Of Cells, Strands, and Networks: Auxin and the Patterned Formation of the Vascular System

A molecular framework for proximal secondary vein branching in the Arabidopsis thaliana embryo

“…These organs form de novo during the post-embryonic growth of the plant. The establishment of the vascular strands in leaves is believed to follow a similar molecular regulation as cotyledons, even if the directionality of the secondary strands differs (Lavania et al 2021). Under NPA treatment, the acropetal transport of auxin that directs the growth of the midvein is altered, resulting in the duplicated formation of midveins (Scarpella et al 2006).…”

“…While auxin has been considered the necessary and sufficient signal for vein formation (Lavania et al 2021), the molecular regulators underpinning the branching of vascular tissues have been poorly explored. Furthermore, the auxin canalization model itself cannot solely explain the appearance of an intermittent vascular network in vein mutants such as cotyledon vascular pattern 2 (cvp2) cvp2-like 1 (cvl1) (Carland and Nelson 2009).…”

“…The evolution of plants is characterized by the selection of a spatial arrangement of vascular strands (vascular patterns) that maximize their overall functionality (Lucas et al 2013). In Arabidopsis thaliana (Arabidopsis), the vascular pattern of foliar organs is generated and maintained through the formation of continuous procambial cell files; these are further organized in vascular bundles comprising the conductive tissues phloem and xylem (Lavania et al 2021). While in most species the patterning of leaf vascular tissues exhibits a high degree of plasticity (Scarpella 2017), the robust and reproducible patterns of the vein network in Arabidopsis cotyledons offer an ideal model to identify the positional and molecular cues underlying this process.…”

“…These vascular cells are surrounded by mesophyll cells, which have been proposed to limit vein propagation in Arabidopsis leaves through their differentiation, terminating the vein path (Scarpella et al 2004). Similar to vascular cells, mesophyll cells derive from ground meristem (GM) cells located in the subepidermal layer of cotyledons (Lavania et al 2021). During the specification of GM cells into either mesophyll or vascular cells, positional cues determine the acquisition of both identities and thus cell types (Lavania et al 2021).…”

“…Similar to vascular cells, mesophyll cells derive from ground meristem (GM) cells located in the subepidermal layer of cotyledons (Lavania et al 2021). During the specification of GM cells into either mesophyll or vascular cells, positional cues determine the acquisition of both identities and thus cell types (Lavania et al 2021). The widely accepted model for cotyledon/leaf vascular cell specification, the auxin canalization model, supports that a directional auxin flow acts as a pre-pattern and reinforces vascular cell identity delineating an incipient vascular path along the cells that have been most exposed to the auxin flow (Scarpella et al 2006; Lavania et al 2021).…”