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 of pathogen.

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.

Acknowledgements

Authors wish to thank Mr. D.G. Goswamy for his assistance in field observations.

Reference

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