Academic journal article Genetics

Rmr6 Maintains Meiotic Inheritance of Paramutant States in Zea Mays

Academic journal article Genetics

Rmr6 Maintains Meiotic Inheritance of Paramutant States in Zea Mays

Article excerpt

ABSTRACT

Paramutation generates heritable changes affecting regulation of specific alleles found at several Zea mays (maize) loci that encode transcriptional regulators of anthocyanin biosynthetic genes. Although the direction and extent of paramutation is influenced by poorly understood allelic interactions occurring in diploid sporophytes, two required to maintain repression loci (rmr1 and rmr2), as well as mediator of paramutation 1 (mop1), affect this process at the purple plant 1 (pl1) locus. Here we show that the rmr6 locus is required for faithful transmission of weakly expressed paramutant states previously established at both pl1 and red1 (r1) loci. Transcriptional repression occurring at both pl1 and booster1 (b1) loci as a result of paramutation also requires Rmr6 action. Reversions to highly expressed, nonparamutant states at both r1 and pl1 occur in plants homozygous for rmr6 mutations. Pedigree analysis of reverted pl1 alleles reveals variable latent susceptibilities to spontaneous paramutation in future generations, suggesting a quantitative nature of Rmr6-based alterations. Genetic tests demonstrate that Rmr6 encodes a common component required for establishing paramutations at diverse maize loci. Our analyses at pl1 and r1 suggest that this establishment requires Rmr6-dependent somatic maintenance of meiotically heritable epigenetic marks.

MEIOTICALLY heritable alterations in gene regulation conditioned by specific allelic interactions are known as paramutations (BRINK 1958). At least four distinct examples of paramutations occur in Zea mays (maize) (ROLLICK et al. 1997; CHANDLER et al. 2000; CHANDLER and STAM 2004). Specific alleles exhibit this unusual inheritance behavior at the red! (rl), boosterl (bl), pericarpl (pi), and purpleplantl (pll) loci (BRINK 1956; COE 1966; DAS and MESSING 1994; HOLLICK et al. 1995), all of which encode transcriptional regulatory proteins of flavonoid biosynthetic pathways. In general, these unique alleles can exist in a dynamic range of regulatory states manifest as heritable differences in visual patterns and levels of colorful red or purple pigments. Both the heritable switching and the stability of these states are influenced by the homologous allele in diploid sporophytes (CoE 1966; STYLES and BRINK 1966; DAS and MESSING 1994; HOLLICK and CHANDLER 1998). Such allelic interactions affecting inheritance of regulatory information has important implications for breeding efforts and represents a potential novel resource of heritable variation in plants (KERMICLE and ALLEMAN 1990; HOLLICK and CHANDLER 1998).

The pll locus encodes a R2R3 MYB domain protein (CONE et al. 1993) that, in combination with basic helixloop-helix domain proteins encoded by either bl or rl loci, promotes transcription of genes encoding enzymes required for anthocyanin biosynthesis (GoFF et al. 1992). The Pll-Rhoades allele confers the highest pigment levels of any previously described pll allele. The high-expression reference state selected in culture (denoted Pl-Rh) is, however, unstable and can spontaneously change to weaker expression states referred to as Pt (Figure IA; HOLLICK et al. 1995). A continuum of Pll-Rhoades expression states can be achieved and is quantified using a visual 1-7 graded anther color score (ACS; Figure IB; HOLLICK et al. 1995). Different^ RNA levels are directly correlated with these anther phenotypes and are considered to reflect differences in transcription rates as measured from isolated husk nuclei (HOLLICK ei a£ 2000).

Pigment levels produced from various Pll-Rhoades expression states are inversely correlated with levels of an experimentally defined activity-termed paramutagenicity-that facilitates heritable changes of the homologous Pll-Rhoades expression state in trans (HoLLiCK et al. 1995). Pt states representing ACS 1-4 classes are highly paramutagenic to the Pl-Kh (ACS 7) reference state: when Pll-Rhoadesalleles of both Pl' and Pl-Kh states are combined in a diploid sporophyte, only Pll-Rhoades alleles of ACS 1-4 Pl' states are sexually transmitted (Figure 1C; HOLLICK et al. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.