*3.9. Expression of Photosynthesis-Related Genes and Genes Encoding Antioxidant Enzymes*

The expression of two genes relevant to the photosynthetic system was investigated in rice cultivars under HS (Figure 4a,b). The treatment of HS downregulated the expression of *psbA* and *psbB* in rice cultivar leaves, whereas heat-treated plants supplemented with Eth, SNP, or NaHS had higher levels of *psbA* and *psbB* transcription than control plants. In addition, under HS, exogenous HT with Eth or SNP significantly down-regulated the expression of *psbA* and *psbB* compared to the heat-treated Eth or SNP-supplemented plants. The expression of *psbA* and *psbB* was reduced in NBD or cPTIO with NaHS-treated heatstressed plants, but the transcription was lowered more sharply in HT-treated plants.

**Figure 4.** Relative expression of (**a**) *psbA* and (**b**) *psbB* of rice (*Oryza sativa* L.) cultivars Taipei-309 and Rasi under control and high temperature stress (HS) supplied with 200 μL L−<sup>1</sup> ethephon (Eth), 100 μM sodium nitroprusside (SNP), 200 μM sodium hydrosulfide (NaHS) or 100 μM hypotaurine (HT), 100 μM 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), or 100 μM norbornadiene (NBD) scavengers of hydrogen sulfide (H2S), nitric oxide (NO), and ethylene action inhibitors, respectively. Data are presented as treatments mean ± SE (*n* = 4). The values followed by the same letters did not differ significantly by LSD test at *p* < 0.05.

The relative expression analysis based on a qRT-PCR method for three SOD isoforms containing *Mn-SOD*, *Cu-SOD*, and *Fe-SOD* and *APX* was carried out on two rice cultivars (Figure 5). The results revealed that the expression of SOD isoforms and *APX* was upregulated in HS-treated rice cultivars compared to control plants. The individual application of Eth, SNP, or NaHS further enhanced the expression of SOD isoforms and APX in rice cultivars exposed to HS compared to heat-treated plants alone.

**Figure 5.** Relative expression of (**a**) *Mn-SOD*, (**b**) *Fe-SOD*, (**c**) *Cu-SOD*, and (**d**) *APX* of rice (*Oryza sativa* L.) cultivars Taipei-309 and Rasi under control and high temperature stress (HS) supplied with 200 μL L−<sup>1</sup> ethephon (Eth), 100 μM sodium nitroprusside (SNP), 200 μM sodium hydrosulfide (NaHS) or 100 μM hypotaurine (HT), 100 μM 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1 oxyl-3-oxide (cPTIO), or 100 μM norbornadiene (NBD) scavengers of hydrogen sulfide (H2S), nitric oxide (NO), and ethylene action inhibitors, respectively. Data are presented as treatments mean ± SE (*n* = 4). The values followed by the same letters did not differ significantly by LSD test at *p* < 0.05.

Meanwhile, in HT treatment with Eth or SNP under HS, no significant difference was observed in the expression levels of SOD isoforms and *APX* compared to control plants. Unlike the above, Eth, SNP, or NaHS treatments significantly increased the transcription level of these genes in heat-exposed plants. In contrast, HT application appears to affect antioxidant defense-related gene expression.
