3.6 Guard cells have smaller photorespiratory activity than
mesophyll cells
Despite the presence of RuBisCO-mediated CO2assimilation in guard cells (Daloso et al. 2015a), the
photorespiratory activity of these cells remains insufficiently
understood. To shed light on the relative importance of photosynthesis
and photorespiration in guard cells, we next evaluated the13C-enrichment into sugars as metabolites of
successful photosynthetic assimilation, and glycine, serine and
glycerate as photorespiratory metabolites. Increased
R13C in sucrose m/z 319 (3,4,5,6-C) was
observed under both dark and light conditions. However, increased
R13C in sucrose m/z 361 (1,2,3,4,5,6-C) was
only observed in the light, when compared to time 0 (Figure 4). This
resulted in a higher R13C in both sucrose m/z319 and m/z 361 in illuminated guard cells, as compared to
dark-exposed guard cells after 60 min of labelling. Similarly, sorbose,
fructose and glucose were preferentially labelled in the light (Figure
7).
Increased R13C in glycine m/z 102 was observed
in both illuminated and dark-exposed guard cells, whilst increased
R13C in serine m/z 204 over time was only
observed in the light (Figure 4). However, no difference in the
R13C of these metabolites between the treatments after
60 min of labelling was observed. Additionally, glycerate was not
labelled in either condition (Figure 7). We next compared the13C-enrichment in different fragments of both glycine
and serine with those obtained in illuminated Arabidopsis rosettes
following provision of 13CO2 (Szecowkaet al. 2013). The R13C in glycine and serine
was 2.1 and 4.4-fold higher in whole Arabidopsis rosettes (where
mesophyll represents the dominant tissue type) than guard cells
(Supplemental Figure S9), suggesting particularly low photorespiratory
activity in guard cells.