Interactions between gibberellins and other hormones

In this chapter we discuss interactions between the gibberellins (GAs) and other plant hormones. Auxin and ethylene exert strong effects on GA levels, and at least some of the physiological responses to auxin are mediated by increased GA content. Abscisic acid (ABA) and GA appear to negatively regulate each other's levels in seeds, consistent with their antagonistic effects on germination. It has been suggested that several non-GA hormones affect the stability of DELLAs, the key signalling proteins that are destabilised by GAs. However, it appears that the reported effects of these additional hormones on DELLA stability are either mediated by changes in GA content, or have not yet been confirmed. Recent papers provide evidence that DELLAs physically interact with transcriptional regulators from the signalling pathways of the brassinosteroids, ethylene, jasmonic acid and ABA. This may represent a physiologically significant way in which GAs interact with other signalling pathways, and helps to explain how DELLAs, which lack a conserved DNA binding domain, can regulate gene transcription.