Slits in the wall! Pulvinar slits confer unique cell wall mechanics for leaf movement

Vascular plants exhibit reversible leaf movements that can take place over timescales from seconds (e.g. folding in mimulus) to hours (circadian movements). Many of these reversible movements are mediated by the pulvinus, a specialized organ at the base of the leaf, leaflet, petiole, or petiolule (Fig. 1A). The cortical motor cells form the swelling part of the pulvinus and execute the growth-independent motion and the bending of leafy appendages (Nakata and Takahara 2022). During leaf movement, pulvinus motion is associated with the active transport of ions from parenchyma cells and sugars from vascular elements, resulting in fluid movement and turgor-induced cell deformation (Kumon and Suda 1984; Sleboda et al. 2023). The cell wall is a dynamic and responsive structure that sustains cell division and expansion, provides structural integrity to each cell, and serves in sensing environmental changes (Oliveira et al. 2020b; Coen and Cosgrove 2023). Cellulose, hemicelluloses, and pectins are the main components of primary cell walls. How do the wall composition and properties of cortical motor cells contribute to the leaf motion has remained a mystery, however. Recent advancements have provided important evidence of cellulose–cellulose bonding and noncovalent cellulose–pectin interactions in the primary walls of Arabidopsis (Arabidopsis thaliana) (Wang et al. 2015; Cosgrove 2018); however, the intraand intermolecular cross-links between cellulose microfibrils with xyloglucan hemicellulose and pectins are still unclear. In this issue of Plant Physiology, Takahara et al. (2023) set out to investigate the cell wall structure of cortical motor cells in the pulvinus of panicled-leaf ticktrefoil (Desmodium paniculatum). As it is not a model plant like Arabidopsis or maize (Zea mays), little is known about its cell wall makeup. The starting point for the authors’ work was the identification of the cell wall structure of cortical motor cells by staining pulvini with calcofluor white, which stains cellulose. Notably, the cortical motor cells presented several slits with no fluorescence in a circumferential orientation in the adaxial/abaxial-mediolateral plane, indicating a lower deposition of cellulose in slits. To determine whether the slits are specific to cortical motor cells, they also stained cortical cells from the petiole and from the boundary region between pulvinus and petiole. Both cases showed no slits. The authors, thus, named the circumferential slits in pulvinus cortical motor cells as “pulvinar slits” (Fig. 1B). The authors wondered to what extent the pulvinar slits are conserved across different legumes with similarly dynamic leaves. To investigate this, they stained vertical sections of pulvini from 11 legume species with calcofluor white and confirmed the presence of slits in both adaxial and abaxial pulvinar cortical motor cells. These results demonstrated that pulvinar slits are innate wall components of cortical motor cells. Consistent with the observation that pulvinar slits are regions deficient in cellulose compared with the surrounding regions, Takahara et al. turned to cellulose-specific staining with pontamine and immunolabeling to confirm that slits have lower cellulose deposition. They concluded that pulvinar slits are regions of low cellulose deposition in the cell walls of both adaxial and abaxial cortical motor cells and are widely conserved in legumes. Interestingly, Sleboda et al. (2023) independently also reported the presence of primary pit fields in pulvini of Mimosa pudica, the structures of which have similar shape and size to pulvinar slits. Therefore, N ew s an d V ie w s