Stomatal Function Requires Pectin De-methyl-esterification of the Guard Cell Wall

Sam Amsbury,Lee Hunt,Nagat Elhaddad,Alice Baillie,Marjorie Lundgren,Yves Verhertbruggen,Henrik V Scheller,J Paul Knox,Andrew J Fleming,Julie E Gray

doi:10.1016/j.cub.2016.08.021

Abstract

SummaryStomatal opening and closure depends on changes in turgor pressure acting within guard cells to alter cell shape [1]. The extent of these shape changes is limited by the mechanical properties of the cells, which will be largely dependent on the structure of the cell walls. Although it has long been observed that guard cells are anisotropic due to differential thickening and the orientation of cellulose microfibrils [2], our understanding of the composition of the cell wall that allows them to undergo repeated swelling and deflation remains surprisingly poor. Here, we show that the walls of guard cells are rich in un-esterified pectins. We identify a pectin methylesterase gene, PME6, which is highly expressed in guard cells and required for stomatal function. pme6-1 mutant guard cells have walls enriched in methyl-esterified pectin and show a decreased dynamic range in response to triggers of stomatal opening/closure, including elevated osmoticum, suggesting that abrogation of stomatal function reflects a mechanical change in the guard cell wall. Altered stomatal function leads to increased conductance and evaporative cooling, as well as decreased plant growth. The growth defect of the pme6-1 mutant is rescued by maintaining the plants in elevated CO2, substantiating gas exchange analyses, indicating that the mutant stomata can bestow an improved assimilation rate. Restoration of PME6 rescues guard cell wall pectin methyl-esterification status, stomatal function, and plant growth. Our results establish a link between gene expression in guard cells and their cell wall properties, with a corresponding effect on stomatal function and plant physiology.

Highlights

Are excluded from the guard cells and that un-esterified HGA is the predominant form of pectin in the guard cell wall
Pme6-1 plants had a reduction in the levels of de-esterified pectin in the guard cell, as indicated by the weaker binding of LM19 (Figures 2C and 2D) and abundant highly esterified pectin, as indicated by LM20 binding (Figures 2G and 2H). These data indicate that the structure of the HGA component of the pectin network has been altered in the pme6-1 knockout line and, in particular, that the pectin of guard cells is more highly methyl-esterified in plants lacking PME6
A restricted ability of pme6-1 stomata to respond to abscisic acid, a classical regulator of stomatal function [17], was observed (Figure S4E), suggesting that the altered pectin methyl-esterification status of the guard cells was affecting a fundamental property of the stomata

Summary

Introduction

Are excluded from the guard cells and that un-esterified HGA is the predominant form of pectin in the guard cell wall. These data indicate that the structure of the HGA component of the pectin network has been altered in the pme6-1 knockout line and, in particular, that the pectin of guard cells is more highly methyl-esterified in plants lacking PME6.

Results

Conclusion