*2.6. Expression of Photorespiratory Proteins Increases after High Temperature Exposure in Wild Type but not in PGLP Overexpressor Lines*

Previously, it was reported that the expression and amounts of some photorespiratory genes and proteins, respectively, changed in response to abiotic stresses [25,35]. Therefore, we analyzed the expression of selected photorespiratory genes and proteins during the high temperature treatment using quantitative real-time polymerase chain reaction (qRT-PCR) and immunoblotting, respectively. As shown in Figure 7, mRNA levels of several photorespiratory genes (*PGLP1*, glutamate:glyoxylate

aminotransferase 1 (*GGT1*), glycine decarboxylase P and T protein (*GDC-P* and *GDC-T*), serine hydroxymethyltransferase 1 (*SHM1*) and peroxisomal hydroxypyruvate reductase 1 (*HPR1*)) were significantly elevated in wild-type leaves after one and three days at 30 ◦C, while after 7 days their expression levels returned almost to the initial levels at 20 ◦C (Figure 7). To prove, whether these transcriptional alterations translated also to changes in the protein abundances, we exemplarily analyzed PGLP1, GDC-P, SHM1 and HPR1 protein amounts at the same time points. As observed before for mRNA expression, all four proteins increased in wild type after exposure to 30◦ (Figure 7). In contrast, both *PGLP* overexpression lines did not display the increased expression of photorespiratory genes and proteins. As expected, lines O9 and O1 showed significantly increased *PGLP* expression on the mRNA and protein level already under control conditions and only minor alterations in course of the temperature treatment (Figure 7A). However, all the other photorespiratory genes analyzed showed constantly lower mRNA and only very minor changes in the protein levels during the entire experiment. Only the genes *GGT1*, *GDC-T*, *GDC-P* and *HPR1* showed higher mRNA amounts in line O1 after 7 days at 30 ◦C (Figure 7B,D,E,F). From these results we conclude that *PGLP* overexpression somehow prepares the plants to cope with the temperature stress without a coordinative upregulation of other genes and enzymes involved in photorespiration.

**Figure 7.** Expression of selected photorespiratory gene on the mRNA and protein level in wild-type Arabidopsis and *PGLP* overexpressors exposed to 30 ◦C. Wild-type and *PGLP* overexpressor (O9 and O1) plants were grown in under standard conditions (20 ◦C) for 6 weeks following exposure to elevated temperature (30 ◦C). Leaf material was harvested at the end of the day (9 h illumination) to analyse mRNA and protein expression of selected photorespiratory genes under control conditions (20 ◦C) and after 1, 3 and 7 days after the transfer to 30 ◦C. Shown are: mRNA expression of (**A**)—*PGLP*, (**B**)—*GGT1*, (**C**)—*GDC-P*, (**D**)—*GDC-T*, (**E**)—*SHM1*, and (**F**)—*HPR1* and protein amounts of (**G**)—PGLP, (**H**)—GDC-P, (**I**)—SHM1 and (**J**)—HPR1. Coomassie stains (10 μg protein per lane) are shown in (**K**) and (**L**) as loading controls. Values are means ± SD from at least 3 biological replicates per genotype (Col.0—green, solid line with circles, O9—black, dotted lines with squares and O1—black, dashed line with triangles). Asterisks indicate values statistically different from the wild type control at 20 ◦C and plusses of the transgenic lines to the respective wild-type time point as determined by Student's *t*-test (*p* < 0.05).
