Abstract
Endosperm and embryo development are coordinated via epigenetic regulation and signaling between these tissues. In maize (Zea mays), the endosperm-embryo signals are not known, but endosperm cellularization is a key event for embryos to form shoots and roots. We screened seed mutants for nonautonomous functions in endosperm and embryo development with genetically nonconcordant seeds and identified the recessive mutant rough endosperm3 (rgh3). The wild-type Rgh3 allele is required in the endosperm for embryos to develop and has an autonomous role in embryo and seedling development. Endosperm cell differentiation is defective in rgh3. Results from endosperm cell culture indicate that rgh3 mutants remain in a proliferative state through mid-seed development. Rgh3 encodes the maize U2AF(35) Related Protein (URP), an RNA splicing factor involved in both U2 and U12 splicing. The Rgh3 allele produces at least 19 alternative splice variants with only one isoform encoding a full-length ortholog to URP. The full-length RGH3α isoform localizes to the nucleolus and displays a speckled pattern within the nucleoplasm, and RGH3α colocalizes with U2AF(65). A survey of alternatively spliced transcripts found that, in the rgh3 mutant, a fraction of noncanonical splicing events are altered. Our findings suggest that differentiation of maize endosperm cell types is necessary for embryos to develop. The molecular cloning of Rgh3 suggests that alternative RNA splicing is needed for cell differentiation, development, and plant viability.
Highlights
Angiosperm seeds develop an embryo and endosperm from double fertilization of the egg and central cell of the megagametophyte (De Smet et al, 2010; Linkies et al, 2010)
Endosperm-to-embryo signaling is supported by two endosperm-specific genes, ABNORMAL LEAF SHAPE1 (ALE1) and RETARDED GROWTH OF EMBRYO1/ZHOUPI (RGE1), which are required for embryos to develop a cuticle and separate from the endosperm (Tanaka et al, 2001; Kondou et al, 2008; Yang et al, 2008)
In mild rgh3 kernels, the aleurone marker labeled only the abgerminal edge of the basal endosperm transfer cell layer (BETL) in a pattern similar to normal kernels except that the labeling was more intense (Figure 6O). These results suggest that aleurone cells are developmentally delayed in mild rgh3 mutants and that the BETL, embryo surrounding region (ESR), and other endosperm cell types are incorrectly specified when the kernel phenotype is severe
Summary
Angiosperm seeds develop an embryo and endosperm from double fertilization of the egg and central cell of the megagametophyte (De Smet et al, 2010; Linkies et al, 2010). We surveyed published primary data for B-A translocation phenotypes and found the vast majority of dek mutants show developmental autonomy (i.e., developmental patterning of the tissue is in accordance to the genotype) (see Supplemental Table 1 online) In these studies, two types of nonautonomous developmental interactions were observed. Globby (glo1), discolored (dsc1), or dek mutant endosperm inhibits wild-type embryo development (Chang and Neuffer, 1994; Scanlon and Myers, 1998; Costa et al, 2003). We show Rgh is required in the endosperm to switch from cellular proliferation to differentiation These data suggest a unique mechanism by which endosperm cell differentiation influences embryo patterning and development. Molecular cloning of Rgh implicates alternative RNA splicing as an essential process to endosperm cell differentiation as well as embryo and plant development
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