The Sorghum Photoperiod Sensitivity Gene , Ma , , Encodes a Phytochrome B '

The Ma, gene is one of six genes that regulate the photoperiodic sensitivity of flowering in sorghum (Sorghum bicolor [L.] Moench). The masR mutation of this gene causes a phenotype that is similar to plants that are known to lack phytochrome B, and masR sorghum lacks a 123-kD phytochrome that predominates in light-grown plants and that is present in non-magR plants. A population segregating for Ma, and masR was created and used to identify two randomly amplified polymorphic DNA markers linked to Ma,. These two markers were cloned and mapped in a recombinant inbred population as restriction fragment length polymorphisms. cDNA clones of PHYA and PHYC were cloned and sequenced from a cDNA library prepared from green sorghum leaves. Using a genome-walking technique, a 7941 -bp partia1 sequence of PHYB was determined from genomic DNA from masR sorghum. PHYA, PHYB, and P H Y C all mapped to the same linkage group. The Ma,linked markers mapped with PHYB more than 121 centimorgans from PHYA and PHYC. A frameshift mutation resulting in a premature stop codon was found in the PHYB sequence from magR sorghum. Therefore, we conclude that the Ma, locus in sorghum is a PHYB gene that encodes a 123-kD phytochrome.