The molecular bases of the tissue culture – tamarillo (Solanum
betaceum) as a case study
Jorge M. Canhoto & Sandra I. Correia
Centre for Functional Ecology, Department of Life Sciences, University of Coimbra,
Calçada Martim de Freitas, 3000-456 Coimbra, Portugal. jorgecan@ci.uc.pt
Plant tissue culture is a set of techniques with a wide range of applications both in
plant cloning and plant development analysis. Protocols for cloning thousands of
species have been developed using axillary shoot proliferation, organogenesis or
somatic embryogenesis. Moreover, tissue culture is of major importance for plant
conservation, production of secondary metabolites and plant genetic transformation.
In spite of the important achievements made on tissue culture in the last years, the
understanding of the molecular events underlying the processes of dedifferentiation
and acquisition of totipotency remain largely unclear. It is true that genes directly
involved on plant regeneration, mainly on somatic embryogenesis induction, have
been found and characterized, such as a SERK and LEC1 among some others.
However, this was achieved mainly for a few model species including Arabidopsis
and carrot. During the last years we have been working in plant tissue culture of
several species, from annuals to trees and from ferns to angiosperms. From these
species we have found that tamarillo (Solanum betaceum, syn. Cyphomandra
betacea), a tree of the solanaceous family has particular features that make it an
interesting system for plant tissue culture and molecular analysis. Tamarillo can be
regenerated through axillary shoot proliferation, organogenesis and somatic
embryogenesis. Protocols for protoplast isolation and culture have been developed
and a method for genetic transformation was also established. Somatic embryogenesis
in this species is particularly interesting since embryogenic and non-embryogenic
cells can be obtained from the same explant (leaves or zygotic embryos and in the
same culture conditions: high sucrose levels and an auxin such as 2,4-D or picloram.
On these conditions, embryogenic and non-embryogenic calli can be separated from
each other and propagated at will to obtain large amounts of embryogeneic and nonembryogenic cells showing the same genetic background but a different
developmental potential. Based on this system proteomic and transcriptomic
approaches have been used trying to identify proteins and transcripts related with
somatic embryogenesis induction. The results have shown that a putative tRNA/rRNA
methyltransferase (NEP-TC 26.5 kDa, GenBank accession number JQ766254) with a
high degree of homology with the Arabidopsis thaliana tRNA/rRNA
methyltransferase (SpoU) family is mainly expressed on non-embryogenic calluses.
NEP-TC is a cytosolic protein mainly expressed around the nucleus of nonembryogenic cells and NEP-TC orthologous in Arabidopsis showed a reduced
expression in embryos when compared with leaves or callus. Besides, nonembryogenic samples with high NEP-TC expression levels show enhanced
methyltransferase activity. The data so far obtained point to an inhibitory role of
NEP-TC on somatic embryogenesis induction contrasting with the promoter effect of
other genes already identified on somatic embryo formation.