The Shape of Plants: exploring developmental transitions

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Plants have evolved sensory and response mechanisms to optimize growth and development under diverse and adverse environmental conditions. Tiny regions of the plant body called meristems maintain stem cells in a quiescent state, and produce the cells that give rise to tissues and organs. Cells are displaced from the meristem, but eventually stop dividing and undergo massive growth. When and where this transition from division to differentiation occurs determines the ultimate size and shape of plant organs, and is strongly influenced by prevailing environmental conditions, and modified by stresses such as drought and salinity. The overarching goal of this research proposal is to identify and understand the mechanisms by which plants determine and coordinate the developmental transition from proliferation to differentiation. It also builds on more than three decades of research on the properties and functions of dynamic sub-cellular filaments called microtubules (MTs). MTs are essential in all eukaryotic organisms for carrying out chromosome separation, intracellular transport, directional growth and motility. The research is divided into three subprograms, each of which focusses on developmental transitions (in the context of MT function). We will first determine how two types of MT-associated proteins, MOR1 and ARK, work in concert to coordinate MT assembly and disassembly. We will determine how these proteins control MT dynamics as cells transition from one stage of development to another, but also in response to salt stress, which involves transient MT disassembly. The second objective will elucidate how the MT-associated protein CLASP and the growth hormone brassinosteroid control the transition from proliferation to differentiation. We recently discovered that CLASP sustains brassinosteroid signalling by fostering the recycling of its receptors but that brassinosteroid signalling also down-regulates CLASP. To explore this negative feedback loop further, we have engineered plants in which the CLASP gene is no longer responsive to brassinosteroids. These 'uncoupled' plants will be powerful tools for identifying the specific function CLASP plays in modulating the proliferation-differentiation transition point. Finally, we will explore how the membrane-anchored cellulose-binding protein COBRA sustains unidirectional growth during cell expansion. COBRA is highly expressed during rapid expansion and is required for the production of cellulose, which is the major tension-bearing component of plant cell walls. We recently confirmed COBRA's presence in the cell wall, but also discovered that it associates with the enzyme complexes that synthesize cellulose, which are distributed along MTs inside the cell. We will use biochemical and transcriptional assays to test our hypothesis that COBRA undergoes cleavage after interacting with cellulose, and that its subsequent association with the cellulose synthase complex modulates cellulose production.