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#plant hormones

167 public questions tagged with this topic.

Transgenic Plant -2

Ethylene synthesis pathway involves which intermediate?

Ethylene synthesis pathway involves ACC as obligate intermediate bridging methionine metabolism and hormonal action. Methionine sulfur is conserved via Yang cycle, while ethylene formation proceeds via S-adenosylmethionine intermediate SAM, then ACC. ACC, chemically 1-aminocyclopropane-1-carboxylic acid, is a strained cyclic nonprotein amino acid accumulating transiently in cytosol and vacuole. It is synthesized in cytosol, can be conjugated to N-malonyl-ACC by ACC N-malonyltransferase for vacuolar storage or to γ-glutamyl-ACC, or transported long distance from flooded roots to shoots via xylem sap causing leaf epinasty. ACC oxidase localized in apoplast and cytosol catalyzes oxidative cleavage of cyclopropane ring, producing ethylene, carbon dioxide and hydrogen cyanide detoxified by beta-cyanoalanine synthase to asparagine. Use of ACC as intermediate explains physiology: flooding induces ACC synthase in roots but ACC oxidase is oxygen-dependent, so ACC transported to aerobic shoots converted to ethylene causing adaptive responses. Identification of ACC solidified targeting strategies using ACC synthase inhibitors for ripening control and flower senescence delay.

Ref: Buchanan Biochemistry Molecular Biology Plants Ch 17 ACC pathway; PMID 11536542 Ethylene biosynthesis.

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Plant tissue culture 1

Gibberellins are mainly involved in:

Gibberellins are tetracyclic diterpenoid hormones with GA1 GA3 GA4 most bioactive functions centered on stem elongation, seed germination and floral transition. Biosynthesis proceeds from geranylgeranyl diphosphate via ent kaurene ent kaurenoic acid to GA12 catalyzed by terpene synthases and cytochrome P450 monooxygenases GA20 oxidase and GA3 oxidase. Signaling involves soluble receptor GID1 binding GA causing conformational change enabling interaction with DELLA repressors which are then ubiquitinated by SCF SLY1 GID2 complex and degraded via proteasome allowing transcription of growth genes. Physiologically gibberellins induce alpha-amylase and protease expression in barley aleurone via GAMYB transcription factor mobilizing starch reserves during germination antagonizing ABA. They promote internode elongation by enhancing cell division in intercalary meristem and wall extensibility through xyloglucan endotransglycosylase and expansin induction. In vitro GA3 at 0.1-1 mg per L elongates dwarf shoots, breaks bud dormancy, induces precocious flowering, though excessive level inhibits rooting and somatic embryo maturation requiring careful titration within medium composition.

Ref: Sun, Annu Rev Plant Biol 2008 GA-DELLA signaling; NCBI NBK10975 gibberellin pathway.

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Plant tissue culture 1

High auxin and low cytokinin ratio promotes:

High auxin coupled with low cytokinin specifies root regeneration from callus or shoots, principle underlying rooting stage of micropropagation and natural adventitious rooting of cuttings. Elevated auxin synthesized via TAA1/YUCCA and transported acropetally creates maximum at basal region where founder cells are specified. Auxin binds nuclear receptors TIR1/AFB forming SCF complex that ubiquitinates Aux/IAA repressors for 26S proteasome degradation, freeing ARF7 and ARF19 to transcribe LBD16, LBD18, LBD29 and PLETHORA family genes converting pericycle-like parenchyma into root primordia. Auxin also enhances H+ ATPase activity acidifying wall and induction of expansins facilitating emergence. Low cytokinin is critical because cytokinin signaling via AHK-ARR pathway induces SHY2/IAA3 which suppresses PIN expression reducing auxin accumulation and inhibits root meristem activity. In vitro typical rooting medium contains IBA or NAA 1-3 mg/L with negligible or zero BAP, yielding functional roots with quiescent center expressing WOX5 and SCROW, essential for absorptive function and successful acclimatization after transfer from heterotrophic sterile environment to soil.

Ref: Perianez-Rodriguez et al., Front Plant Sci 2014 auxin root specification; Taiz & Zeiger Chap. 19.

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Plant tissue culture 1

Low auxin and high cytokinin ratio promotes:

Low auxin combined with high cytokinin directs differentiation toward shoot regeneration, fundamental to shoot multiplication media. Cytokinin dominance activates signaling cascade: cytokinin binding to sensory histidine kinases AHK2, AHK3, AHK4 in endoplasmic reticulum triggers histidine-to-aspartate phosphorelay via AHPs to nucleus where type-B ARR transcription factors ARR1, ARR10, ARR12 switch on genes for shoot meristem identity. Key targets include WUSCHEL maintaining stem cell pool, SHOOT MERISTEMLESS KNOX gene preventing differentiation and CYCD3;1 promoting cell cycle in peripheral zone. Simultaneously cytokinin represses auxin transport by downregulating PIN1 expression in shoot apex and upregulates type-A ARR which negatively modulates cytokinin oversignaling creating balanced meristem size. Reduced auxin prevents lateral root program mediated by ARF7/19 and LBD16 from dominating. Morphologically resulting tissue organizes shoot apical meristem dome with leaf primordia and provascular strands. Routinely formulation containing BAP 2-5 mg per L and NAA 0.1-0.2 mg per L achieves caulogenesis across diverse species including banana, apple, rose, coffee enabling rapid multiplication.

Ref: Campbell Biology Chap. 39; Teale et al., Nature Rev Mol Cell Biol 2006 auxin-cytokinin crosstalk.

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Plant tissue culture 1

Intermediate auxin : cytokinin ratio induces:

The concept of auxin cytokinin interaction governing morphogenesis was established by Skoog and Miller using tobacco pith cultures, showing distinct concentration ratios produce different developmental outcomes. Intermediate ratio where both hormones are present in approximately equal proportions sustains cell division without organ determination, leading to formation of unorganized callus mass. Mechanistically balanced signaling maintains expression of D-type cyclins particularly CYCD3;1 and cyclin dependent kinases driving G1 to S transition while preventing dominant activation of shoot promoting type-B ARRs or root promoting ARF-LBD module. Auxin supports cell expansion through acid growth and induction of expansins and cell wall biosynthesis enzymes, while cytokinin enhances cytokinesis by regulating phragmoplast formation and suppressing KRP inhibitors. Neither pathway overwhelms the other, so cells stay in dedifferentiated meristematic condition comprising thin-walled parenchyma with dense cytoplasm and prominent nuclei capable of later redifferentiation. This callus state provides reservoir of totipotent cells exploited for subsequent directed differentiation into shoots, roots or embryos upon changing hormonal balance in subculture sequence.

Ref: Skoog & Miller 1957; NCERT Class XI Biology, Chap. Plant Growth, tissue culture.

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