Hormone interactions and the regulation of seedling growth

An increased understanding in recent years of the biosynthesis and signal transduction pathways for various plant hormones, including gibberellin (GA), is providing an important tool for understanding interactions between these hormones and their role in plant physiology. In this thesis, interactions between auxin and gibberellin in regulating root growth, and between abscisic acid (ABA) and gibberellin in regulating shoot growth, are explored using the model species, pea. The GA signalling pathway, in particular the involvement of “DELLA” proteins, has received much attention in the last decade. The GAs act by destabilising the growth inhibitory DELLAs; essentially, GA acts as an “inhibitor of an inhibitor”. In Arabidopsis, DELLA proteins have been shown to promote the biosynthesis of active GAs, with the DELLA mutant rga displaying elevated expression of the biosynthesis gene GA4. The recently-sequenced pea DELLA genes LA and CRY are used in this thesis to show that in roots also, DELLA proteins effectively promote GA synthesis gene expression, including a new member of the pea GA 3-oxidase family which appears to play a major role in these organs. Furthermore, these DELLA mutants are used to investigate the role of GA signalling in the interactions with auxin and ABA to regulate growth. Auxin has been shown to promote GA biosynthesis in the above-ground parts of pea (Ross et al., 2000). However, it cannot be assumed that the same interaction also occurs in pea roots. Indeed, another study indicates that auxin acts by enhancing the capacity of GA to destabilise DELLAs in roots of Arabidopsis (Fu and Harberd, 2003). According to the Fu and Harberd model, auxin would down-regulate GA synthesis, the opposite of the up-regulation found by Ross et al. (2000) in stems and consequently, the Ross et al. (2000) and Fu and Harberd (2003) models predict opposite effects of auxin on the expression of GA synthesis genes. Here, to understand the interactions between auxin and GAs in pea roots, wild-type pea roots were treated with the inhibitors of auxin action and auxin transport. These compounds generally down-regulated GA synthesis genes and up-regulated GA deactivation genes, and reduced the level of the bioactive GA1, suggesting that in pea roots, auxin at normal endogenous levels stimulates GA biosynthesis, agreeing with the Ross et al. (2000) model. It is also shown that supra-optimal levels of exogenous auxin reduce the endogenous level of bioactive GA in roots, although the effect appears too small to account for the strong growth-inhibitory effect of high auxin levels. ABA is a known inhibitor of plant growth and historically, ABA and GA have generally been shown to act antagonistically. Previous evidence indicates that ABA inhibits GA synthesis while GA inhibits ABA synthesis. However, the evidence presented here suggests otherwise. GA synthesis gene expression and endogenous levels were not altered in ABA-treated shoots, indicating that, at least in pea, ABA does not regulate GA biosynthesis. The effects of GA deficiency on ABA levels were also investigated. Furthermore, ABA has been reported to act via the GA signalling pathway to inhibit root growth. However, there are conflicting reports on whether ABA acts on GA signalling via the DELLA proteins or downstream of these proteins. Here it is shown that ABA inhibits shoot growth in both the WT and pea DELLA mutants to a similar degree, suggesting that the DELLA proteins are not involved in the ABA-induced inhibition of pea shoot growth. The results presented in this thesis clarify a number of conflicting reports on the auxin-GA and the ABA-GA interactions and how they influence the growth of the plant.