Viola R, Roberts A G, Haupt S, Gazzani S, Hancock R D, Marmiroli N, Machray G C, Oparka K J
Unit of Plant Biochemistry, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, United Kingdom.
Plant Cell. 2001 Feb;13(2):385-98. doi: 10.1105/tpc.13.2.385.
Phloem unloading was studied in potato plants in real time during the early stages of tuberization using carboxyfluorescein (CF) as a phloem-mobile tracer, and the unloading pattern was compared with autoradiography of tubers that had transported (14)C assimilates. In stolons undergoing extension growth, apoplastic phloem unloading predominated. However, during the first visible signs of tuberization, a transition occurred from apoplastic to symplastic transport, and both CF and (14)C assimilates subsequently followed identical patterns of phloem unloading. It is suggested that the switch to symplastic sucrose unloading may be responsible for the upregulation of several genes involved in sucrose metabolism. A detailed analysis of sugar levels and (14)C sugar partitioning in tuberizing stolons revealed a distinct difference between the apical region of the tuber and the subapical region. Analysis of invertase activity in nontuberizing and tuberizing stolons revealed a marked decline in soluble invertase in the subapical region of swelling stolons, consistent with the switch from apoplastic to symplastic unloading. However, cell wall-bound invertase activity remained high in the apical 1 to 2 mm of tuberizing stolons. Histochemical analysis of potato lines transformed with the promoter of an apoplastic invertase gene (invGE) linked to a reporter gene also revealed discrete gene expression in the apical bud region. Evidence is presented that the apical and lateral tuber buds function as isolated domains with respect to sucrose unloading and metabolism.
在块茎形成的早期阶段，以羧基荧光素（CF）作为韧皮部可移动示踪剂，对马铃薯植株中的韧皮部卸载进行了实时研究，并将卸载模式与运输过¹⁴C同化物的块茎的放射自显影进行了比较。在进行伸长生长的匍匐茎中，质外体韧皮部卸载占主导。然而，在块茎形成的最初可见迹象出现时，发生了从质外体运输到共质体运输的转变，随后CF和¹⁴C同化物都遵循相同的韧皮部卸载模式。有人提出，向共质体蔗糖卸载的转变可能是参与蔗糖代谢的几个基因上调的原因。对块茎形成期匍匐茎中糖水平和¹⁴C糖分配的详细分析揭示了块茎顶端区域和亚顶端区域之间的明显差异。对未形成块茎和正在形成块茎的匍匐茎中转化酶活性的分析表明，肿胀匍匐茎亚顶端区域的可溶性转化酶显著下降，这与从质外体卸载到共质体卸载的转变一致。然而，在形成块茎的匍匐茎顶端1至2毫米处，细胞壁结合的转化酶活性仍然很高。对用与报告基因相连的质外体转化酶基因（invGE）启动子转化的马铃薯品系进行的组织化学分析也显示，在顶芽区域有离散的基因表达。有证据表明，顶端和侧生块茎芽在蔗糖卸载和代谢方面起着独立区域的作用。
