nemhauser lab
department of biology
university of washington
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Using time and space to parse the photomorphogenetic network into simpler circuits

Disruptions in the brassinosteroid pathway lead to aberrant patterning. GL2, a marker of non-hair cells, is mis-expressed in roots lacking the brassinosteroid receptor BRI1. Cells in the hair position (black arrow) and in the non-hair position (white arrow) are indicated. Several cortical cells are outlined in orange.
Summary: One concern in drawing the network using a ‘bird’s eye’ view is that spatial and temporal information is frequently lost. This could be compared with looking at all of the frames of a movie at once or listening simultaneously to all of the phone conversation passing across a fiberoptic cable at a given moment. To address this, the Nemhauser Lab is using differences in timing and location within the organism to subdivide the events of seedling development.

Development: Cells of the root epidermis differentiate into one of two possible fates. Epidermal cells positioned over the boundary between two underlying cortical cells become hair-bearing cells. Cells in contact with only one cortical cell differentiate into non-hair cells. Our previous genomic studies of whole seedlings showed that brassinosteroids increased expression of a gene controlling root cell fate. We used a variety of molecular genetic approaches to demonstrate that reduced levels or perception of brassinosteroids cause disruptions in spatial organization of gene expression and aberrant distribution of cell types. Our study adds a new role for brassinosteroids in normal root epidermal development and provides an excellent second system to use to understand cell-type specificity in hormone response.

Timing: In recent work, we have used high-resolution temporal analysis to parse the development of young seedlings into discrete phases. Surprisingly, many facets of growth—including speed of development, growth duration, and maximal growth rate—are exquisitely sensitive to genetic and environmental perturbations. We have found that daily growth peaks are not fixed, but rather can be reset by small changes in growth conditions. Our work reveals the complexity of seedling growth patterns and highlights the potential of using timing to subdivide a complex process. We are beginning to investigate several key questions, including: what aspects of growth are under hormone control and how hormones relate to one another.

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