The "Specific Tissue" (ST) are proteins of unknown function present only in some plant families, mainly Fabaceae and
Asteraceae. They are included in the PF10950 protein family and characterized by the presence of at least one domain of
unknown function (DUF)2775. In this work we studied the involvement of the six members of the Medicago truncatula ST
family (ST1 to ST6) in the development of flowers, fruits, and seeds by analysing the activity of their promoters (pST) after
the construction of M. truncatula transgenic plants expressing the b-glucuronidase (GUS) reporter gene under the control
of the six pSTs. The GUS activity was analysed in whole flowers and fruits and also in histological sections of these organs.
The pST expression in the reproductive organs was mainly associated with the vascular bundles, especially throughout
fruit development. These results pointed to an important role of ST proteins during the reproductive development stage,
related to nutrient mobilization during the fruit and seed formation, that could be facilitated by their presence in the pod
vascular bundles, as well as in the connective tissue of the anthers (ST3, ST4, ST6), in the placenta, the funiculus, and
the outer parts of the developing seed (ST2, ST3, ST6). The observations made in this study were in agreement with the
functions previously established for the three groups of M. truncatula ST proteins, as in the proposed function for ST1 in
the transport and assimilation of nutrients, or the involvement of ST4, ST5, and ST6 in floral defence.
