THE PLANT CELL, Vol 3, Issue 6 593-604, Copyright © 1991 by American Society of Plant Biologists
Plasmodesmatal Function Is Probed Using Transgenic Tobacco Plants That Express a Virus Movement Protein
S. Wolf, C. M. Deom, R. Beachy and W. J. Lucas
Botany Department, University of California, Davis, California 95616-8537
A gene encoding a temperature-sensitive mutant (MPP154A) of the
30-kilodalton movement protein (MP) of tobacco mosaic virus (TMV) was
transformed into Nicotiana tabacum cv Xanthi. Transgenic plants expressing
the MPP154A gene complemented local and systemic movement of an
MP-defective mutant of TMV (U3/12MPfs) at the permissive temperature of
24[deg]C but not at 32[deg]C, the nonpermissive temperature. A
microinjection procedure was used to investigate the effects of the
modified TMV MP on plasmodesmatal size-exclusion limits. Movement of
fluorescein isothiocyanate-labeled dextran (F-dextran), with an average
molecular mass of 9.4 kilodaltons, was detected between leaf mesophyll
cells of the transgenic plants at 24[deg]C; however, no movement of either
3.9-kilodalton or 9.4-kilodalton F-dextrans was detected when the
transgenic plants were held for 6 hours (or longer) at 32[deg]C. When these
plants were shifted back to 24[deg]C for 6 hours, cell-to-cell movement of
the F-dextrans was again observed. Accumulation of MPP154A was not affected
by the temperature regime, nor was the subcellular distribution of the MP
altered. These results are consistent with a change in the protein
conformation of MPP154A at the nonpermissive temperature, which gives rise
to a protein that fails to modify the molecular size-exclusion limits of
plasmodesmata to the same extent as wild-type MP. Surprisingly, at
32[deg]C, movement of the F-dextrans was inhibited in transgenic plants
expressing the wild-type MP gene; however, the inhibition was transient and
was no longer detected after 48 hours at this elevated temperature. This
transient inhibition of plasmodesmatal function was alleviated with
Sirofluor, an inhibitor of callose ([1->3]-[beta]-D-glucan) synthesis.
This result provides experimental evidence that callose deposition is
involved in regulating the molecular size-exclusion limit of plasmodesmata
in plants. Sirofluor had no effect on the inhibition of F-dextran movement
at 32[deg]C in plants expressing the MPP154A gene, indicating that callose
formation was not responsible for the failure of the temperature-sensitive
mutant protein to alter the size-exclusion limit of plasmodesmata.
