Collaborative Research: Dynamic zonation in the plant root

Plant roots grow through the soil, thereby anchoring the plant while foraging for nutrients and water. The growing root tip is divided into two main regions: the meristem zone, which produces new cells for the root body, and the elongation zone, where cells elongate rapidly, thus propelling the root tip through the soil. The separation into these functional growth zones is complicated by the fact that cells at the edges of the zones change identity; that is, meristem cells become elongating cells and elongating cells become mature cells.

These zones are therefore inherently dynamic, yet retain their location. In addition, as the root responds to factors such as temperature, the zones change in length, growth rate, and cell number. An intriguing question is how the root maintains the integrity and stability of the zones despite the underlying cell dynamics.

This question is being addressed experimentally using genetic methods and temperature changes to study responses of the root, image the growth and quantify the processes. These studies will help explain how the zones are established and regulated, providing fundamental knowledge about the mechanisms and resilience of root growth. The outcomes could provide access to new tools for modifying root growth through breeding more resilient root responses and could provide baseline information for predicting impacts of temperature change on root behavior.

Undergraduate students will be trained and will gain skills and experience in interdisciplinary approaches that integrate engineering with biology to solve scientific problems.

To understand how stable zones emerge from dynamic cells, this project uses Arabidopsis thaliana acclimating to temperature. Investigating thermomorphogenesis in roots, principal investigator Baskin found that although the root grows faster at 25ºC compared to 15ºC, the growth zone has the same length and the cortex produces cells at the same rate.

Thus, growth zone length and cortical cell production rate acclimate to counteract the acceleration of reactions by temperature. Interestingly, acclimated cell production rate is specific to cortex: epidermis produces cells faster at the warmer temperature. Also, the growth zone is truncated when an inhibitor of cell division is expressed in the cortex but not when expressed in other tissues.

The project aims first to elucidate the pathway regulating root zonation during thermomorphogenesis. Comparing 15 and 25ºC, the team will screen extant mutants, determine whether implicated genes act in the root directly, and obtain transcriptomes specifically for epidermis and cortex. The second aim is to elucidate the role of the cortex in root zonation.

Genes that inhibit cell division will be expressed from inducible promoters in specific tissues; then, division, elongation, and endoreduplication will be quantified as a function of temperature. The third aim will use advanced image analysis to quantify cell division rates throughout the meristem at both temperatures, resolving, apparently for the first time, division behavior in separate tissues.

Overall, the project unites molecular dissection of growth-regulating pathways with quantitative analysis of cellular behavior to characterize the hierarchical organization of a living system.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.