Fruit Development in Trillium1 : Dependence on Stem Carbohydrate Reserves

Leaves are the main source of carbon for fruit maturation in most species. However, in plants seeing contrasting light conditions such as some spring plants, carbon fixed during the spring could be used to support fruit development in the summer, when photosynthetic rates are low. We monitored carbohydrate content in the rhizome (a perennating organ) and the aboveground stem of trillium (Trillium erectum) over the entire growing season (May–November). At the beginning of the fruiting stage, stems carrying a developing fruit were harvested, their leaves were removed, and the leafless stems were maintained in aqueous solution under controlled conditions up to full fruit maturation. These experiments showed that stem carbohydrate content was sufficient to support fruit development in the absence of leaves and rhizome. This is the first reported case, to our knowledge, of complete fruit development sustained only by a temporary carbohydrate reservoir. This carbohydrate accumulation in the stem during the spring enables the plant to make better use of the high irradiances occurring at that time. Many other species might establish short-term carbohydrate reservoirs in response to seasonal changes in growing conditions.

Regulation of Tomato Fruit Polygalacturonase mRNA Accumulation by Ethylene: A Re-Examination1

Polygalacturonase (PG) is the major enzyme responsible for pectin disassembly in ripening fruit. Despite extensive research on the factors regulating PG gene expression in fruit, there is conflicting evidence regarding the role of ethylene in mediating its expression. Transgenic tomato (Lycopersicon esculentum) fruits in which endogenous ethylene production was suppressed by the expression of an antisense 1-aminocyclopropane-1-carboxylic acid (ACC) synthase gene were used to re-examine the role of ethylene in regulating the accumulation of PG mRNA, enzyme activity, and protein during fruit ripening. Treatment of transgenic antisense ACC synthase mature green fruit with ethylene at concentrations as low as 0.1 to 1 μL/L for 24 h induced PG mRNA accumulation, and this accumulation was higher at concentrations of ethylene up to 100 μL/L. Neither PG enzyme activity nor PG protein accumulated during this 24-h period of ethylene treatment, indicating that translation lags at least 24 h behind the accumulation of PG mRNA, even at high ethylene concentrations. When examined at concentrations of 10 μL/L, PG mRNA accumulated within 6 h of ethylene treatment, indicating that the PG gene responds rapidly to ethylene. Treatment of transgenic tomato fruit with a low level of ethylene (0.1 μL/L) for up to 6 d induced levels of PG mRNA, enzyme activity, and protein after 6 d, which were comparable to levels observed in ripening wild-type fruit. A similar level of internal ethylene (0.15 μL/L) was measured in transgenic antisense ACC synthase fruit that were held for 28 d after harvest. In these fruit PG mRNA, enzyme activity, and protein were detected. Collectively, these results suggest that PG mRNA accumulation is ethylene regulated, and that the low threshold levels of ethylene required to promote PG mRNA accumulation may be exceeded, even in transgenic antisense ACC synthase tomato fruit.

Better fruit.

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Better fruit.

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Better fruit.

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Better fruit.

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Better fruit.

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Better fruit.

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