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.