The interaction of auxin and cytokinin signalling regulates primary root procambial patterning, xylem cell fate and differentiation in Arabidopsis thaliana.
Plants contribute to the Earths atmosphere by binding carbon dioxide and releasing oxygen. Trees produce biomass, which is a renewable source of energy. The Arabidopsis root vasculature is a good model system for studying biomass formation, as it contains the same cell types that are also found in trees: xylem, phloem and intervening pluripotent procambial cells. In Arabidopsis thaliana roots, these cells arise from stem cells within the root meristem. The wild type root radial pattern is bisymmetric, and the regulation of xylem formation is controlled by phytohormones, especially auxin and cytokinin.
Our findings show that the vascular pattern is set by a symmetry-breaking event during embryogenesis and is initiated by auxin accumulation and signalling at the cotyledon initials. As the embryo grows, the high auxin signalling promotes the expression of AHP6. Upregulation of AHP6 in specific cells leads to inhibition of cytokinin signalling and might be a key factor in symmetry breakage. Mutants with altered cotyledon numbers or altered cotyledon anatomy fail to establish the bisymmetric pattern and often show altered root symmetry. In growing roots, the bisymmetric pattern is actively reinforced by polar auxin transport and long distance cytokinin transport/translocation from the apical parts of the plant. Cytokinin movement via the phloem and unloading at the root apical meristem promotes cytokinin signalling in the procambial cells in the proximal meristem. Both cytokinin and auxin are required during root procambial patterning, and the interaction of these two phytohormones is mutually inhibitory. According to our model (described in the first part of this thesis), auxin signalling is critical for protoxylem identity formation. In turn, the results from the procambial re-patterning experiments (second part of this thesis) show that cytokinin is the key hormone in promoting cell proliferation in the proximal meristem. Epistasis experiments illustrate that a fine balance between these two hormones affects the fate of all vascular cells.
We are beginning to understand the complexity and interdependencies of signalling pathway interactions during proximal meristem vascular patterning, yet the temporal aspect is still largely unexplored. In the last part of this thesis, I discuss the role ROS signalling might have in stele patterning and temporal regulation of programmed cell death. While our published GRI-MC9-PRK5 module might not be directly linked to primary root proximal meristem procambial patterning, one cannot exclude the possibility that it might be required in the final stages of protoxylem differentiation or that a similar signalling mechanism could regulate initial stele patterning and meristem growth dynamics.
This thesis describes the auxin-cytokinin interaction in vascular initial patterning and the mechanism by which the hormonal signalling domains are maintained in the proximal meristem. The unpublished data demonstrate how procambial cells can be manipulated to generate new tissues by affecting the homeostasis of auxin and cytokinin signalling. The last part of the thesis describes a cell death signalling module and speculates that it (or similar module) might be involved with primary root meristem maturation.