Is there a role for trihelix transcription factors in embryo maturation?

Abstract

The development of the angiosperm seed includes the accumulation of storage products, the loss of most of its water and the establishment of dormancy. While much is known about the pathways that initiate maturation during mid-embryogenesis or repress it after germination, only recently has it been shown that other mechanisms repress the program during early embryogenesis.Two recent reports have shown that microRNAs are critical regulators of maturation in Arabidopsis early embryogenesis. Two closely related trihelix transcription factors, ASIL1 and ASIL2, were identified as probable partially redundant repressors of early maturation downstream of the microRNA-synthesizing enzyme DICER-LIKE1. An overlap between the genes upregulated in asil1-1 and dcl1-15 mutants support this conclusion. ASIL2 orthologs are found across seed plants, indicating that their role in maturation might be conserved. ASIL1 arose from the ancestral ASIL2 clade by a gene duplication event in the Brassicaceae, although it is not clear whether its function has diverged.

Figure 1. Comparison of microarray data for dcl1-15 and asil1-1. The overlaps were generated using Partek Genomics Suite 6.5. The statistical test used to show significance of the overlap was a Fisher’s exact test.

Figure 2. Neighbor-joining phylogenetic tree for the ASIL1/2 and At3g24490 clades. Full-length protein sequences were aligned using ClustalW, and the tree was built using MEGA4. For each protein, the first number is the one assigned by the PlantTFDB, while the second number is the one assigned by the corresponding sequencing project. Arabidopsis genes are highlighted in gray boxes. Species are identified as follow: Aan, Artemisia annua; Ahy, Arachis hypogaea; Ath, Arabidopsis thaliana; Aly, Arabidopsis lyrata; Bdi, Brachypodium distachyon; Bna, Brassica napus; Bra, Brassica rapa; Cpa, Carica papaya; Csg, Cucumis sativus; Csi, Citrus sinensis; Ghi, Gossypium hirsutum; Ppe, Prunus persica; Lja, Lotus japonicas; Mes, Manihot esculenta; Mtr, Medicago truncatula; Osj, Oryza sativa subsp Japonica; Pta, Pinus taeda; Pgl, Picea glauca; Pth, Populus trichocarpa; Sbi, Sorghum bicolor; Sly, Solanum lycopersicon; Smo, Selaginella moellendorffii; Stu, Solanum tuberosum; Vvi, Vitis vinifera; Zma, Zea mays.

Figure 3. Alignment of the trihelix domains of selected proteins of the ASIL1/2 clade. The trihelix domains were aligned using ClustalW. The position of the helices is based on. The numbers identifying the proteins correspond to those on Figure 2. Differences present in only one lineage (dicot, rice or pine) are boxed in red. Arabidopsis genes are highlighted in gray boxes.