*2.2. BSR1 Is Involved in Regulating MAMP-Responsive Genes*

In MAMP-treated suspension-cultured cells, the transcriptional activation of defense-related genes was analyzed. Transcript levels of four defense-related genes, diterpenoid phytoalexin (DP) momilactone biosynthetic gene*KAURENE SYNTHASE-LIKE 4* (*KSL4*), DP biosynthetic key transcription factor-encoding gene *DITERPENOID PHYTOALEXIN FACTOR* (*DPF*) [41], the representative defense marker gene *PROBENAZOLE-INDUCIBLE PROTEIN 1* (*PBZ1*), and flavonoid phytoalexin and lignin biosynthetic gene *PHENYLALANINE AMMONIA-LYASE 1* (*PAL1*) were determined. After treatment with peptidoglycan, the inductions of *KSL4*, *DPF*, and *PBZ1* in knockout cells were significantly weaker than in wild-type, although significant changes in *PAL1* transcript level were not detected (Figure 2a). Knocking out *BSR1* resulted in a decrease in *PBZ1* transcript levels under mock-treatment conditions (Figure 2). Our liquid cultivation conditions slightly induced *PBZ1* transcriptional activation, which was mediated by BSR1.

**Figure 2.** MAMP-induced transcript levels of defense-related genes were suppressed in *BSR1*-knockout suspension-cultured rice cells. The *PBZ1*, *PAL1*, *KSL4*, and *DPF* transcript levels at 3-h post treatment with peptidoglycan (**a**) and LPS (**b**) were normalized against the *RUBQ1* internal control levels. Values are presented as the means ± standard deviations of three biological replicates. Experiments were conducted two times with similar results. Different letters indicate significant differences (Tukey's test; *p* < 0.05). PGN, peptidoglycan; KO, knockout line; KO#1, *bsr1-1*#13-1; KO#2, *bsr1-2*#16-2; KO#8, *bsr1-8*#5-1; WT, wild-type; LPS, lipopolysaccharide.

Knocking out *BSR1* suppressed the elicitation of *KSL4* and *DPF* by LPS (Figure 2b). The significant suppression of *PBZ1* and *PAL1* were not reproducibly detected. As shown in Figures 1 and 2, BSR1 appears to function in defense responses after the plant perceives peptidoglycan and LPS.
