Gibberellin – Mechanism of Action

Abstract

Since their discovery, gibberellin (GA) diterpenoid phytohormones have been used by agriculturalists and horticulturalists to control plant growth and to improve yield quantity and quality. Coordinated breeding efforts after World War II have led to strong yield increases following the introduction of dwarfing alleles that impair GA signalling in wheat and GA biosynthesis in rice, later named the ‘Green Revolution’. In the last two decades, the identity of the basic components of the GA signal transduction pathway has been elucidated. Core to GA action is the regulation of DELLA protein levels, repressors that control plant growth by negatively interfering mainly, but as it emerges not exclusively, with transcription regulator activities. Here, the author summarises the current knowledge of GA signal transduction, thereby putting an emphasis on the crosstalk of GA signalling with the light and jasmonic acid signalling pathways and with microtubule organisation.

Key Concepts:

  • Manipulation of gibberellin (GA) signalling has contributed to the ‘Green Revolution’.

  • DELLA proteins are GA‐labile growth repressors that mainly, but not exclusively, repress transcription factors.

  • GA inactivates DELLAs by targeting them for degradation by the ubiquitin–proteasome system.

  • Alternative GA‐independent DELLA inactivation mechanisms have been described.

  • GA and DELLAs repress PIF and BZR1 transcription factors to control light‐regulated development.

  • GA and DELLAs antagonistically control different steps in the signal transduction of the phytohormone jasmonic acid.

  • GA and DELLAs modulate microtubule formation by modulating prefoldin activity.

Keywords: arabidopsis; gibberellin; green revolution; hormone; Jasmonic acid; microtubules; prefoldin; receptor; rice; transcription factor

Figure 1.

DELLA inactivation mechanisms. (a) Scheme of the conventional GA‐dependent DELLA interaction mechanism where GA‐dependent interactions between DELLAs and GID1GA receptors promote the degradation of DELLAs by the 26S proteasome as indicated here by the attachment of a ubiquitin (Ubi) protein molecule. DELLAs generally but not exclusively repress transcription factor (TF) activities (see Table ). SCFSLY1 is the dominant E3 ubiquitin ligase in Arabidopsis that promotes DELLA degradation. (b) GID1 proteins with a histidine (His) at the position of a proline in other GID1 proteins can promote the DELLA‐independent degradation of DELLAs. GID1b from Arabidopsis is a naturally occurring variant of such GID1 proteins (GID1b His) and this sequence variation is also conserved in other species. (c) In the absence of a functional protein degradation machinery, GA‐dependent or GA‐independent interactions between DELLAs and GID1 can at least partially suppress the DELLA‐mediated repression by binding DELLAs to the GID1 receptors.

Figure 2.

The DELLA‐PIF4‐BZR1 network in photomorphogenic growth control. In the dark, DELLAs are largely absent due to the high levels of GA and BZR1 and PIF4 can activate the expression of downstream genes including that of the elongation regulatory PRE genes. PREs inhibit bHLH transcription factors that restrain elongation growth. In the light, DELLAs become stabilised and prevent PIF4 and BZR1 DNA‐binding allowing for reduced elongation growth by releasing the growth‐inhibitory function of PRE‐interacting bHLH transcription factors. Ubi (ubiquitin) symbolises the targeting of a protein for proteasomal degradation.

Figure 3.

DELLAs and JAs control the balance between defence and growth. (a) GA and JA antagonistically regulate defence and growth in response to pathogen attack (symbolised by the caterpillar). DELLAs released after JA‐induced JAZ degradation will restrain growth by interacting with PIF transcription factors. (b) In the context of TPS gene expression both, GA and JA, promote TPS expression by promoting DELLA and JAZ degradation, respectively. Both, DELLA and JAZ, directly inhibit MYC2 activity. (c) JA and MYC2 regulate DELLA expression at the transcriptional level and this should feedback to MYC2 activity as DELLAs can repress MYC2. (d) In the context of stamen filament elongation, GA promotes the expression of JA biosynthetic genes (e.g. DAD1) and consequently the degradation of JAZ proteins. In this context, JAZ proteins directly inhibit MYB transcription factors as well as the activity of a transcription factor (TF) for the control of MYB gene expression. MYB function is required for stamen filament elongation. Ubi (ubiquitin) symbolises the targeting of a protein for degradation.

Figure 4.

DELLAs restrain elongation growth by promoting prefoldin in the nucleus. In the absence of GA, DELLAs will retain prefoldin (PFD) in the nucleus. Reduced prefoldin activity in the cytoplasm will determine the abundance of free tubulin subunits, lead to disorganised cortical microtubule arrays and reduced plant growth. In the presence of GA, the release of prefoldin to the cytoplasm results in the increased production of tubulin dimers and the formation of a transverse alignment of cortical microtubules required for longitudinal cell expansion.

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Further Reading

Davies PJ (2011) Plant Hormones: Biosynthesis, Signal Transduction, Action! Houten, Netherlands: Springer.

Hedden P (2003) The genes of the Green Revolution. Trends in Genetics 19: 5–9.

Merritt JM, Merck & Co. and Rahway NJ (1958) Gibberellins for agriculture. Journal of Agricultural and Food Chemistry 6: 184–187.

Taiz L and Zeiger E (2010) Plant Physiology. Basingstoke, England: Palgrave Macmillan.

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Claus, Schwechheimer(Mar 2014) Gibberellin – Mechanism of Action. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023921]