Annual Report 2006

phosphatase activity by SIPK depended
partially on the putative common docking
domain of SIPK. On the other hand, conversion
of Lys41 and Arg43 of NtMKP1 to Ala (K41A/R
43A) abolished the interaction with SIPK.
Expression of constitutively active MAPK
kinase in induced
activation of SIPK and cell death. Simultaneous
expression of either NtMKP1 (Fig. 6) or NtMKP
1 L443R, which was unable to bind CaM,
compromised the constitutively active MAPK
kinase-induced responses, whereas that of
NtMKP1K41A/R43A did not. These results
indicate that the regulation of NtMKP1 activity
by SIPK binding, but not by CaM binding, is
important for the function of NtMKP1.
that the nodule infected cells were
significantly smaller than those of wild type
plants, contained enlarged symbiosomes with
multiple bacteroids, and underwent deterioration
of the symbiosomes prematurely as well as
disintegration of the whole infected cell
cytoplasm (Fig. 7C and D). These results
indicate that the ineffectiveness of the
nodules is primarily due to impaired
growth of infected cells accompanied with the
premature senescence induced at early stages
of nodule development. We delimited the
locus in a few hundred kb region on
the upper portion of chromosome 4, and are in
progress towards molecular identification of the
gene.
Symbiotic nitrogen fixation
A novel Fix- symbiotic mutant of
shows impaired development and
premature deterioration of nodule
infected cells and symbiosomes
Nitrogen-fixing symbiosis between legume
plants and rhizobia is established through
complex interactions between two symbiotic
partners. To identify the host legume genes
that play crucial roles in such interactions, we
isolated a novel Fix- mutant from a
model legume MG-20 by
somaclonal mutation through extensive culture
of suspension cells followed by regeneration of
the plant. The mutants displayed
nitrogen deficiency symptoms after inoculation
with under nitrogen free
conditions, but their growth recovered when
supplied with nitrogen-rich nutrients (Fig. 7A).
The mutant formed an increased
number of small and pale-pink nodules (Fig. 7B).
Nitrogenase (acetylene reduction) activity per
nodule fresh weight was low but retained more
than 50% of that of the wild type nodules. Light
and electron microscopic observations revealed
Fig. 7
(A, B) Growth and nodulation phenotypes of wild type
MG-20 (left) and (right) plants inoculated
with Bars=10mm(A)and4mm(B).
(C, D) Micrographs of nodule infected cells of wild type
MG-20 (C) and (D). Wild-type nodule
infected cells are densely packed with bacteroids,
whereas infected cells show less-dense and
aggregated bacteroids (arrow). U, uninfected cells; V,
vacuoles. Bars = 10 µm.