*2.1. PGLP Overexpressors Sustain CO2 Assimilation and Electron Transport at Combined High Light and Temperature*

In a first series of experiments, we characterized the photosynthetic performance of *PGLP* overexpressors using high light intensities and elevated temperature during photosynthetic measurements. To this end, the wild type and *PGLP* overexpression lines O9 and O1 were grown under standard conditions to growth stage 5.1 [33] and then subjected to gas exchange measurements (Figure 1A,B). First, we monitored net photosynthetic CO2 uptake rates (*A*) in response to increasing light intensities (PAR: 0, 20, 50, 100, 200, 400, 600, 800, 1200 and 1600 μmol m−<sup>2</sup> s−1) at the standard growth temperature of 20 ◦C. In parallel, stomatal conductance (*gs*), transpiration (*E*) and relative electron transport rates (rETR) were recorded. As shown in Figure 2A, we did not detect significant differences in any of these parameters between the transgenic lines and the wild type, which is in good agreement with previous findings [10]. However, an increase of the incubation temperature from 20 ◦C to 30 ◦C caused significant and systemic short-term changes. In comparison to the wild type, we observed significantly increased *A* at higher light intensities, ranging from 1200 to 1600 in O9 and from 600 to 1600 μmol m−<sup>2</sup> s−<sup>1</sup> in our best performing line O1 (Figure 2B). Similarly, both lines were able to maintain higher *gs* at the specified light intensities and displayed somewhat elevated *E* at 1200 and 1600 μmol-2 s-1 light (Figure 2B). Moreover, an increase in rETR was measured, but it was significant only in line O1 (400 to 1600 μmol m−<sup>2</sup> s<sup>−</sup>1). Collectively, our results suggest that high PGLP activity is beneficial for photosynthetic CO2 fixation and chloroplastidal electron transport at high light intensities in conjunction with elevated temperatures.

**Figure 1.** Scheme of the experimental strategy. **(A**) Following 6 weeks of growth under standard conditions, plants were used for (**B**) gas exchange and chlorophyll a fluorescence measurements. Light response curves were recorded at 20 ◦C (standard) and 30 ◦C (elevated temperature). (**C**) A subset of plants was exposed to water shortage and used for gas exchange measurements (*A*/*Ci* curves) after 13 days or (**D**) to elevated temperature for gas exchange measurements (*A*/*Ci* curves) after 1 and 7 days and metabolite and expression analysis.
