75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

147 Systemic signal transduction of flowering and protein trafficking of FT
Megumi Takahashi, Eiko Himi, Koji Goto
Research Inst. Bio Scineces, Okayama
Arabidopsis TERMINAL FLOWER 1 (TFL1) and FLOWERING LOCUS T (FT) are homologous proteins, that is
54% of amino residues are conserved in the whole region. However, the developmental role of these proteins are opposite;
while tfl1 mutants show early flowering, ft mutants show late flowering. In addition to the early flowering phenotype,
tfl1 shows terminal flower, suggesting that TFL1 maintains inflorescence meristem (IM) and represses floral meristem
(FM) formation. Since coordinate differentiation of three layers are essential for the floral primordia formation, TFL1
function is supposed to be required in the whole region of IM, but its transcript was localized only inner region of L3
layer of IM. We have revealed non-cell-autonomous function of TFL1 is conferred by its protein trafficking in the shoot
apical meristem (SAM).
We have also found that FT moves among cell layers of IM. The endogenous FT is, however, expressed in the
vascular bundle of leaf apical region, and recent study revealed that FT interacts with FD and functions in the SAM.
Taken together, the following scenario can be deduced; after long distance travel from leaves via vascular bundle, FT is
unloaded near the SAM (vascular bundle is undifferentiated in the SAM) and then FT protein moves to spread into the
SAM and interacts with FD to activate target genes.
Another recent study suggests that FT RNA moves from the leaf to the SAM. To confirm this, we have developed
grafting system using tobacco. As expected, transgenic tobacco carrying Arabidopsis FT gene showed early flowering,
we used it as a rootstock and wild type scion was grafted. We found the scions acquired early flowering phenotype. Now
we are investigating FT RNA and/or protein are transmitted from rootstock to scion by using modified FT genes.
148 Deletion of core components of the plastid protein import machinery causes differential
arrest of embryo development in Arabidopsis thaliana
Bianca Hust, Michael Gutensohn
Institute of Plant Physiology, Martin-Luther-University Halle-Wittenberg, Germany
Among the genes that have recently been pinpointed to be essential for plant embryo development a large number
encodes plastid proteins suggesting that embryogenesis is linked to plastid localized processes. However, nuclear
encoded plastid proteins are synthesized as precursors in the cytosol and subsequently have to be transported across the
plastid envelopes by a complex import machinery. We supposed that deletion of components of this machinery should
allow a more general assessment of the role of plastids in embryogenesis since it will not only affect single proteins but
instead inhibit the accumulation of most plastid proteins. Therefore we have characterized three Arabidopsis thaliana
mutants lacking core components of the Toc complex, the protein translocase in the outer plastid envelope membrane,
which indeed show embryo lethal phenotypes. Remarkably, embryo development in the atToc75-III mutant, lacking the
pore forming component of the translocase, was arrested extremely early at the two-cell stage. In contrast, despite the
complete or almost complete lack of the import receptors Toc34 and Toc159, embryo development in the atToc33/34
and atToc132/159 mutants proceeded slowly and was arrested later at the transition to the globular and the heart stage,
respectively. These data demonstrate a strict dependence of cell division and embryo development on functional plastids
as well as specific functions of plastids at different stages of embryogenesis. In addition, our analyses suggest that not
all components of the translocase are equally essential for plastid protein import in vivo.