Rishikesh P. Bhalerao - Seasonal Control of Growth in Perennial Plants and Regulation of Cell Elongation

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Survival of perennial plants, including long-lived trees, depends on their ability to adapt to seasonal changes in environmental conditions. These plants have developed sophisticated mechanisms to sense changes in environmental conditions and modulate their growth and development accordingly. The first project in my lab focuses on understanding how perennial plants sense the changing environmental conditions, by analysing the molecular basis of short-day-induced growth cessation and dormancy in hybrid aspen. The second project is focused on understanding how cell elongation is regulated in plants.
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Cell elongation in plant cells presents a unique problem, as plant cells are enclosed in a rigid cell wall that needs to be remodelled in order for cell elongation to occur. The research in my group aims to identify the key components that are involved in regulation of cell elongation and cell wall remodelling in the model plant Arabidopsis.
Molecular basis of growth cessation and dormancy in perennial plants
Perennial plants need to undergo growth cessation and establish dormancy prior to the onset of winter, in order to survive low temperatures. These plants anticipate the approach of winter by sensing the reduction in day length. Reduction in day length (short day signal) induces growth cessation that is apparent in the form of bud formation at the apex and eventually the establishment of dormancy. Once dormancy is established, prolonged exposure to chilling temperatures is essential for release from dormancy. Once release from dormancy has occurred, warm temperatures can reinitiate new growth. In my group, we are investigating the molecular basis of short-day-induced growth cessation and dormancy. In particular we are addressing the following questions:

1. What are the signal transduction components mediating short day regulated growth cessation?
2. How is dormancy established and what is the molecular mechanism regulating the release from dormancy?

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To address these questions, we are applying a combination of genomics, genetics and biochemical approaches with a model perennial plant, hybrid aspen. Using full genome microarrays and metabolic profiling, we have outlined the transcriptional and metabolic networks underlying the distinct stages of induction of growth cessation, establishment and release from dormancy. Using the information from transcriptional and metabolic profiling, we have identified a set of candidate genes that could be key regulators of growth cessation and dormancy acting downstream of the environmental and hormonal signals. We are now analyzing the role of these candidate genes in hybrid aspen by using RNAi and microRNAs to down-regulate their expression, as well as by overexpressing and misexpressing these genes and investigating the effects of modulating their expression on growth cessation and dormancy.
Elucidating the control of cell elongation in Arabidopsis thaliana
The final size of plants and their constituent organs is determined by cell division and cell expansion. While cell division serves to increase the number of cells, cell expansion serves to increase the cell size. Mechanistically, cell expansion in plants poses a unique problem compared with animal animal cells. Unilike animal cells, plant cells are surrounded by a rigid cell wall that encompasses the plasma membrane. For cell expansion to occur, the cell wall structure must be remodelled. The process of cell wall remodelling involves loosening of the cell wall by alteration of the interactions between the three major components of the primary plant cell wall, namely cellulose, hemicellulose (of which xyloglucans are the major components) and pectins. This process involves the breakdown of linkages between the various cell wall components, accompanied by the addition of newly synthesized components to growing cell walls as cell size increases. Importantly, at the cellular level cell expansion can be polar. This is the case in root hair formation where cell wall loosening and eventual elongation is localised to the basal end of trichoblast cells. Here the cell wall components and the proteins needed for cell wall remodeling need to be delivered to the specific locations where cell wall expansion will take place. Thus, to understand how cell expansion is regulated, an important question that needs to be answered is:
How do cells regulate the delivery of various cell wall components and the proteins involved in cell wall remodelling to their site of action during cell expansion?
We are currently addressing this question in the model plant Arabidopsis. We have identified several Arabidopsis mutants altered in cell elongation and these are being characterized using genetic and cell biology techniques. Our results so far indicate a key role for the components of trafficking machinery in the regulation of cell elongation and cell wall remodelling.

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Key Publications

Druart N, Johansson A, Baba K, Schrader J, Sjödin A, Bhalerao RR, Resman L, Trygg J, Moritz T, Bhalerao RP (2007) Environmental and hormonal regulation of the activity-dormancy cycle in the cambial meristem involves stage-specific modulation of transcriptional and metabolic networks. Plant J 50: 557-573

Rohde A, Bhalerao RP (2007) Plant dormancy in the perennial context. Trends Plant Sci 12: 217-223

Ubeda-Thomas S, Swarup R, Coates J., Swarup K, Laplaze L, Beemster G, Hedden P, Bhalerao R and Bennett M (2008) Root growth in Arabidopsis requires gibberellin/DELLA signaling in endodermis. Nature Cell Biol 10: 625-628

Nilsson J, Karlberg A, Antii H, Lopez-Vernaza M, Mellerowicz E, Rechenmann C, Sandberg G and Bhalerao RP (2008) Dissecting the Molecular Basis of the Regulation of Wood Formation by Auxin in Hybrid Aspen. Plant Cell 20: 843-855

Swarup R, Perry P, Hagenbeek D, Van Der Straeten D, Beemster GT, Sandberg G, Bhalerao R, Ljung K, and Bennett MJ (2007) Ethylene upregulates auxin biosynthesis in Arabidopsis seedlings to enhance inhibition of root cell elongation. Plant Cell 7: 2186-2196

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May 2012

Friday, May 18, 2012
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