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We analyzed the Arabidopsis thaliana genome sequence to detect Late Embryogenesis Abundant (LEA) protein genes, using as reference sequences proteins related to LEAs previously described in cotton or which present similar characteristics. We selected 50 genes representing nine groups. Most of the encoded predicted proteins are small and contain repeated domains that are often specific to a unique LEA group. Comparison of these domains indicates that proteins with classical group 5 motifs are related to group 3 proteins and also gives information on the possible history of these repetitions. Chromosomal gene locations reveal that several LEA genes result from whole genome duplications (WGD) and that 14 are organized in direct tandem repeats. Expression of 45 of these genes was tested in different plant organs, as well as in response to ABA and in mutants (such as abi3, abi5, lec2 and fus3) altered in their response to ABA or in seed maturation. The results demonstrate that several so-called LEA genes are expressed in vegetative tissues in the absence of any abiotic stress, that LEA genes from the same group do not present identical expression profile and, finally, that regulation of LEA genes with apparently similar expression patterns does not systematically involve the same regulatory pathway.

The maize abscisic acid (ABA)-responsive gene rab28 has been shown to be ABA-inducible in embryos and vegetative tissues, expression being mostly restricted to vascular elements during late embryogenesis. In the course of an expressed sequence tags (ESTs) programme, we have isolated an Arabidopsis thaliana gene, Atrab28, encoding the orthologue of maize rab28. The Atrab28 cDNA is 1090 bp long, including a poly(A)+ stretch, and encodes a polypeptide of 262 amino acids. Atrab28 antibody against the recombinant protein recognizes a polipeptide of about 30 kDa and pI 6, in close agreement with the predicted molecular mass and pI. As for maize rab28, expression studies with Atrab28 revealed high specificity for embryo tissues, transcription being stimulated by the transcriptional activator abi3. In contrast, Atrab28 was not induced in vegetative tissues by ABA, osmotic stress or dehydration. The expression of Atrab28 mRNA and the accumulation of Atrab28 protein was largely restricted to provascular tissues of mature embryos and in the seed coat outer tegument and embryo and silique epidermis, as revealed by in situ hybridization and immunocytochemistry with anti-Atrab28 antibodies.

We have isolated and sequenced a 9.5 kb genomic region from A. thaliana, located on chromosome 2, which contains two tandemly arranged closely related genes (AtM10 and AtM17) coding for a new family of LEA proteins. The deduced proteins have a molecular mass of 11 and 29 kDa, respectively, are extremely hydrophilic except at their N-termini and share 70% amino acid (aa) identity. A 47 aa motif containing a 6-cysteine domain is present once in AtM10 and four times in AtM17. The short intergenic region, the identical position of the intron and the overall sequence homology suggest that these two genes evolved through a duplication event. This conclusion is supported by the presence of two homologous strictosidine synthase-like (pseudo)genes downstream from AtM17 and AtM10. Expression studies, using AtM10 and AtM17 cDNAs, revealed that both transcripts accumulate exclusively in seeds from late embryogenesis until two days after imbibition. Expression of both genes in young seedlings is repressed during ABA, salt or drought treatment, whereas a cold stress induces the expression of AtM17 only. In situ hybridization revealed that AtM10 transcripts are detected throughout the embryo while those of AtM17 are more localized to cotyledon cells.

Wheat germin is a protein expressed during germination which possesses an oxalate oxidase activity. Germin-type oxalate oxidases have been extensively studied in monocotyledons (wheat and barley) where they are thought to have important functions for development, stress response and defence against pathogens. In contrast, almost nothing is known about the germin-like proteins found in dicotyledons, gymnosperms and myxomycetes. In this work, cDNA clones for three genes (ATGER1, ATGER2 and ATGER3) encoding germin-like proteins, initially characterized as expressed sequence tags (ESTs), from Arabidopsis thaliana cDNA libraries were further characterized. In addition, we isolated and sequenced a Brassica napus cDNA which was strongly homologous to the cDNA for ATGER1. Sequence analysis and secondary structure predictions of the proteins encoded by these cDNAs showed that they possess all the characteristic features of members of the germin family and of the germin/seed globulins/sucrose binding protein superfamily. Sequence comparisons and mapping demonstrated the existence of at least two different gene families in the A. thaliana genome encoding a minimum of three genes for germins. These three genes have been mapped in three different location on the Arabidopsis genome. By northern blot hybridizations we found that these genes are differentially regulated. ATGER1 was expressed during germination, like wheat germin, but also in leaves whereas ATGER2 transcripts were exclusively found in developing embryos, like wheat pseudo-germin. ATGER3 mRNAs were found in leaves and flowers and their abundance was shown to vary during the circadian cycle.

In order to identify genes involved in cold acclimation, we have constructed a cDNA library from Brassica napus (cv. Samouraï) cold-acclimated etiolated seedlings. By differential screening, a cDNA clone named pBnC24 (Brassica napus Cold), corresponding to a new cold-inducible plant gene, was isolated. Northern blot hybridizations using total RNA from acclimated and unacclimated seedlings confirmed that BnC24 represents a cold-regulated gene. In contrast with a number of cold-inducible plant genes, BnC24 does not seem to be responsive to abscisic acid (ABA). In addition, further screening of the 'cold-acclimated' cDNA library using pBnC24 cDNA as a probe, allowed the isolation of a second type of homologous cDNA. Sequence analysis showed that the two BnC24 genes encode basic 24 kDa proteins, which are highly hydrophilic and rich in alanine, lysine and arginine. The nucleotide and deduced amino acid sequences of these clones do not show any homology with other previously described cold-induced plants genes. However they have strong homology with a recently discovered human tumour gene, bbcl (breast basic conserved), which seems to be highly conserved in eukaryotes.

In order to analyse the cruciferin gene family in radish a cDNA library was screened either with heterologous rapeseed probes or by differential screening and sequencing. We could identify six partial cDNA clones belonging to two different groups of cruciferin genes which do not cross-hybridize, and probably three distinct subfamilies. One of these classes corresponds to the previously described cruciferin from rapeseed and Arabidopsis. A gene corresponding to the second group, as well as its border sequences, was isolated from a radish genomic library and analysed in more detail. The cruciferin gene (cruRS) contains three introns and encodes a 479 amino acid protein. The transcription initiation site was determined. The expression of the different group of genes was studied by northern blot analysis: genes of both classes are expressed simultaneously and roughly at the same level between 25 and 35 days after flowering. Cruciferin gene copy number was estimated by Southern blot analysis. There appear to be seven or eight genes in one class and three in the other, located at different loci.