Starch biosynthesis by AGPase, but not starch degradation by BAM1/3 and SEX1, is rate‐limiting for CO₂‐regulated stomatal movements under short‐day conditions

Abstract

Starch in guard cells functions in osmoregulation during stomatal movements. Starch metabolism is controlled by the circadian clock. We investigated the role of starch metabolism in stomatal responses to CO under different photoperiodic conditions. Guard cell starch levels correlate with low/high [CO ] exposure. Starch biosynthesis-deficient AGPase (ADG1) mutants but, unexpectedly, not the starch degradation-deficient BAM1, BAM3, and SEX1 mutants alone, are rate-limiting for stomatal conductance responses to [CO ]-shifts. Interestingly, AGPase is rate-limiting solely under short- but not long-day conditions. These findings suggest a model of enhanced AGPase activity in guard cells under short days such that starch biosynthesis becomes rate-limiting for CO -induced stomatal closing.

Arabidopsis guard cell chloroplasts import cytosolic ATP for starch turnover and stomatal opening

“…7d ). Previous studies reported that starch biosynthesis in GCs is involved in stomatal closing, where starch would serve as a sink for metabolites previously stored in the vacuole, which then need to be removed to reduce cell turgor 47 , 48 . Our findings are in line with this idea and highlight the importance of gluconeogenesis and ATP import to GCCs to promote the conversion back to starch of organic acids and sugars previously accumulated by the GCs.…”

Leaf starch metabolism sets the phase of stomatal rhythm

“…On the other hand, sex1 is defective in the plastidial -glucan water, dikinase (GWD1) that is responsible for the first step of glucan phosphorylation necessary for starch breakdown (Caspar et al, 1991; Yu et al, 2001; Ritte et al, 2002). In sex1 , daytime starch synthesis largely exceeds the residual nighttime degradation, resulting in a severe starch-excess phenotype (Streb and Zeeman, 2012; Figure 1A-B), which can be observed in mesophyll cells and also in the guard cells (Azoulay-Shemer et al, 2018; Figure 1C-D). Both pgm and sex1 mutations result in very low starch turnover and generate similar diel variations in the carbon status: during the day, photosynthesis-derived sugars accumulate massively compared to the wild type, but then at night the absence or unusability of starch triggers a rapid depletion of available sugars, resulting in carbon starvation until dawn (Streb and Zeeman, 2012).…”

Metabolomics of red‐light‐induced stomatal opening in Arabidopsis thaliana: Coupling with abscisic acid and jasmonic acid metabolism

“…We identified high humidity as another key factor to observe red‐light‐promoted stomatal opening in Arabidopsis epidermal peels from growth‐chamber‐grown plants (Figure ). Previous studies have detected growth‐condition‐dependent responses of guard cells to other stimuli that regulate stomatal apertures, particularly CO 2 and blue light (Talbott et al , , ; Frechilla et al , , ; Azoulay‐Shemer et al , ). Particularly, enhanced CO 2 response was observed in greenhouse‐grown V. faba leaves when relative humidity was elevated, indicating air relative humidity is a key environmental factor mediating the stomatal sensitivity to CO 2 (Talbott et al , ).…”