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Plant Hormone Action

Researchers: Dr. Karen Osmont, Dr. Stephen Depuydt, Dr. Paula Salinas, Emanuele Scacchi, Julien Beuchat

Plant hormones are at the heart of many plant developmental processes. They determine critical growth parameters, such as the rate of cell division or elongation, as well as developmental decisions, for instance where along a primary root a lateral root should be formed.

Different plant hormones control different developmental processes to varying degrees, sometimes in the same, sometimes in opposite directions. Whether and how the overlapping influence of different hormones on traits is coordinated is largely unknown. Part of the research in our lab aims to determine how the major plant hormone auxin, arguably the most important in shaping the plant, regulates downstream events that lead to discrete morphological change. After all, those changes require structural rearrangements on the cellular and organ level after hormone signaling. One tool to answer this question are the hy5 and hyh mutants of Arabidopsis, which display constitutively increased auxin signaling.

Left, middle: expression pattern of the HY5 gene in root tips and cotyledons as revealed by a transgenic reporter gene (green fluorescence). Right: rosette stage phenotypes of hy5 hyh double mutants. Cotyledons (on the left) are aligned next to each other instead of opposite, typical of the generally defective lateral organ formation in this double mutant.

Another important question is the nature of crosstalk between hormone pathways, that is the impact of more than one hormone signaling pathway on the same trait or molecular event. Does such crosstalk simply reflect a quantitative add up of different hormone signals at the point of convergence (for example, in the way the water flow rate through a river is determined by the contribution of its two tributaries downstream of their junction), or is there real crosstalk happening upstream (for example, do the two tributaries that will meet have built-in dams, and if so, do the dam gatekeepers communicate with each other to intentionally regulate the water flow beyond the river junction)?

Model of a BRX-mediated feedback loop between brassinosteroid biosynthesis and auxin action.

Much to our surprise, we isolated an important regulator of hormone pathway interaction in the root through our natural variation project: loss of function of the BRX gene results in reduced root growth because BRX is required for the biosynthesis of the hormone brassinolide, whose concentration is in turn rate-limiting for auxin action. Since BRX is itself under auxin control, this creates a feedback loop required for optimal root growth. Whether an equivalent loop works in the shoot, and whether this feedback is conserved in other species are some of the questions we are currently interested in with regard to this project, generously funded by the Swiss National Science Foundation.

 


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