Häusler R E, Baur B, Scharte J, Teichmann T, Eicks M, Fischer K L, Flügge U I, Schubert S, Weber A, Fischer K
Botanisches Institut der Universität zu Köln, Gyrhofstrasse 15, D-50931 Köln, Germany.
Plant J. 2000 Nov;24(3):285-96. doi: 10.1046/j.1365-313x.2000.00876.x.
The inducible crassulacean acid metabolism (CAM) plant Mesembryanthemum crystallinum accumulates malic acid during the night and converts it to starch during the day via a pathway that, because it is located in different subcellular compartments, depends on specific metabolite transport across membranes. The chloroplast glucose transporter (pGlcT) and three members of the phosphate translocator (PT) family were isolated. After induction of CAM, transcript amounts of the phosphoenolpyruvate (PEP) phosphate translocator (PPT) and the glucose-6-phosphate (Glc6P) phosphate translocator (GPT) genes were increased drastically, while triose phosphate (TP) phosphate translocator (TPT) and the pGlcT transcripts remained unchanged. PPT- and GPT-specific transcripts and transporter activities exhibited a pronounced diurnal variation, displaying the highest amplitude in the light. pGlcT transcripts were elevated towards the end of the light period and at the beginning of the dark period. These findings, combined with diurnal variations of enzyme activities and metabolite contents, helped to elucidate the roles of the PPT, GPT, TPT and pGlcT in CAM. The main function of the PPT is the daytime export from the stroma of PEP generated by pyruvate orthophosphate:dikinase (PPDK). The increased transport activity of GPT in the light suggests a higher requirement for Glc6P import for starch synthesis rather than starch mobilization. Most likely, Glc6P rather than 3-phosphoglycerate or triose phosphates is the main substrate for daytime starch biosynthesis in M. crystallinum plants in which CAM has been induced (CAM-induced), similar to non-green plastids. In the dark, starch is mobilized both phosphorylytically and amylolytically and the products are exported by the GPT, TPT and pGlcT. The transport activities of all three phosphate translocators and the transcript amounts of the pGlcT adapt to changing transport requirements in order to maintain high metabolic fluxes during the diurnal CAM cycle.
诱导型景天酸代谢（CAM）植物冰花在夜间积累苹果酸，并在白天通过一条途径将其转化为淀粉，该途径由于位于不同的亚细胞区室，依赖于特定代谢物跨膜转运。分离出了叶绿体葡萄糖转运体（pGlcT）和磷酸转运体（PT）家族的三个成员。诱导CAM后，磷酸烯醇式丙酮酸（PEP）磷酸转运体（PPT）和葡萄糖-6-磷酸（Glc6P）磷酸转运体（GPT）基因的转录本量急剧增加，而磷酸丙糖（TP）磷酸转运体（TPT）和pGlcT转录本保持不变。PPT和GPT特异性转录本及转运体活性呈现出明显的昼夜变化，在光照下振幅最大。pGlcT转录本在光照期结束时和黑暗期开始时升高。这些发现与酶活性和代谢物含量的昼夜变化相结合，有助于阐明PPT、GPT、TPT和pGlcT在CAM中的作用。PPT的主要功能是在白天将由丙酮酸磷酸双激酶（PPDK）产生的PEP从基质中输出。光照下GPT转运活性的增加表明淀粉合成而非淀粉动员对Glc6P输入的需求更高。很可能，Glc6P而非3-磷酸甘油酸或磷酸丙糖是诱导了CAM的冰花植物（CAM诱导型）白天淀粉生物合成的主要底物，类似于非绿色质体。在黑暗中，淀粉通过磷酸解和淀粉分解两种方式被动员，产物由GPT、TPT和pGlcT输出。所有三种磷酸转运体的转运活性和pGlcT的转录本量都适应不断变化的转运需求，以便在昼夜CAM循环中维持高代谢通量。
