Characterization of the nv mutation in L. esculentum

The recessive nv mutation was introgressed into L. esculentum from L. peruvanum during a breeding program for resistance against tobacco mosaic virus (TMV) and the gene was mapped to chromosome 9 (Hall, 1980). Plants homozygous for nv typically exhibit yellow leaves and are stunted as compared to wildtype plants. By applying biochemical, electron microscopial and molecular biological methods we have analyzed the physiological defects underlying the phenotype of nv. Since the manifestation of the characteristic phenotype is crucially dependent on the physiological stage of the mutant plants, wildtype (GCR 26) and nv (GCR 236) plants of L. esculentum var. Craigella were harvested every two weeks after germination. The plants were monitored for their chlorophyll content and the morphology of the chloroplasts. Representative results for plants 7 weeks after germination are summarized in Table 1. The mutant plastids display a significant reduction in chlorophyll content, the number and the height of the grana stacks are also decreased by a factor of about 2 in the mutant as compared to wildtype plants. On the other hand, analysis of the chloroplast proteins on 2D gels revealed that the mutant shows the same overall protein pattern as the wildtype, disregarding a reduction in the total amount of protein which is paralleled by the decrease in chlorophyll content. This result is, however, not caused by a reduction of transcript levels for chloroplast proteins, because in vitro translation of poly A+RNA isolated from mutant as well as from wild type plants yielded comparable levels of radiolabelled proteins. Rather we could demonstrate by pulse chase experiments that the lower accumulation of various chloroplast proteins is due to an increased protein turnover in mutant plants. As an example, the results for LHCP are given in Fig. 1. Thus, in contrast to the Su-mutation in tobacco (Kawata and Cheung, 1990), the nv mutation in tomato does not seem to affect the accumulation of transcripts for the photosynthetic proteins, but rather results in a reduced stability of chloroplast proteins.

Literature cited:

Hall, T.J. 1980 Euphytica 29:189-197.

Kawata, E.E. and Cheung, A.Y. 1990 EMBO J. 9:4197-4203.

Table 1. Physical properties of tomato plants 7 weeks after germination.

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genotype                  GCR26    GCR236
                          (+/+)    (nv/nv)  
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size (cm)                  26      10  
chlorophyll (mg/g fw)      2.8      1.2  
chloroplast size:    
        length (um)        2.8-4.8  2.6-3.8  
        height (um)        1.7-2.3  0.9-1.5  
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