Cues and mechanisms of daily cell wall thickening

Most plant biomass is made up of a thick wall that surrounds some types of cell. A tree trunk or a corn stalk are mostly this form of secondary cell wall. This growth, where plants increase in mass, is distinct from the type of growth that results in changes in plant volume which is the result of changes in cell size.

The rate of volume growth often changes throughout the day, where stems may elongate much more rapidly at the end of the night than any other time of day. Plants respond to daily changes in temperature and light and their circadian clocks play a role in orchestrating the timing of those rhythms. The goal of this project is to understand the environmental cues and regulatory mechanisms that control the synthesis of cell wall mass.

Plant cell wall biomass is an abundant renewable resource and the cornerstone to the pulp and papers, timber and textile, and lignocellulosic biofuel industries. The knowledge gained will enable conventional breeding and biotechnology approaches to improve the production of ecologically and economically sustainable forestry and agriculture. This project will also provide interdisciplinary training in development, genetics, genomics, and biochemistry for graduate students and facilitate an internship program with the National Technical Institute for the Deaf.

This project calls for genetic and biochemical approaches to understand the environmental conditions and the molecular mechanisms that control the timing of cell wall biosynthesis in the model grass Brachypodium distachyon. Three integrated hypotheses will be pursued: (1) Secondary wall biosynthesis is a time-of-day specific behavior. Several observations support a model in grasses where the rate of cell elongation and wall thickening is inversely timed and influenced by daily changes in temperature.

Rates of plant biomass accumulation, polymer precursor metabolites, and cell wall gene expression will be quantified. (2) Gene promoter sequences determine the timing of secondary cell wall gene expression. Although several specific protein-DNA interactions that regulate wall biosynthesis have been reported, the functional outcome of the presence of these sequences has on the timing of gene expression remains unknown.

The specific function of these DNA sequences will be investigated in vivo with synthetic promoters and native promoters using real-time monitoring of luciferase reporters. (3) Phytochrome proteins play a role in time-of-day specific cell wall thickening. Phytochrome protein dimers are reversibly active and inactive, and the rate of that process is influenced by red/far-red light ratios and by temperature.

This mechanism may play a role in the regulation of secondary cell wall gene expression and wall thickening rhythms. Wildtype B. distachyon and phytochrome mutants will be examined for the effects of red/far-red light and temperature on stem secondary wall thickness and associated gene expression.

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.