Expression of a fungal endo-α-1,5-l-arabinanase during stolon differentiation in potato inhibits tuber formation and results in accumulation of starch and tuber-specific transcripts in the stem

Potato (Solanum tuberosum cv. Posmo) was transformed with a fungal endo-α-1,5-l-arabinanase gene (E.C. 3.2.1.99) and the enzyme secreted to the apoplast. Using promoters that are known to be sucrose inducible in potato-the granule bound starch synthase promoter and the promoter of the patatin storage protein gene-strong expression of the arabinanase was observed in developing sinks, notably the meristems giving rise to tubers, stolons and side shoots. Expression of the arabinanase interfered with normal meristem function and depending on the promoter used, different phenotypes were obtained devoid of tubers or both tubers, stolons and side shoots. The altered ratio of source to sink tissues in the transformants was used to study the regulation of photoassimilate allocation to different tissues. Tuberless transformants were unable to fully balance supply with demand and excess photoassimilates were deposited as starch granules with the structure of tuber starch in above ground tissues, notably in the stem pith. Transcripts normally associated with tuber development were detected in the starch accumulating tissues and the tuberlike transcript profile was not limited to transcripts that are directly sucrose inducible. These observations are discussed in relation to organ identity and source-sink regulation in potato.