Paramutation alters regulatory control of the maize pl locus

The maize purple plant (pl) locus encodes a transcription factor required f or anthocyanin pigment synthesis in vegetative and floral tissues. The stro ngly expressed Pl-Rhoades (Pl-Rh) allele is unstable, spontaneously changin g to weaker expression states (PEI) at low frequencies and exclusively chan ging to PII in Pl'/Pl-Rh heterozygotes. The weakly expressed Pl' state is m itotically and meiotically stable, vet reversible. This type of allele-depe ndent, heritable alteration of gene control is called paramutation. Express ion studies herein demonstrate that visible differences in anthocyanin pigm ent levers mirror pi RNA abundance and that pi paramutation is associated w ith reduced transcription of the pl gene. This transcriptional alteration i s accompanied by acquisition of light-dependent regulation. Restriction end onuclease mapping indicates that these changes in pi gene regulation are no t associated with detectable DNA alterations or with extensive changes in c ytosine methylation patterns. Genetic tests show that Pl-Blotched in pi par amutation. This result suggests that if cis-acting sequences are required f or pl paramutation they are distinct from the protein coding and immediatel y adjacent regions. A model is discussed in which Pl paramutation results i n heritable changes of chromatin structure that fundamentally alter regulat ory interactions occurring during plant development.