Suppression of AGPase in transgenic strawberry plants abolishes the leaf starch diurnal cycle, but has no effect on normal vegetative and reproductive development

Sweetness is one of the most important traits determining strawberry fruit quality. Strawberry leaves show a diurnal pattern of starch accumulation. It was hypothesised that preventing starch synthesis in source leaves would change the source-sulk relationships in the fruiting plant, increasing sugar transport and availability to the developing strawberry fruit sink. The enzyme ADP-glucose pyrophosphorylase (AGPase) is a key enzyme in starch synthesis. In order to reduce AGPase activity in strawberry plants transgenic strawberry plants were generated with antisense repression of the AGPase small subunit under the control of the constitutive 35S promoter. Several transgenic plants showed a 90% reduction in AGPase activity and consequently had only traces of starch content in their leaves. The diurnal starch accumulation pattern in the leaves was abolished. During daytime the leaves of transgenic plants accumulated soluble sugars, especially sucrose, thereby compensating for the starch, and the total diurnal carbohydrate level was not decreased. Despite the dramatic reduction of starch level in the transgenic plant leaves and in other plant tissues, no obvious effect on vegetative or reproductive development was observed. Also, the reduction of the diurnal leaf starch accumulation did not affect fruit yield (fruit size and number) or fruit quality, as total soluble solids (TSS). However, photosynthesis at elevated CO₂ was severely inhibited in the starchless transgenic plants. It was concluded that the capability to synthesize starch in strawberry leaves during daytime was not necessary for normal plant growth and development but could contribute to photoassimilation at high CO₂ concentrations.