Background: ShooT specific/Specific Tissue (ST) belong to a protein family of unknown function characterized by
the DUF2775 domain and produced in specific taxonomic plant families, mainly Fabaceae and Asteraceae, with the
Medicago truncatula ST family being the largest. The putative roles proposed for this family are cell elongation, biotic
interactions, abiotic stress and N reserve. The aim of this work was to go deeper into the role of three M. truncatula
ST proteins, namely ST1, ST2 and ST3. Our starting hypothesis was that each member of the family could perform a
specific role, and hence, each ST gene would be subjected to a different type of regulation.
Results: The search for cis-acting regulatory elements (CREs) in silico in pST1, pST2 and pST3 promoters showed
prevalence of tissue/organ specific motifs, especially root- and seed-specific ones. Light, hormone, biotic and abiotic
related motifs were also present. None of these pSTs showed the same combination of CREs, or presented the same
activity pattern. In general, pST activity was associated with the vascular cylinder, mainly in roots. Promoter activation
was highly specific and dissimilar during reproductive development. The ST1, ST2 and ST3 transcripts accumulated in
most of the organs and developmental stages analysed - decreasing with age - and expression was higher in the roots
than in the aerial parts and more abundant in light-grown plants. The effect of the different treatments on transcript
accumulation indicated that ST1 behaved differently from ST2 and ST3, mainly in response to several hormones and
dehydration treatments (NaCl or mannitol), upon which ST1 transcript levels decreased and ST2 and ST3 levels increased.
Finally, the ST1 protein was located in the cell wall whereas ST2 and ST3 were present both in the cytoplasm and in the
cell wall.
Conclusions: The ST proteins studied are ubiquitous proteins that could perform distinct/complementary roles in plant
biology as they are encoded by differentially regulated genes. Based on these differences we have established two
functional groups among the three STs. ST1 would participate in processes affected by nutritional status, while ST2 and
ST3 seem to act when plants are challenged with abiotic stresses related to water stress and in physiologically controlled
desiccation processes such as the seed maturation.
