1. 5% CO 2 + HL Normal Air + HL Supplemental Figure S1. Conditional lethal effect of MSX on Arabidopsis wild- type plants. A. Plants were grown on the selection medium with 5% CO 2 at 150 mol photons m –2 s –1 (HL) for 2 weeks and then kept in ambient air or 5% CO 2 for 150 h. B. Plants were grown on the selection medium with 5% CO 2 and 50 mol photons m –2 s –1 (LL) for 2 weeks, and then kept in ambient air conditions at LL or HL for 150 h. A B 5% CO 2 + LL Normal Air + LL Normal Air + HL

2. WTnara3 WTnara7 134 bp 579 bp 425 bp 157 bp 1004 bp WTnara10 76 bp 114 bp 38 bp WTnara5 618 bp 403 bp 215 bp Supplemental Figure S2. Detection of nara3, nara5, nara7, and nara10 mutations using dCAPS markers. Gene segments containing nara mutations were amplified by PCR using the following dCAPS primer sets: nara3, 5-TGGAGTACTAATCTCAATCG- 3/5-TGGAGTACTAATCTCAATCG-3; nara5, 5-AACACTTTAGTTGCTCTCGC-3/5- GTACCCGTAAAAGCTACCG-3; nara7, 5-CGGTCTTAGAGCTGCTCTTG-3/5- ACTTCATACCGCGAAATGGG-3; nara10, 5-ATGGCCAACCCGTTT-3/5- CCAAGATCCTCGAGTGT-3. Each PCR product was then digested by restriction enzymes, Bsr DI for nara3, Mnl I for nara5, Pvu II for nara7, and SfcI for nara10.

3. Normal air (0.037 % CO 2 ) WT nara7 Supplemental Figure S3. Lethal photorespiratory phenotype of nara7. Plants were grown on soil in ambient air or 0.3% CO 2 for 14 days. Bar = 1 cm. High CO 2 (0.3 % CO 2 )

4. Col F2 progeny nara5-1 (homozygote) × nara5-2 (heterozygote) Yellow (nara5 )Green (WT) D + E ( + MnlI ) A + B B + C Supplemental Figure S4. Allelism between nara5-1 and nara5-2. DNA was extracted from yellow (nara5) and green (WT) F1 progenies in a seed capsule resulting from outcrossing between nara5-1 and nara5-2. nara5 mutation was confirmed by PCR using dCAPS primers (D and E) as described in Supplemental Figure S2. T-DNA insertions were detected by PCR using the following primers: forward primer A, 5-TTTCGGAGCTTCTGGGAACT-3; reverse primer B, 5- ATGATAGGCTAGTTTGACGCCTTA-3; forward primer C, 5- CCCATTTGGACGTGAATGTAGACAC-3. NARA5 Exon1 Exon 2 ABC D E G to A T-DNA

5. AB shootrootRLCLSTI Relative transcript level Relative transcript level Supplemental Figure S5. Relative NARA5 mRNA levels in different tissues of wild-type Arabidopsis. A. Transcript level of NARA5 in shoots or roots of 8-d-old seedlings cultured in MS medium supplemented with 3% sucrose. Values shown are relative to those of expression in shoots. B. Transcript level of NARA5 in 4-week-old soil-grown plants in rosette leaves (RL), cauline leaves (CL), stems (ST), and inflorescences (I). Inflorescences include inflorescence meristems, closed flower buds, and open flowers. Values shown are relative to transcript levels in R. Data represent means ± SD (n = 3). Each transcript was normalized against Act8 transcript, and the relative transcript level represents the transcript level in each sample relative to that in shoot or RL. Act8 was used to standardize the amount of cDNA from the different tissues because it shows relatively uniform distribution in nearly all vegetative tissues in Arabidopsis thaliana (An et al., 1996)

6. SUPPLEMENTAL LITERATURE CITED An YQ, McDowell JM, Huang S, McKinney EC, Chambliss S, Meagher RB (1996) Strong, constitutive expression of the Arabidopsis ACT2/ACT8 actin subclass in vegetative tissues. Plant J 10:107–121