Guard cell photosynthesis is critical for stomatal turgor production, yet does not directly mediate CO₂‐ and ABA‐induced stomatal closing

Abstract

SUMMARY Stomata mediate gas exchange between the inter-cellular spaces of leaves and the atmosphere. CO2 levels in leaves (Ci) are determined by respiration, photosynthesis, stomatal conductance and atmospheric [CO2]. [CO2] in leaves mediates stomatal movements. The role of guard-cell photosynthesis in stomatal conductance responses is a matter of debate, and genetic approaches are needed. We have generated transgenic Arabidopsis plants that are chlorophyll-deficient in guard cells only, expressing a constitutively active chlorophyllase in a guard-cell specific enhancer trap-line. Our data show that more than 90% of guard cells were chlorophyll-deficient. Interestingly, approximately ~ 45% of stomata had an unusual, previously not-described, morphology of thin-shaped chlorophyll-less stomata. Nevertheless, stomatal size, stomatal index, plant morphology, and whole-leaf photosynthetic parameters (PSII, qP, qN, FV′/FM′) were comparable to wild-type plants. Time-resolved intact leaf gas exchange analyses showed a reduction in stomatal conductance and carbon assimilation rates of the transgenic plants. Normalization of CO2 responses showed that stomata of transgenic plants respond to [CO2] shifts. Detailed stomatal aperture measurements of normal kidney-shaped stomata, which lack chlorophyll, showed stomatal closing responses to [CO2] elevation and abscisic acid (ABA), while thin-shaped stomata were continuously closed. Our present findings show that stomatal movement responses to [CO2] and ABA are functional in guard cells that lack chlorophyll. These data suggest that guard-cell CO2 and ABA signal transduction are not directly modulated by guard-cell photosynthesis/electron transport. Moreover, the finding that chlorophyll-less stomata cause a “deflated” thin-shaped phenotype, suggests that photosynthesis in guard cells is critical for energization and guard-cell turgor production.

Photorespiration Is Crucial for Dynamic Response of Photosynthetic Metabolism and Stomatal Movement to Altered CO 2 Availability

“…We can therefore only speculate on the reduced abundance of the BL receptors in the guard cells. In addition, it has been recently shown that guard cell photosynthesis (Azoulay-Shemer et al, 2015) and mobilization of starch in guard cells (Prasch et al, 2015;Horrer et al, 2016) are both central in stomatal turgor generation and opening. Inhibition of photosynthesis in the strong PR mutants, as indicated by reduced ETR (Figure 5A), is likely associated with reduced accumulation of starch in stomatal guard cells (Figure 6).…”

Distinct Cellular Locations of Carbonic Anhydrases Mediate Carbon Dioxide Control of Stomatal Movements

“…Chloroplast reactions and a chloroplast HCO 3 2 -CO 2 sink may contribute to the CO 2 responses by enhancing the plasma membrane cellular influx of CO 2 -HCO 3 2 into guard cells, and additional research will be needed to investigate putative chloroplast contributions to the CO 2 response. Note that guard cell chloroplast functions during CO 2 control of stomatal movements may be independent of Rubisco (von Caemmerer et al, 2004) and are not dependent on guard cell photosynthesis (Azoulay-Shemer et al, 2015). Guard cell chloroplasts could function in malate-starch interconversion during stomatal movements (Outlaw and Manchester, 1979).…”

A Tale of Two Sugars: Trehalose 6-Phosphate and Sucrose

“…Guard cells are usually the only cells in the leaf epidermal layers that contain chloroplasts, but the photosynthetic capacity and physiological significance of guard cell chloroplasts have been a matter of debate for many years (Lawson et al, 2014). Guard cell-specific expression of chlorophyllase abolished the photosynthetic capacity of most guard cells but did not affect stomatal sensitivity to ABA or CO 2 stimuli (Azoulay-Shemer et al, 2015). However, guard cell turgor and energization were impaired, indicating that the photosynthetic capacity of guard cell chloroplasts is necessary for the guard cells to function properly.…”