Differential expression of the Arabidopsis genes coding for Em-like proteins

Late embryogenesis abundant (lea) genes are a large and diverse group of genes highly expressed during late stages of seed development. Five major groups of LEA proteins have been described. Two Em genes (group I lea genes) are present in the genome of Arabidopsis thaliana L., AtEm1 and AtEm6. Both genes encode for very similar proteins which differ basically in the number of repetitions of a highly hydrophilic amino acid motif. The spatial patterns of expression of the two Arabidopsis Em genes have been studied using in situ hybridization and transgenic plants transformed with the promoters of the genes fused to the beta-glucuronidase reporter gene (uidA). In the embryo, AtEm1 is preferentially expressed in the pro-vascular tissues and in meristems. In contrast, AtEm6 is expressed throughout the embryo. The activity of both promoters disappears rapidly after germination, but is ABA-inducible in roots of young seedlings, although in different cells: the AtEm1 promoter is active in the internal tissues (vasculature and pericycle) whereas the AtEm6 promoter is active in the external tissues (cortex, epidermis and root hairs). The AtEm1 promoter, but not AtEm6, is also active in mature pollen grains and collapsed nectaries of young siliques. These data indicate that the two Em proteins could carry out at least slightly different functions and that the expression of AtEm1 and AtEm6 is controlled at, at least, three different levels: temporal, spatial and hormonal (ABA).