Chemical dissection of hormone signalling in Arabidopsis

The plant hormone gibberellin (GA) regulates many developmental processes during a plant’s life cycle, including root and hypocotyl growth. Bioactive GAs promote GA-responsive growth and development by targetting DELLA proteins for degradation. Whilst the early steps of GA signalling are well understood it is not yet clear how the DELLA proteins alter the expression of GA-responsive genes. As other steps of the signalling pathway are encoded by multi-gene families it is possible that genetic redundancy is masking the transcription factors that act downstream of DELLAs. Using a chemical screen based on DELLA protein’s control of GA biosynthesis, 28 chemicals which blocked the GA-mediated downregulation of GA20ox1::GUS activity were identified. Using GA-mediated RGA degradation as a marker, 11 chemicals were identified as acting downstream of DELLAs in the GA signalling pathway.

One of the chemicals (N23) identified in the screen was found to induce agravitropic root growth, a response more often associated with perturbation of auxin signalling. However, N23 had no effect on auxin signalling based on the characterisation of its effect on auxin-inducible genes and AUX/IAA degradation. The mode of action of N23 requires further investigation. However, N23 represents a potential for studying the role of GA in modulating gravitropism.

The compound N16 potentially perturbs GA signalling by altering GA transport. It was found to block the uptake of both radiolabelled and fluorescent labelled GA into the root. Five days of exposure to N16 was required before any inhibition was observed on Col-0 roots but root elongation in ga1-3 seedlings was inhibited after only 24 hours suggesting that roots of wild type plants are saturated for GA. The site of action of N16 was not identified, but a putative oligopeptide transporter OPT6 was which is rapidly downregulated in the roots in response to GA application was investigated as a potential novel GA transporter. However, GA uptake assays in yeast strains overexpressing OPT6 proved inconclusive.