Organ accumulation and subcellular location of Cicer arietinum ST1 protein

Three members of Medicago truncatula ST family (MtST4, MtST5 and MtST6) are specifically induced by hormones involved in biotic interactions

In this work, we study the function of the Medicago truncatula ST4, ST5 and ST6 proteins that belong to a protein family of unknown function characterized by the DUF2775 domain. Thus, we analyse their promoter sequence and activity, their transcript accumulation, and their subcellular location. The analysis of the three promoters showed different combination of cis-acting regulatory elements and they presented different activity pattern. Throughout development only ST6 mRNAs have been detected in most of the stages analysed, while ST4 was faintly detected in the roots and in the flowers and ST5 was always absent. The addition of MeJA, ET and SA revealed specific responses of the STs, the ST4 transcript accumulation increased by MeJA; the ST5 by MeJA and ET when applied together; and the ST6 by ET and by SA. Finally, the ST4 and ST5 proteins were in the cell wall whereas the ST6 had a dual location. From these results, we can conclude that the ST4, ST5 and ST6 RNAs are specifically and differentially up-regulated by MeJA, ET and SA, plant regulators also involved in the plant defence, pointing that ST4, ST5 and ST6 proteins might be involved in specific biotic interactions through different signalling pathways.

Three members of Medicago truncatula ST family are ubiquitous during development and modulated by nutritional status (MtST1) and dehydration (MtST2 and MtST3)

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.