Patterning of plant cell wall deposition by cortical microtubules

Abstract

Plant cells are surrounded by cell walls. Cell walls provide strength to protect and shape the cells they surround. Structural strength is particularly important in the water conducting tissue of the xylem. Water and nutrient transport is driven by water evaporation at the leaves, generating a negative pressure in the tubular xylem vessels. Patterned cell wall reinforcements are deposited during the development of these cells that function in counteracting the negative pressure. The cell wall reinforcements are deposited in a diverse number of patterns. Secondary cell wall thickenings in the form of rings and spirals provide physical strength, while allowing elongation of the surrounding tissue in the newly grown plant parts. Plant cell walls consist of a mixture of polysaccharides, forming an intricate network with cellulose microfibrils as the main load-bearing component. The cortical microtubule array dictates the direction and location of newly polymerised cellulose microfibrils in primary cell wall deposition. In this thesis, I focus on understanding if and how microtubule dynamics control the patterning of secondary cell wall position. Which self-organizing properties of microtubules change to generate a microtubule pattern and how is this pattern translated to a patterned cell wall deposition during protoxylem formation? We measured the microtubule dynamicity parameters growth velocity, shrink velocity, rescue rate and catastrophe rate which are important for the self-organizational capability of the microtubule array. We show that the rescue and catastrophe rate increases in the microtubule gap area compared to the microtubule band area. Via stochastic computer simulations we show that a 2-fold increase of the catastrophe rate is sufficient to generate microtubule bands and gaps. Furthermore we investigate if gravity is a factor able to influence the reorganization of the microtubule array during protoxylem formation. Altogether, the results indicate that local changes in the rescue and catastrophe rate are responsible for the generation of microtubule bands and gaps over time during protoxylem formation.