Regulation of tuber formation and ADP-glucose pyrophosphorylase (AGPase) in sweet potato (Ipomoea batatas (L.) Lam.) by nitrate

The relationship between nitrate fertilization and tuber-bulking in roots from single-leaf explants was examined to clarify the mechanism of tuber formation. Cuttings of sweet potato leaves were grown on a medium containing 1, 10, 30, and 50 mM nitrate and analyzed with regard to morphological and biochemical traits. The cuttings grown on higher concentrations (≥ 10 mM) showed inhibited root growth, with no apparent effect on leaf growth or root length. The roots accumulated starch in a medium containing a lower nitrate concentration (≤ 10 mM). Under this culture condition, both the ADP-glucose pyrophosphorylase (AGPase) transcript level and the starch content in the root decreased, while there was no decrease in the starch content in the leaf. The greater AGPase transcript level in roots growing with a higher nitrate concentration (≥ 10 mM) might be caused by nitrogen-mediated signalling and/or changes in the levels of starch metabolism. Starch synthesis via AGPase and cell proliferation may work together for tuber formation in sweet potato root.