SALT-STRESS INDUCES PARTIAL RIPENING OF THE NOR TOMATO MUTANT BUT EXPRESSION OF ONLY SOME RIPENING-RELATED GENES

Tomato plants homozygous for the character non-ripening (nor) when grown in normal nutritional conditions produce fruit that change colour at maturity only very slowly, in a typical non-climacteric fruit behaviour pattern. However, by subjecting the plants to salinity stress, at the end of development the fruit redden and partially ripen. Wild type cv. Ailsa Craig plants and an isogenic selection containing the alleles nor/nor were grown in deep solution culture at conductivity levels of either 3 or 8 mS cm-1 (mS) in the first season, and 3, 9 or 15 mS in a subsequent year. Various physiological and chemical parameters concerning fruit development were measured. As salt-stress was increased, the average fruit weight decreased along with the firmness index, while the percentage dry matter, the tomato colour index (TCI), acidity, sugar levels and K+ ion concentrations in the fruit sap increased. Na+ ion concentrations reached a plateau at moderate salinities and then decreased at even higher levels. No activity for the pectic enzyme polygalacturonase (PG) was detected in any nor fruit, whereas salt-stress induced high levels of invertase activity. Total cellular RNA was extracted from both wild type and nor fruit from plants grown normally, and from nor fruit of plants grown at high salinity. The expression of four ripening-related genes was examined using cDNA probes. One mRNA, corresponding to clone pTOM4, was at similar levels in 3 mS nor and wild type fruit, whilst two, corresponding to pTOM5 and pTOM13, were present in substantially reduced levels in nor tomatoes compared to parental lines. No mRNA was detected in nor fruit corresponding to pTOM6, a cDNA clone for PG, which is abundant in normal ripe tomatoes. Salt-stress on nor plants had no apparent effect on pTOM4, pTOM6 or pTOM13 mRNA levels in the fruit, but the mRNA for pTOM5 was greatly induced, in parallel with colour development. The results emphasize that there is strict control of development at the physiological, enzymic and gene expression levels, but high salinity can partially overcome the nor lesion without the involvement of PG.