Differential expression of the Arabidopsis genes coding for Em-like proteins1

Introduction Seed maturation is characterized by a desiccation process. Late embryogenesis abundant ( lea) genes are a large During desiccation, a number of specific proteins referred and diverse group of genes highly expressed during to as LEA (Late Embryogenesis Abundant proteins) late stages of seed development. Five major groups of accumulate in the embryo (Baker et al., 1988; Hughes LEA proteins have been described. Two Em genes and Galau, 1989). According to their accumulation pat(group I lea genes) are present in the genome of tern and physico-chemical proprieties, it has been sugArabidopsis thaliana L., AtEm1 and AtEm6. Both genes gested that LEA proteins could be involved in seed encode for very similar proteins which differ basically desiccation tolerance (Dure et al., 1989; Hughes and in the number of repetitions of a highly hydrophilic Galau, 1989; Dure 1993; Leprince et al., 1993; Xu et al., amino acid motif. The spatial patterns of expression 1996; Kermode, 1997). LEA proteins have a widespread of the two Arabidopsis Em genes have been studied distribution among plant species. Five major groups of using in situ hybridization and transgenic plants transLEA proteins have been described on the basis of their formed with the promoters of the genes fused to the amino acid sequence homologies (Dure et al., 1989; b-glucuronidase reporter gene (uidA). In the embryo, Delseny et al., 1993). Expression of many lea genes can AtEm1 is preferentially expressed in the pro-vascular be precociously induced in immature seeds or in vegetative tissues and in meristems. In contrast, AtEm6 is tissues upon ABA treatment or by osmotic or waterexpressed throughout the embryo. The activity of both deficit stress (Skriver and Mundy, 1990; Jakobsen et al., promoters disappears rapidly after germination, but is 1994). ABA-inducible in roots of young seedlings, although in The first LEA protein was identified in wheat, and different cells: the AtEm1 promoter is active in the corresponded to the so-called Em protein (Cuming and internal tissues (vasculature and pericycle) whereas Lane, 1979). Since then, several Em-like coding genes the AtEm6 promoter is active in the external tissues have been isolated and characterized in many monocot (cortex, epidermis and root hairs). The AtEm1 proand dicot species (Vicient et al., 1998), all of them moter, but not AtEm6, is also active in mature pollen encoding proteins having the highly conserved hydrophilic grains and collapsed nectaries of young siliques. 20 amino acid motif (Dure et al., 1989). In some species, These data indicate that the two Em proteins could the Em genes are encoded by multigene families, somecarry out at least slightly different functions and that times encoding proteins with different numbers of the 20 the expression of AtEm1 and AtEm6 is controlled at, amino acid motif arranged in tandem, as for example in at least, three different levels: temporal, spatial and cotton (Galau et al., 1992), maize ( Williams and Tsang, hormonal (ABA). 1992), barley (Espelund et al., 1992; Stacy et al., 1995), Arabidopsis (Gaubier et al., 1993), mung bean (Manickam et al., 1996) or soybean (Calvo et al., 1997). Em genes