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Higher plants are multicellular organisms that produce the major volume of the world biomass. This production is mostly due to radial growth in trees – a process driven by cambium stem cell activity in trunks. During radial growth, cambium produces two vascular tissues: xylem and phloem. In Arabidopsis thaliana, the model plant species, thousands of cells are produced by cambium just in a couple of weeks. During cell proliferation, expansion and differentiation, cells acquire specific morphologies shaping thus vascular tissues. Cell and tissue morphogenesis can be regulated by molecular signalling and mechanical stimuli. Cell walls play the role of an interface for these mechanical stimuli, as well as an extracellular matrix and a skeleton for the whole plant. The current study aims to quantify cell and tissue morphogenesis and to understand how it is influenced by different factors of plant development. The role of the cell wall was tested with the help of inducible tissue-specific expression of cell wall-related genes. Cell and tissue morphogenesis was analysed by a quantitative histology protocol that was established in this study. This protocol was tested on two different alleles of phloem intercalated with xylem (pxy) mutants. Radial growth was affected by the cambium and xylem-specific ectopic expression of cell wall-related genes HAESA-INFLORESCENCE DEFICIENT IN ABCISSION coding for a receptor-ligand module and VANGUARD 1 coding for a pectin-methyl esterase. Quantitative histology revealed previously unseen properties of vascular tissue morphogenesis and subtle differences between pxy alleles. The study demonstrated that the cell wall is a key player in plant morphogenesis during radial growth, as it was previously shown in other plant organs. Quantitative histology is a powerful phenotyping tool that revealed tissue topology as an important factor in morphogenesis. A mechanistic theory on how PXY could regulate vascular tissues patterning was presented. Overall, the knowledge on radial growth can be used in agronomic and forestry applications to mitigate the dangers of the whole humanity like food shortages or climate crisis.

Precise organization of growing structures is a fundamental process in developmental biology. In plants, radial growth is mediated by the cambium, a stem cell niche continuously producing wood (xylem) and bast (phloem) in a strictly bidirectional manner. While this process contributes large parts to terrestrial biomass, cambium dynamics eludes direct experimental access due to obstacles in live cell imaging. Here, we present a cell-based computational model visualizing cambium activity and integrating the function of central cambium regulators. Performing iterative comparisons of plant and model anatomies, we conclude that the receptor- like kinase PXY and its ligand CLE41 are part of a minimal framework sufficient for instructing tissue organization. By integrating tissue-specific cell wall stability values, we moreover probe the influence of physical constraints on tissue geometry. Our model highlights the role of intercellular communication within the cambium and shows that a limited number of factors is sufficient to create radial growth by bidirectional tissue production.Impact statementRadial plant growth produces large parts of terrestrial biomass and can be computationally simulated with the help of an instructive framework of intercellular communication loops.