Fig. S1 Apparatus used for mutant screening. (a) The stainless net used for mutant screening. Each well contains 40-50 seeds. (b) The plastic box with a fitted net. (c) A diagram showing the water level during ethylene treatment. (d) Typical phenotypes for mutant seedlings and WT seedlings in a segregated population after ethylene treatment. Red arrows indicate mutants and blue arrows indicate WT seedlings. (a) (b) (c) (d) C2H4=10ppm water

Fig. S2 Ethylene response of additional mhz7 mutants and identification of mutation sites. (a) Ethylene response of the mhz7-3, mhz7-4 and mhz7-5 mutants compared to WT in the presence or absence of 10 ppm of ethylene. (b) Confirmation of mutation sites in mhz7-3, mhz7-4 and mhz7-5 by PCR-based analyses using cDNA as templates. The point mutation in mhz7-3 was confirmed using a pair of dCAPS primers (5’-AAAATGGTGGCAGCGACGAAG-3’ and 5’-CGAAGCTGACAACGAGGGAG-3’). The PCR products were digested with BssH II, resulting in a 26bp-deletion in WT but not in the mutant. The point mutation in mhz7-4 was confirmed using a pair of dCAPS primers (5’-ACAAACTCCATTCCAAGGCAgATC-3’ and 5’-TATTGTTGTCCCTTGCTCGAGAGC-3’). The PCR products were digested with Bgl II, resulting in a 20bp-deletion in the mutant but not in WT. The 87bp-deletion in mhz7-5 was examined using RT-PCR with primers (5’-ACATTGACCTCGGGAAATGG-3’ and 5’-AATAGAAGATTGAATCCAAGTGCTATCCC-3’). (a) WT mhz7-3 mhz7-4 mhz7-5 C2H4 (10ppm) - + - + - + - + (b) mhz7-3 mhz7-4 mhz7-5 mhz7-3 mhz7-4 mhz7-5 M WT WT WT bp 300 200 100

Fig. S3 Phenotypic comparison of light-grown seedlings. (a) Phenotypes of WT, mhz7-1, mhz7-2 and MHZ7-overexpressing seedlings grown in water under light condition. The seeds were germinated in PCR plates with wells cut from bottom and floating in water. The seedlings were then grown in water for one month. (b) Chlorophyll a contents of WT, mhz7 mutants and MHZ7-overexpressing lines . Each column is average of three measurements and bars indicate SD. ‘**’ indicate significant difference compared to WT (P<0.01). (a) WT mhz7-1 mhz7-2 OX2 OX3 OX4 OX44 (b) **