THE SORGHUM PHOTOPERIOD SENSITIVITY GENE, MA(3), ENCODES A PHYTOCHROME-B

The Ma(3) gene is one of six genes that regulate the photoperiodic sen sitivity of flowering in sorghum (Sorghum bicolor [L.] Moench). The ma (3)(R) mutation of this gene causes a phenotype that is similar to pla nts 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 i s present in non-ma(3)(R) plants. A population segregating for Ma(3) a nd ma(3)(R) was created and used to identify two randomly amplified po lymorphic DNA markers linked to Ma(3). These two markers were cloned a nd mapped in a recombinant inbred population as restriction fragment l ength polymorphisms. cDNA clones of PHYA and PHYC were cloned and sequ enced from a cDNA library prepared from green sorghum leaves. Using a genome-walking technique, a 7941-bp partial sequence of PHYB was deter mined 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 wit h PHYB more than 121 centimorgans from PHYA and PHYC. A frameshift mut ation resulting in a premature stop codon was found in the PHYB sequen ce 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.