The Sos1-Reference allele arose spontaneously (Doebley et al., 1995). Seeds were obtained from the Maize Genetics Coop and backcrossed six times to B73. The mutant had previously been reported to map to the short arm of chromosome 4 (Doebley et al., 1995). To generate a mapping population, Sos1 (in the B73 background) was crossed to the Mo17 geneti...

The most striking defect in Sos1 mutants is that the SPM initiates one instead of two SMs. In addition, Sos1 mutants have defects in SPM initiation, which we propose is due to the overall reduction in apical IM size. Sos1 mutants also produce fewer branches. However, the branches that are produced are not normal branches but are mixed branches, whi...

Plant Growth and Mature Phenotype Characterization
The Sos1-Reference allele was obtained from the Maize Genetics Cooperation Stock Center (stock 427I) and backcrossed six times into the B73 genetic background of maize (Zea mays). Analysis of mature phenotype was carried out on plants grown at Rock Springs, PA, during the summer. The plants were genotyped as Sos1 heterozygotes or homozygotes using SSR marker umc1294, which is the closest genetic marker identified so far (umc1294F, 5′-GCC GTC AAC GGG CTT AAA CT-3′ and umc1294R, 5′-GCC TCC ACG...

We thank Erik Vollbrecht for pointing out the similarity between Sos1; ra2 and ra1; ra2 double mutants, David Barnes and Sarah Hake for first discovering the Sos1; ra2 interaction, and two anonymous reviewers for their helpful suggestions. We thank Deb Grove of the Huck Institutes Nucleic Acid Facility for conducting the real-time RT-PCR experiment...