| The Arabidopsis thaliana genome encodes 29 AT-hook Motif Nuclear Localized (AHL) genes, which evolved into two phylogenic clades. The AHL proteins contain one or two AT-hook motif(s) and one PPC/DUF296 domain. Seedlings lacking both SOB3/AHL29 and ESC/AHL27 confer a subtle long-hypocotyl phenotype when compared to the wild type or either single-null mutant. In contrast, the missense allele sob3-6 confers a dramatic long-hypocotyl phenotype in the light. In this study, we examined the dominant-negative feature of sob3-6 and found that it encodes a protein with a disrupted AT-hook motif that abolishes binding to AT-rich DNA. A loss-of-function approach demonstrated different, yet redundant, contributions of additional AHL genes in suppressing hypocotyl elongation in the light. We showed that AHL proteins interact with each other and themselves via the PPC/DUF296 domain. AHLs also share interactions with other nuclear proteins, such as transcription factors, suggesting that these interactions also contribute to the functional redundancy within this gene family. The coordinated action of AHLs requires an AT-hook motif capable of binding AT-rich DNA, as well as a PPC/DUF296 domain containing a conserved Gly-Arg-Phe-Glu-Ile-Leu region. Alteration of this region abolished SOB3/AHL29's physical interaction with transcription factors and resulted in a dominant-negative allele in planta that was phenotypically similar to sob3-6. We propose a molecular model where AHLs interact with each other and themselves, as well as other nuclear proteins, to form complexes in order to modulate plant growth and development. We also identified AHL genes from a variety of land plants, ranging from the moss Physcomitrella patens and lycophyte Selaginella to a variety of monocot and dicot flowering plants. Alignment of the amino acid sequences from the identified AHL proteins revealed three types of AHLs (Type-I, -II and -III), classified based on the combinations of the number and type of AT-hook motif(s) as well as the type of PPC/DUF296 domain. With these results, we also suggest an evolutionary scenario for the AHL gene family in land plants. |
