Abstract
Within the MADS-box gene family, the AGAMOUS-subfamily genes are particularly important for plant reproduction, because they control stamen and carpel identity. A number of studies in the last three decades have demonstrated that the AGAMOUS (AG) function has been conserved during land plant evolution. However, gene duplication events have led to subfunctionalization and neofunctionalization of AG-like genes in many species. Here we show that alternative splicing in Oryza sativa produces two variants of the AG ortholog OsMADS3 which differ in just one serine residue, S109. Interestingly, this alternative splicing variant is conserved and specific to the grass family. Since in eudicots the S109 residue is absent in AG proteins, stamen and carpel identity determination activity of the two rice isoforms was tested in Arabidopsis thaliana. These experiments revealed that only the eudicot-like OsMADS3 isoform, lacking the serine residue, had ability to specify stamens and carpels in ag mutant flowers, suggesting an important functional role for the serine residue at position 109 in AG proteins of grasses.
Highlights
The evolution of flowers as reproductive structures is the most remarkable feature of angiosperms, which currently are by far the most dominating group of plants on Earth, counting over 350,000 species1
Since the length of the N terminal region of AG lineage proteins has only been experimentally determined for Arabidopsis AtAG (Riechmann et al, 1999), we refer to this serine amino acidic position as S109, counting from the more conserved methionine residue just preceding the MADS domain (M1, Figure 1C)
We tested the interaction of AtAG, OsMADS3+S109 and OsMADS3 with Arabidopsis amplified using the primers SEP1f/SEP1r (AtSEP1), AtSEP2 and AtSEP3 proteins using the yeast-2-hybrid assay (Table 2 and Figure 7)
Summary
The evolution of flowers as reproductive structures is the most remarkable feature of angiosperms (flowering plants), which currently are by far the most dominating group of plants on Earth, counting over 350,000 species. The flower is a bisexual structure composed of perianth organs surrounding the reproductive organs, the male stamens and the female gynoecium, which are located in the center of the flower. All organs are arranged either in a spiral phyllotaxy or in whorls, depending on the angiosperm lineage. The gynoecium can be composed of one or more pistils made by one carpel each, or by one pistil formed by multiple fused carpels. The pistil consists of a stigma, style and ovary which contains the ovules. The pistil upon pollination eventually becomes a fruit, and the ovules develop into seeds (Bowman, 1997)
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