactones serve two roles: an endogenous hormonal role inside the producing plant (branching, root architecture) and an exogenous signaling role once exuded into the rhizosphere, where they recruit beneficial mycorrhizal fungi. Striga seeds evolved chemoreceptors that detect this exuded signal specifically because it reliably indicates a living host root is nearby — the parasite is eavesdropping on a signal that evolved for an entirely different, mutualistic purpose. This is a textbook example of how a molecule's ecological "meaning" depends entirely on which organism is listening.
If BRI1 is a receptor kinase, why is it sometimes described as working through a phosphatase (BSU1) rather than only kinases?
Signal transduction cascades commonly alternate kinases and phosphatases because biological "off" switches need their own dedicated machinery — a kinase can't easily reverse its own phosphorylation. In the BR pathway, BRI1's kinase activity is transmitted through further kinases (BSK, CDG1) that then activate the phosphatase BSU1, whose actual job is to remove an inhibitory phosphate from BIN2, thereby switching BIN2 off. This kinase-to-phosphatase handoff is what ultimately achieves de-repression: phosphorylation events build toward a step whose final effect is dephosphorylation of the pathway's central brake.
Are these four hormones the only "non-classical" plant signaling molecules?
No — other well-established plant signals include ethylene precursor-related peptides, small signaling peptides (e.g., CLE peptides in meristem maintenance), nitric oxide, reactive oxygen species used as second messengers, and the more recently characterized karrikins (smoke-derived germination signals perceived by a D14 paralog, KAI2). Brassinosteroids, jasmonates, salicylic acid, and strigolactones are simply among the best-mechanistically-resolved and most agriculturally exploited of this larger, still-expanding signaling vocabulary.
Why does the same JAZ-COI1-style "molecular glue" logic show up in both auxin and jasmonate signaling?
Both pathways rely on SCF-type E3 ubiquitin ligase complexes (SCF^TIR1 for auxin, SCF^COI1 for jasmonate) that mark a repressor protein (Aux/IAA for auxin, JAZ for jasmonate) for proteasomal degradation, with the hormone itself completing the receptor's binding pocket for its repressor target. This shared architecture likely reflects the ancient, general utility of ubiquitin-proteasome-based rapid protein degradation for making growth and defense decisions reversible and fast, and it suggests these two hormone-perception systems may share evolutionary and mechanistic ancestry within the broader family of plant hormone receptors, distinct from the receptor-kinase mechanism BRI1 uses.