Strigolactone- and Karrikin-Independent SMXL Proteins Are Central Regulators of Phloem Formation

Plant stem cell niches, the meristems, require long-distance transport of energy metabolites and signaling molecules along the phloem tissue. However, currently it is unclear how specification of phloem cells is controlled. Here we show that the genes SUPPRESSOR OF MAX2 1-LIKE3 (SMXL3), SMXL4, and SMXL5 act as cell-autonomous key regulators of phloem formation in Arabidopsis thaliana. The three genes form an uncharacterized subclade of the SMXL gene family that mediates hormonal strigolactone and karrikin signaling. Strigolactones are endogenous signaling molecules regulating shoot and root branching [1] whereas exogenous karrikin molecules induce germination after wildfires [2]. Both activities depend on the F-box protein and SCF (Skp, Cullin, F-box) complex component MORE AXILLARY GROWTH2 (MAX2) [3-5]. Strigolactone and karrikin perception leads to MAX2-dependent degradation of distinct SMXL protein family members, which is key for mediating hormonal effects [6-12]. However, the nature of events immediately downstream of SMXL protein degradation and whether all SMXL proteins mediate strigolactone or karrikin signaling is unknown. In this study we demonstrate that, within the SMXL gene family, specifically SMXL3/4/5 deficiency results in strong defects in phloem formation, alteredsugar accumulation, and seedling lethality. By comparing protein stabilities, we show that SMXL3/4/5 proteins function differently to canonical strigolactone and karrikin signaling mediators, although being functionally interchangeable with those under low strigolactone/karrikin signaling conditions. Our observations reveal a fundamental mechanism of phloem formation and indicate that diversity of SMXL protein functions is essential for a steady fuelling of plant meristems.Peer reviewe

Computational modeling of cambium activity provides a regulatory framework for simulating radial plant growth

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 stiffness 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 are sufficient to create radial growth by bidirectional tissue production