The sorghum photoperiod sensitivity gene, Ma(3), encodes a phytochrome B

The Ma(3) gene is one of six genes that regulate the photoperiodic sensitivity of flowering in sorghum (Sorghum bicolor [L.] Moench). The ma(3)(R) mutation of this gene causes a phenotype that is similar to plants that are known to lack phytochrome B, and ma(3)(R) sorghum lacks a 123-kD phytochrome that predominates in light-grown plants and that is present in non-ma(3)(R) plants. A population segregating for Ma(3) and ma(3)(R) was created and used to identify two randomly amplified polymorphic DNA markers linked to Ma(3). 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 partial sequence of PHYB was determined from genomic DNA from ma(3)(R) sorghum. PHYA, PHYB, and PHYC all mapped to the same linkage group. The Ma(3)-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 ma(3)(R) sorghum. Therefore, we conclude that the Ma(3) locus in sorghum is a PHYB gene that encodes a 123-kD phytochrome.