Many plants accumulate transitory starch reserves in their leaves during the day to buffer their carbohydrate supply against fluctuating light conditions, and to provide carbon and energy for survival at night. It is universally accepted that transitory starch is synthesized from ADP-glucose (ADPG) in the chloroplasts. However, the consensus that ADPG is made in the chloroplasts by ADPG pyrophosphorylase has been challenged by a controversial proposal that ADPG is made primarily in the cytosol, probably by sucrose synthase (SUS), and then imported into the chloroplasts. To resolve this long-standing controversy, we critically re-examined the experimental evidence that appears to conflict with the consensus pathway. We show that when precautions are taken to avoid artefactual changes during leaf sampling, Arabidopsis thaliana mutants that lack SUS activity in mesophyll cells (quadruple sus1234) or have no SUS activity (sextuple sus123456) have wild-type levels of ADPG and starch, while ADPG is 20 times lower in the pgm and adg1 mutants that are blocked in the consensus chloroplastic pathway of starch synthesis. We conclude that the ADPG needed for starch synthesis in leaves is synthesized primarily by ADPG pyrophosphorylase in the chloroplasts.

许多植物白天在叶子中积累临时淀粉储备，以缓冲其碳水化合物供应以应对光照条件的波动，并为夜间生存提供碳和能量。普遍认为临时淀粉是由叶绿体中的 ADP-葡萄糖 (ADPG) 合成的。然而，ADPG 是由 ADPG 焦磷酸化酶在叶绿体中产生的共识受到了一项有争议的提议的挑战，该提议认为 ADPG 主要在胞质溶胶中产生，可能由蔗糖合酶 (SUS) 产生，然后导入叶绿体。为了解决这一长期存在的争议，我们批判性地重新审查了似乎与共识途径相冲突的实验证据。我们表明，当采取预防措施以避免在叶片取样过程中发生人为变化时，在叶肉细胞中缺乏 SUS 活性的拟南芥突变体（四重sus1234）或没有 SUS 活性（六重sus123456）具有野生型水平的 ADPG 和淀粉，而 ADPG 在 pgm和adg1突变体中低 20 倍，这些突变体在淀粉合成的一致叶绿体途径。我们得出结论，叶中淀粉合成所需的 ADPG 主要由叶绿体中的 ADPG 焦磷酸化酶合成。