Mathematical modeling of auxin transport in protoxylem and protophloem of Arabidopsis thaliana root tips.

Phytohormone auxin is the main regulator of plant growth and development. Nonuniform auxin distribution in plant tissue sets positional information, which determines morphogenesis. Auxin is transported in tissue by means of diffusion and active transport through the cell membrane. There is a number of auxin carriers performing its influx into a cell (AUX\LAX family) or efflux from a cell (PIN, PGP families). The paper presents mathematical models for auxin transport in vascular tissues of Arabidopsis thaliana L.root tip, namely protophloem and protoxylem. Tissue specificity of auxin active transport was considered in these models. There is PIN-mediated auxin efflux in both protoxylem and protophloem, but AUX1-mediated influx exists only in protophloem. Optimal values of parameters were adjusted for model solutions to fit the experimentally observed auxin distributions in the root tip. Based on simulation results we predicted that shoot-derived auxin flow to protophloem is lower than one to protoxylem, and the efficiency of PIN-mediated auxin transport in protophloem is higher than in protoxylem. In summary, our simulation showed that despite the same auxin distribution pattern, provascular tissues in the root tip differ in dynamics of auxin transport.