Two plant hormones, auxins and gibberellins (GAs) regulate many biological processes, to ensure optimal growth and development. Since both hormones can enhance crop yield, understanding their interaction could provide information to manipulate further the performance of crop plants. The levels of bioactive GA are tightly regulated by external and internal factors including GA itself through metabolic regulation. Altered auxin status (AAS) (as a consequence of manipulating auxin level or distribution) affects GA metabolic and signaling pathways, although the mechanism(s) mediating these effects remain unknown. Therefore, the effects of AAS on GA homeostasis in Arabidopsis thaliana seedlings were studied using reporter gene constructs and semi-quantitative reverse transcription-polymerase chain reaction analysis. Results showed that expression of GA metabolic enzymes including the stem-expressed GA 20-oxidase (AtGA20ox1) is up-regulated in shoots but not roots under AAS, and only at specific developmental stages. Enhanced AtGA20ox1 expression may result from altered flow through the GA biosynthetic pathway, or from blocked GA signaling. Confocal microscopic analysis of the GA signaling component, Repressor of ga1-3 (RGA), showed that in AAS green-fluorescent-protein-fused-RGA accumulated in shoots and roots, and this accumulation was counteracted by application of GA4. In AAS the accumulation of RGA may be a consequence of reduced bioactive GA levels. It was concluded that different layers of AtGA20ox1 regulation are enforced in a strict hierarchy: spatial (organ-, tissue-, cell-specific) > developmental > metabolic > auxin regulation. Therefore, auxin regulation of AtGA20ox1 expression is superseded by three other levels of regulation in intact A. thaliana seedlings.
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