Kneen, B. E. and T. A. EaRue
Boyce Thompson Institute, Ithaca, NY 14853 USA
The availability of both bacterial and plant mutants defective in
nodulation and fixation is essential to studying the molecular basis of
nitrogen fixing symbioses. To obtain host mutants, seeds of Pisum sativum
cv 'Sparkle' were treated with gamma radiation or EMS. M2 populations were
screened with Rhizobium leguminosarum strain 128C53 for nodulation defec-
tive plants, and selections which were stable and fertile to M3 are being
characterized. We found well nodulated Ineffective mutants which have
little or no nitrogenase (C2H2 ) activity. There are 29 selections with few
or no nodules; four of these have shortened roots and/or shoots. F1 and F2
progeny from reciprocal crosses among mutants are being examined to deter-
mine allelism.
Six mutants, five obtained by EMS and one by gamma radiation, are allelic
with the sym-5 gene of our first reported non-nod mutant (2). The reason
for the high frequency of mutation at this locus is unknown.
Three new non-nodulating lines have single gene recessive mutations at
unique loci. All the F1 plants from crosses between the mutants and
Sparkle or 'Rondo' nodulate, and segregation of the progenies ap-
proximates 3:1 non:non-nod. Crosses among selections E69, R2 5, R72, and
sym-2 ('Afghanistan' [3]) and sym-5 lines yield F1 nodulating plants. The
new loci are designated sym-7, sym-8, and sym-9. Two mutants, one from EMS
and one from gamma radiation, are allelic at sym-8. None of these mutants
exhibits strain specificity in infectivity tests with nine strains of _R.
leguminosarum including four Middle Eastern strains. The non-nodulating
phenotypes were not temperature dependent in an experiment comparing plants
grown at a 2 5C/20C day/night regime or plants whose roots were kept at 9C
with plants grown at the normal 20C/15C regime.
Two mutants with few nodules and abnormal roots are monogenic reces-
sives, based on crosses with Sparkle or 'Trapper'. R50 has shortened
internodes and shortened lower lateral roots. The leaves are pale with
raised veins. Very few (0-5) white nodules are found near the crown of the
roots. E151 appears normal in shoot growth, but the lower lateral roots
are variable in length, averaging approximately one-half that of Sparkle.
The number of pale pink or white nodules varies (0-30). These low nodula-
tion phenotypes are always associated with shortened roots in segregating
F2 populations.
Jacobsen (I) found two nodulation defective EMS-induced mutants of
Rondo: K5 and K24.We are crossing these selections with our mutant lines
to determine if they are allelic.
1. Jacobsen, E. 1984. Plant and Soil 82:427-438.
2. Kneen, B. E. 1984. J. Heredity 75:238-240.
3. Kneen, B. E., T. A. LaRue, and N. Weeden. 1984. PNL 16:31-34.