Pulvinar Slits: Cellulose-deficient and De-Methyl-Esterified Homogalacturonan-Rich Structures in a Legume Motor Cell

Abstract

The cortical motor cells (CMCs) in a legume pulvinus execute the reversible deformation in leaf movement that is driven by changes in turgor pressure. In contrast to the underlying osmotic regulation, the cell wall structure of CMCs that contributes to the movement has yet to be characterized in detail. Here, we report that the cell wall of CMCs has tangential slits with low levels of cellulose deposition. This structure is unique and distinct from any other primary cell wall reported so far; thus, we named them "pulvinar slits." Notably, we predominantly detected de-methyl esterified homogalacturonan inside pulvinar slits, with a low deposition of highly methyl-esterified homogalacturonan inside the slits, as with cellulose. In addition, Fourier-transform infrared spectroscopy analysis indicated that the cell wall composition of pulvini is different from that of other axial organs, i.e. petioles or stems. Moreover, monosaccharide analysis showed that pulvini are pectin-rich organs like developing stems. Computer modeling suggested that pulvinar slits facilitate anisotropic extension in the direction perpendicular to the slits in the presence of turgor pressure. Mechanical tests revealed that the proximo-distal extensibility of pulvini is superior to that of the other axial organs. In this study, we thus established the distinctive cell wall structure of CMCs that contribute to plant movement, adding new knowledge to the structural diversity and function of the plant cell wall.

Mechanics of Reversible Deformation during Leaf Movement and Regulation of Pulvinus Development in Legumes

“…ε and P c can also be affected by the matrix structure between the cellulose microfibrils. Our recent study [ 9 ] found that the pulvinus was rich in pectin compared to other axial organs. The cell walls of pulvinar motor cells also had many slits (pulvinar slits) running perpendicular to the proximodistal axis, in which the amount of cellulose was low and de-methyl-esterified pectin was abundant.…”

“…The cell walls of pulvinar motor cells also had many slits (pulvinar slits) running perpendicular to the proximodistal axis, in which the amount of cellulose was low and de-methyl-esterified pectin was abundant. Computer simulations indicated that the slits were capable of converting changes in turgor pressure into cellular deformation along an axis perpendicular to the slits [ 9 ]. The association between pulvinar slits and the proximodistal-biased deformation of pulvinar motor cells is further supported by the results of a mechanical test and the finding that pulvinar slits are widely conserved in legumes [ 9 ].…”

“…Computer simulations indicated that the slits were capable of converting changes in turgor pressure into cellular deformation along an axis perpendicular to the slits [ 9 ]. The association between pulvinar slits and the proximodistal-biased deformation of pulvinar motor cells is further supported by the results of a mechanical test and the finding that pulvinar slits are widely conserved in legumes [ 9 ]. Meanwhile, a mechanical test of the onion epidermis after enzymatic pectin esterification indicated that the de-methyl-esterification of pectin increased the plasticity but not the elasticity of the cell wall [ 10 ].…”