MADS-box gene evolution beyond flowers: expression in pollen, endosperm, guard cells, roots and trichomes

MADS-box genes encode transcriptional regulators involved in diverse aspect s of plant development. Here we describe the cloning and mRNA spatio-tempor al expression patterns of five new MADS-box genes from Arabidopsis: AGL16, AGL18, AGL19, AGL27 and AGL31. These genes will probably become important m olecular tools for both evolutionary and functional analyses of vegetative structures. We mapped our data and previous expression patterns onto a new MADS-box phylogeny. These analyses suggest that the evolution of the MADS-b ox family has involved a rapid and simultaneous functional diversification in vegetative as well as reproductive structures. The hypothetical ancestra l genes had broader expression patterns than more derived ones, which have been co-opted for putative specialized functions as suggested by their expr ession patterns. AGL27 and AGL31, which are closely related to the recently described flowering-time gene FLC (previously AGL25), are expressed in mos t plant tissues. AGL19 is specifically expressed in the outer layers of the root meristem (lateral root cap and epidermis) and in the central cylinder cells of mature roots. AGL18, which is most similar in sequence to the emb ryo-expressed AGL15 gene, is expressed in the endosperm and in developing m ale and female gametophytes, suggesting a role for AGL18 that is distinct f rom previously characterized MADS-box genes. Finally, AGL16 RNA accumulates in leaf guard cells and trichomes. Our new phylogeny reveals seven new mon ophyletic clades of MADS-box sequences not specific to flowers, suggesting that complex regulatory networks involving several MADS-box genes, similar to those that control flower development, underlie development of vegetativ e structures.