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

Abstract

Stomatal 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.

Keywords: Arabidopsis; cell wall; guard cell; pectin; stomata.

Graphical abstract

Figure 1

Guard Cells Show Specific Patterns of Wall Epitopes (A) Ubiquitous presence of pectin in cell walls. The JIM7 antibody binds to HGA with a broad range of methyl-esterification and shows labeling in all cell walls in a cross-section through the epidermis (ep) encompassing guard cells (gc) above a sub-stomatal cavity (ssc). (B) Calcium cross-linked HGA is restricted to cell interstices. The 2F4 antibody indicates cell walls containing calcium cross-linked HGA characterized by stretches of unesterified HGA residues. (C) Unesterified HGA is present in GC walls. Binding of the LM19 antibody indicates that HGA with no or little esterification is prevalent in all cell walls of the epidermis. (D) Highly methyl-esterified pectin is absent from the guard cell wall, as indicated by the lack of binding of the LM20 antibody. (E) Binding of the LM25 antibody indicates that xyloglucan is present in all cell walls of the epidermis. (F) A control with no primary antibody (Ab) showing low levels of autofluorescence against the Calcofluor White staining of the cell wall. In all panels, the green signal shows binding of the specific primary antibody indicated, and the magenta signal (false color) indicates Calcofluor White fluorescence of cell walls. Scale bars, 20 μm. See also Figure S1 and Table S1.

Figure 2

Guard Cell Wall Pectin Composition Is Altered in pme6-1 Plants (A–D) The high level of unesterified HGA in WT guard cells indicated by LM19 antibody binding in both cross-sections (A) and paradermal sections (B) is greatly diminished in pme6-1 (C and D). ep, epidermis; gc, guard cells. (E–H) Highly methyl-esterified HGA is absent in WT guard cell walls (E and F) but accumulates in the guard cell walls of the pme6-1 mutant, as revealed by binding of the LM20 antibody (G and H). (I–L) The general distribution of HGA (indicated by the JIM7 antibody) is similar in the WT (I and J) and the pme6-1 mutant (K and L). (M–P) Control sections not hybridized with primary antibody but stained with Calcofluor White indicate the signal specificity of the immunolabeling experiments in (A)–(L) and the general distribution of the cell wall material. In all panels, the green signal shows binding of the specific primary antibody indicated, and the magenta signal (false color) indicates Calcofluor White fluorescence of cell walls. (Q–S) Counting of stomata showing the patterns of labeling with each antibody indicate the switch in LM20/LM19 labeling pattern between WT and the pme6-1 mutant guard cells. Localization of fluorescence in transverse sections after antibody binding was scored as fully covering guard cells (as in I), partially covering guard cells (as in C), or limited to guard cell-epidermal cell junctions (as in E). Data are shown for LM19 (Q), LM20 (R), and Jim7 (S) immunolabeling. Quantification was based on scoring patterns from 50 stomata, with five stomata scored from each of ten plants. Scale bars, 20 μm. See also Figures S1–S3.

Figure 3

pme6-1 Plants Have Altered Guard Cell Physiology and Water Relationships (A) Guard cell opening/closure response to changing CO₂ concentration is lost in the pme6-1 mutant. Pore area was measured from stomata in epidermal peels taken from the genotypes indicated (WT, pme6-1, and pme6-1 complemented with a _proPME6::PME6 construct) after incubation of the peels with either CO₂-free air (0 ppm CO₂; solid bars), ambient CO₂ (hatched bars) or high CO₂ (1,000 ppm; open bars). Each column shows the mean and SEM. (n = 6), with statistical differences determined by ANOVA with a post hoc Tukey test. Columns indicated with identical letters cannot be distinguished from each other (p < 0.01). (B) pme6-1 plants are less able to adjust leaf temperature under drought conditions. Thermal images are shown of well-watered plants of the genotypes indicated (top images) taken at day 0 post-drought. Images of equivalent plants at day 5 post-drought (lower panel) show that the pme6-1 plants have a lower leaf temperature than the WT or the complemented pme6-1 mutant. (C) Quantification of thermal image data shows that pme6-1 leaf temperature does not change significantly under drought conditions, while the WT and the complemented mutant leaf temperature increases. Each bar represents the mean temperature for the rosette with error bars indicating SEM (n = 6). Statistical differences were determined by ANOVA with a post hoc Tukey test. Columns indicated with identical letters cannot be distinguished from each other (p < 0.05). See also Figure S4.

Figure 4

pme6-1 Plants Show a Limited Dynamic Range of Stomatal Movement and Decreased Growth under Ambient CO₂, which Is Rescued by Elevated CO₂ (A) pme6-1 leaves show a limited dynamic range in stomatal conductance (g_s) in response to changing CO₂ level. Gas exchange data for WT and pme6-1 leaves show that, under ambient CO₂ conditions, the pme6-1 leaves have higher g_s than the WT. Following exposure to elevated (1,000 ppm) CO₂, g_s in both mutants and WTs fall. Exposure to a low (100-ppm) CO₂ regime induces increased g_s, but the pme6-1 g_s trace plateaus to a lower value than for WT leaves. Error bars indicate the SEM (n = 8). (B) pme6-1 stomata show a differential pore size response after incubation in high osmoticum. Stomatal pore areas were measured in epidermal peels from either WT or pme6-1 leaves incubated either in resting buffer (solid bars) or resting buffer with addition of mannitol to 0.5 M (hatched bars). Statistical differences were determined by ANOVA and a post hoc Tukey test. Columns indicated with identical letters cannot be distinguished from each other (p < 0.01, n = 3, with 40 stomata counted from a total of four plants, repeated on 3 consecutive days). Error bars indicate the SEM. (C) The more limited dynamic range in g_s exhibited by pme6-1 leaves is maintained after the growth of plants at elevated CO₂. Gas exchange data for WT and pme6-1 leaves taken from plants grown continually under elevated CO₂. The traces for the WT and pme6-1 as the CO₂ level is altered during gas exchange analysis are comparable to those shown in (A), with the pme6-1 trace again reaching a lower plateau after exposure to sub-ambient CO₂ level (n = 8). (D) At elevated CO₂ levels, the pme6-1 leaves have a greater potential to assimilate CO₂ than WT leaves. A/Ci curve analysis of WT and pme6-1 leaves indicates that the instantaneous C assimilation rate at ambient CO₂ levels is comparable but that as C_i increases, the pme6-1 leaves show a greater maximum potential assimilation rate (n = 5 for WT; n = 6 for pme6-1; error bars indicate the SEM). (E–G) pme6-1 plants are smaller than WTs under ambient CO₂, but growth at elevated CO₂ leads to plants attaining a similar size. Images of plants (genotypes as indicated) under ambient CO₂ are shown in (E, top row) and under elevated (1,000 ppm) CO₂ in (E, bottom row). Quantitation of total rosette area of plants grown under ambient CO₂ (F) shows that pme6-1 plants achieve a smaller final size, whereas growth in elevated CO₂ (G) leads to all plants reaching a similar mean size. In (F and G), error bars indicate the SEM, n = 8. See also Figure S4.