Disease lesion mimic mutants display phenotypes similar to the ones caused
by the pathogen. Several such mutants have been isolated in corn (Neuffer
and Calvert 1975), Arabidopsis (Greenberg and Ausubel 1993) and
rice (Takahashi et al. 1999). They are caused by either recessive
or dominant gene mutations. Molecular analysis of these mutants revealed
that the host factors alone are sufficient to cause cell death, which
are potentially analogous to that associated with pathogen attack. Kawasaki
et al. (1999) found that a small GTP binding protein Rac regulates
cell death in rice.
A gamma ray induced, stable disease lesion mimic mutant was isolated from
the rice variety 'Basmati 370'. Dark red lesions appeared on the leaves
from seedling to flowering stage. The lesions first appeared on 12-day
old seedlings. Younger leaves were free of lesions, but at the later stages
of growth, lesions appeared on all the leaves (Fig. 1). The spread of
lesions was restricted to an area of about 1-2 cm2. The appearance
of lesion was similar to those of disease spots caused by rice blast (Magnaporthe
grisea). The mutant was free of blast disease under the field conditions,
whereas controls grown side by side were found to be infected. This appears
due to the inbuilt resistance of the mutant. The resistance of this mutant
to blast disease was confirmed by challenging the mutant with pathogen
under controlled conditions. Similar lesion mimic mutants of rice resistant
to blast were isolated by Takahashi et al. (1999) where cellular
and molecular markers of defence response are activated in the absence
When the mutant was crossed with its parent, the F1 plants
had only normal green leaves suggesting the recessive nature of the mutant.
Of 528 F2 plants, 391 had normal
green leaves and 137 displayed typical lesions. The segregation agreed
with 3:1 ratio (chi2 = 0.25; P = 0.75- 0.50), indicating that
the disease lesion mimic mutant is overned by a single recessive nuclear
gene. Similar lesion mimic mutants have also been reported in rice (Kiyosawa
1910, Takahashi et al. 1999).
Basmati lesion mutant and control plants were grown in the field and leaf
samples were collected at 45 and 80 days of plant growth to study the
protein profile. The soluble protein extracted from leaf was precipitated
with acetone and was used for SDS-PAGE. The electropherogram showed at
least 20 polypeptides with molecular weights ranging from 16 to 94 kDa,
which were arranged in four zones. The major polypeptides were present
in zone 2 (Z2). A protein doublet of 56 kDa and 53 kDa (Fig. 2) was observed
in the mutant, while in the control only a single polypeptide was present.
An additional polypeptide of about 47 kDa was also present in the mutant.
In zone 3 (Z3), a 35 kDa polypeptide appeared in the control, which was
absent in the mutant. Rest of the polypeptides were identical for both
control as well as the mutant.
The formation of the doublet in the mutant may be due to a proteolytic
cleavage of 55 kDa polypeptide which appears to be the Rubisco large subunit
suggested by Navarre and Wolpert (1999) in oat leaf extracts resulting
from plant senescence under various stress
conditions. The additional
polypeptide of 47 kDa can be attributed to its synthesis as a PR (pathogenesis-related)
or stress protein. This lesion mimic mutant was found to be similar to
the mutants isolated by Takahashi et al. (1999), which showed alteration
in early signaling events of defence in the absence of pathogen. Such
disease lesion mimic mutants are of immense use in studying the defence
responses as well as molecular mechanism regulating cell death.
Authors wish to thank Mr. D.G. Goswamy for his assistance in field observations.
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