Role of gibberellins’ metabolism and signaling in coordination of above-belowground growth in Populus

Background/Question/Methods

Gibberellins (GAs) represent a major class of growth hormones in plants that regulate a number of growth and developmental processes. Because of their pivotal role in regulation of commercially and environmentally important traits, GAs metabolism and signaling pathways have been extensively dissected, predominantly in the model Arabidopsis plants. We study the genetics and genomics aspects of GAs role in coordination the above and belowground growth processes in Populus – the model taxon for woody perennial development. Availability of genome sequence and facile transformation for Populus allow intensive dissection of traits at gene and genome level. Using the genome sequence we identified by homology the genes encoding key enzymes in GA metabolism and signaling pathways. We then studied the expression of these genes in different organs and environmental conditions. Using transgenic manipulations we generated GA metabolic and response mutants and studied the effect of these manipulations under greenhouse and field conditions.

Results/Conclusions

Our work shows that increase in concentration of bioactive GAs and signaling has positive effect on aboveground but negative on belowground growth. In contrast, decrease in GAs concentrations and signaling leads to decreased aboveground growth but promotes root development. These responses are mediated by tissue-specific and stress-induced transcription regulation of the genes encoding key enzymes and factors in the GAs metabolism and response. These enzymes and factors are encoded by small gene families with tissue-specific and environmentally-sensitive expression patterns. We have concentrated our work on GA 2-oxidase and DELLA domain protein gene families as they represent the main routes for decreasing GA concentrations and blocking the GA signaling. We show that specific gene members are expressed predominantly in roots while others in shoots. Furthermore, many of the members are highly induced by drought stress. In support to our expression analysis, we find that when we upregulated the function of these genes in transgenics, the modified plants display hyper sensitive inhibition of aboveground growth coupled with reinforcements of stress related physiological readjustments.  Using microarrays, we have characterized the genome-wide changes in the transgenic plants. Consistent with our phenotypic and physiological studies, we find upregulation of processes and gene involved in stress response in the shoots. In roots, we find strong genome-wide signatures of processes involved in lateral root formation. Our work shows a key but yet poorly understood role of GA in regulation of above and belowground growth adjustments in response to stress.