A mathematical model integrates diverging PXY and MP interactions in cambium development

Abstract

The cambium is a secondary meristematic tissue in plant stems, roots and hypocotyls. Here, cell divisions occur that are required for radial growth. In most species that undergo secondary growth, daughters of cell divisions within the cambium differentiate into woody xylem cells towards the inside of the stem, or phloem towards the outside. As such, a pattern of xylem-cambium-phloem is present along the radial axis of all secondary vascular tissues, whether in stem, hypocotyl or root. A ligand-receptor tracheary element, trans-differentiation inhibitory factor (TDIF)-PHLOEM INTERCALATED WITH XYLEM (PXY) promotes cell division in the cambium, as do the phytohormones, cytokinin and auxin. An auxin response factor, MONOPTEROS (MP), has been proposed to initiate cambial cell divisions by promoting PXY expression, however, MP has also been reported to repress cambial cell divisions later in development where TDIF-PXY complexes are also reported to suppress MP activity. Here, we used a mathematical modelling approach to investigate how MP cell division-promoting activity and cell division-repressing activity might be integrated into the same network as a negative feedback loop. In our model, this feedback loop improved the ability of the cambium to pattern correctly and was found to be required for normal patterning as the stability of MP was increased. The implications of this model in early and late cambium development are discussed.