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#auxin

33 public questions tagged with this topic.

plant Development -ll

MP and NPH4 promote expression of:

MP/ARF5 and NPH4/ARF7 auxin response factors become transcriptionally active when auxin triggers TIR1/AFB-mediated ubiquitination and proteasomal degradation of BODENLOS and other AUX/IAA repressors containing EAR motifs. Activated ARFs bind canonical auxin response elements TGTCTC in promoters of PLETHORA genes particularly PLT1, PLT2, PLT4/BABYBOOM, PLT5/AIL5 rapidly inducing their expression in basal and root pole during embryogenesis. MP-PLT module directly converts auxin accumulation into root stem cell specification program, stabilizing PIN expression creating self-reinforcing auxin-PLT-PIN loop crucial for embryonic root establishment and post-embryonic maintenance.

Ref: Berleth & Jürgens, Development 1993: MP and NPH4/ARF7 promote PLT expression establishing root fate downstream of auxin.

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plant Development -ll

Auxin-induced gene promoting root identity:

PLETHORA family AP2-domain transcription factors PLT1, PLT2, PLT3, PLT5, PLT7 are directly transcriptionally induced by auxin maxima at root pole through ARF5, ARF7, ARF10, ARF16 after auxin-triggered degradation of AUX/IAA repressors via TIR1/AFB pathway. PLT proteins accumulate in graded manner highest near quiescent center promoting stemness, intermediate sustaining mitotic activity, low permitting differentiation and elongation. PLTs activate WOX5, PIN genes, and cell cycle regulators while repressing shoot determinants. Auxin-PLT-PIN positive feedback stabilizes auxin maximum defining stem cell niche patterning and regeneration.

Ref: Aida et al., Cell 2004; Galinha et al., Development 2007: PLT genes are auxin-induced root identity transcription factors.

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Plant tissue culture 2

Somatic embryogenesis is induced commonly using the auxin:

Somatic embryogenesis induction most frequently relies on auxin 2,4 dichlorophenoxyacetic acid 2,4-D applied at 0.5 to 10 mg per L because this synthetic phenoxy auxin resists degradation by IAA oxidases maintains prolonged auxinic signaling. High 2,4-D imposes stress and auxin response causing differentiated somatic cells to re-enter cell division and acquire totipotency through global epigenetic reprogramming involving DNA demethylation histone acetylation and activation of chromatin remodeling factors. It stimulates expression of embryogenic competence markers SERK1 glutathione S transferase and ABA responsive genes elevating endogenous auxin via upregulation of YUCCA genes. Auxin response factors ARF5 LEAFY COTYLEDON2 are induced initiating embryonic program while suppressing photosynthetic leaf identity genes. Persistent high concentration inhibits progression beyond globular stage because polar auxin transport via PIN1 required for bilateral symmetry cannot establish; therefore after competence achievement cultures are transferred to low auxin or hormone free medium often supplemented with ABA to permit maturation through heart torpedo cotyledonary stages. IAA NAA IBA are weaker inducers used for embryo maintenance rather than induction.

Ref: Dudits et al., J Exp Bot 2011 2,4-D embryogenesis; NCBI Book somatic embryogenesis.

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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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