The Plant Vascular System: Evolution, Development and FunctionsF

Insights into Plant Vascular Biology 345
Figure 21. Delivery of phloem-mobile transcripts to sink tissues requires formation of stable ribonucleoprotein complexes.
(A) Model illustrating the protein composition of the pumpkin CmRBP50-based ribonucleoprotein (RNP) complex that binds specifically to
five phloem transcripts that encode transcription factors which are delivered into sink tissues (from Ham et al. 2009).
(B) Phosphorylation of four serine residues at the C-terminus of CmRBP50 is essential for assembly of a stabilized RNP complex (left image).
Mutating these serine residues prevents RNP complex assembly in the sieve tube system (right image) (from Li et al. 2011).
turn white (Palauqui et al. 1996). Subsequently, this process
moved up the body of the plant in a source-to-sink pattern
reflective of phloem translocation.
An insightful follow-up series of experiments revealed that a
graft-transmissible signal moved into the scion where it caused
the turnover of Nia transcripts. A deficiency in fixed nitrogen
then caused the leaves of the scion to turn white (Palauqui
et al. 1997) (Figure 22A). Cell-to-cell movement of the Nia
silencing signal was tested by placement of a WT stem seg-
ment between the silenced stock and the non-silenced scion.
That white leaves still developed in these scions confirmed
the involvement of the phloem (Figure 22B). This conclusion
was further supported by grafting Nia-silenced scions onto
non-silenced root stocks. As the direction of the phloem is
from the stock to the scion, this graft combination did not
result in the generation of white leaves (Figure 22C), again
consistent with transmission of the silencing signal through the
phloem.