FURTHER CHARACTERIZATION OF MUTANT nod-3, HIGHLY NODULATING IN THE PRESENCE OF NITRATE

PNL Volume 16 1984 RESEARCH REPORTS 23

FURTHER CHARACTERIZATION OF MUTANT nod-3, HIGHLY NODULATING IN THE

PRESENCE OF NITRATE

Jacobsen, E. Dept. of Genetics, University of Groningen

Haren , The Netherlands

In pea, nodulation is strongly inhibited when plants are grown and

inoculated on a nitrogen-containing medium (1). A mutant (nod-3) was

found which is highly nodulating both on nitrate-free and on nitrate-

containing media, whereas acetylene reduction is much higher than that

of parent variety 'Rondo' (2).

The effect of ammonium and nitrate on nodulation and on taproot

length of both genotypes is shown in Table 1. In these experiments,

nodulation of cv Rondo on nitrogen-free medium was relatively high and

the negative effect of nitrate on nodule number and nodule mass was

stronger than that of ammonium. In mutant nod-3, nodulation expressed

in nodule number and nodule mass is also inhibited by nitrate and am-

monium, but to a much lower extent. In contrast with Rondo, nodulation

of nod-3 seems to be affected more by ammonium than by nitrate. It is

clear that mutant nod-3 is highly nodulating on nitrogen-free medium and

in the presence of both nitrate and ammonium.

The taproot length of plants of Rondo is essentially the same in

all experiments (Table 1). Mutant nod-3 had a much shorter taproot

compared with Rondo, and showed differences in length when grown on

ammonium (43.3) than on KC1 or KNO₃ (29.8 cm and 32.2 cm respectively).

Root branching of mutant nod-3 and Rondo was compared after growth of

seedlings without Rhizobium on nitrate-free and on nitrate containing

medium. The number of secondary lateral roots appeared to be much

higher in mutant nod-3 (data not shown). All of these observations

could be an indication that the phytohormone balance in the two

genotypes is different.

Table 1. Nodulation of cv Rondo and mutant nod-3 inoculated with R_.

leguminosarum strain PF„ and cultured on standard mineral solution (SMS)

plus 15 mM KC1, 15 mM KNO₃ or 7.5 mM

(NH₄)₂S0₄-(n=5).

24 PNL Volume 16 1984

RESEARCH REPORTS

Mutant nod-3 was originally selected on nitrate containing medium

ai i er inoculation with R. leguminosarum strain PF2 (2). An important

question is whether the mutant properties of nod-3 are restricted to

strain PF2. Therefore, the mutant and Rondo were inoculated with strain

PRE (W. Europe), TOM (Turkey) and P8 (kindly supplied by Dr. Lie,

Wageningen, the Netherlands). Rondo was nodulated by strain P8, but the

nodules were ineffective (3). Table 2 shows that Rondo was nodulated by

all strains used and that nodulation was always strongly affected by

nitrate. Mutant nod-3 was highly nodulating with all strains on

nitrogen-free medium, whereas the effect of nitrate on nodulation was

comparable with the relatively weak effect found for strain PF2.

Acetylene reduction was measured for Rondo and mutant nod-3 after

nodulation with strain P8. In both genotypes, the nodules appeared to

be ineffective, indicating that strain P8 is acting in the same way in

both genotypes.

Table 2. Nodulation of cv Rondo and mutant nod-3 cultured on nitrogen-

free and on 15 mM KNO₃ containing standard mineral solution

(SMS), inoculated with different strains of Rhizobium.

The observations (1) that mutant nod-3 is highly nodulating in the

presence of both nitrate and ammonium, (2) that the mutant characters of

nod-3 are strain independent, and (3) that the taproot length of nod-3

is reduced, are of importance when considering the use of nod-3 in

agriculture and breeding.

1. Jacobsen, E. and I. van Dongen. 1982. PNL 14:23-24.

:. Jacobsen, E. and H. Nijdaai. 1983. PNL 15:31-32.

3. Lie, T. A., D. Hille, R. Lambers, and A. Houwers. 1975. In:

Nutman, P. S. (ed.). Symbiotic Nitrogen Fixation in Plants. JBP

7, Cambridge University Press, London, p. 319-333.