1. Exploitation of phloem transcriptome data
1-1. Phloem transcriptome, in celery, in standard growth conditions. An initial study was performed on celery because in this species the phloem can be readily separated from other tissues by peeling. This enabled us to describe the transcription profile of a subset of 989 sequences, out of which 73 displayed preferential expressions in the vascular tissues (Vilaine et al., 2003). Some major classes of mRNAs found in this tissue encode proteins related to stress responses and degradation or turnover of proteins. These observations stress out possible mechanisms for a role of phloem in systemic responses.
1-2. Phloem transcriptome, in celery, in response to stresses. Genes involved in phloem signalling during the response to stress, were identified using a similar transcriptome analysis of the phloem, in various environmental conditions, including infestation by aphids, infection by virus and nitrogen starvation. For example, in celery, (Fanchon Divol PhD thesis), we established that the plant sets up a complex systemic response in the phloem, in response to an aphid infestation (Divol et al., 2005). Subsequent studies on selected genes in celery or on the closest relative putative orthologues on Arabidopsis provided further information on their role in plant-aphid interaction, such as a role in cell wall modifications involved in plant defence mechanisms (Divol et al., 2007).

2. Functional analysis of phloem genes
Orphan genes, i.e. genes of unknown function, represent nearly 2/3 of the total genes found to be expressed in the phloem cells. They comprise in particular a large number of genes that appear to be unique to plant kingdom or that have largely diverged from their counterpart in other organisms. Breakthroughs in the understanding of phloem processes are expected from the functional analysis of genes presenting highly specific patterns of expression, in particular those having similar tissue specificity in unrelated species. Based on these data we focus on the analysis of orphan genes encoding two main classes, i.e. putative trans-membrane proteins and structural proteins.
2.1 Phloem structural proteins. In the phloem, the sieve elements represent a unique case of specialization. Their differentiation and the acquisition of functionality are associated with phloem-specific proteins (P-proteins) that occur in a variety of forms including filaments, tubules, and crystalline aggregates. One major constituent of P-proteins is the phloem lectin PP2 (PP2), which presents also an RNA binding property, suggesting that it has a role in the regulation of long distance translocation of macromolecules and in signalling. Indirect evidence supports the hypothesis that PP2s polymerize in vivo, property that is supposed to be related to macromolecular trafficking. We established that the two main genes of the family, PP2-A1 and PP2-A2, are specifically expressed in the sieve element-companion cell-complexes (Dinant et al., 2003). Further experiments on the function and the structure of these genes are ongoing. The objectives are to characterize the properties of PP2 polymerization, their role in regulating phloem sap transport and their function in macromolecule long distance trafficking. This project encompasses a study of the self-assembly of PP2 as well as a functional analysis of PP2 genes (link with PhD project).
2.2. Putative membrane proteins. A large subset of genes determined as preferentially expressed in the phloem encode putative membrane proteins, the functions of which are unknown. The conservation of the tissue specificity in unrelated species (celery, poplar, Arabidopsis) suggests a central function in processes that are unique to phloem tissues. From this subset of genes, we selected a subset encoding distinct trans-membrane protein. The functional characterization of these genes is ongoing. A combination of physiological, cytological and genetic analysis should give us evidence on their possible involvement in signalling, transport or phloem homeostasis.
To pinpoint the genes intervening in the adaptation of the plants to the stress biotic, an analysis of the transcriptome of the phloem was equally undertaken on plants grown after a massive infestation by the phloem feeders aphids: Myzus persicae. This study allowed the identification of many genes deregulated in response to aphid attack (Divol et al., 2005).
The comparison of the expression profiles of the genes identified on celery as phloem- specific and genes phylogenetically closely related and found on other plant species, such as Arabidopsis thaliana or Poplar, is on going in order to determine what is the impact of evolution on the conservation of tissue specific regulations of the gene expression.