Wang Jiangxin, Sommerfeld Milton, Hu Qiang
Department of Applied Biological Sciences, Arizona State University, Polytechnic Campus, 7001 E. Williams Field Road, Mesa, AZ 85212, USA.
Planta. 2009 Jun;230(1):191-203. doi: 10.1007/s00425-009-0932-4. Epub 2009 May 1.
The plastid terminal oxidase (PTOX) is a plastoquinol oxidase involved in carotenoid biosynthesis in higher plants, and may also represent the elusive oxidase in chlororespiration. Haematococcus pluvialis is a green alga that has the ability to synthesize and accumulate large amounts of the red carotenoid astaxanthin (ca. 2% of dry weight) under various stress conditions. However, the occurrence and function of PTOX in astaxanthin synthesis and the stress response in this organism is unknown. In this study, two ptox cDNAs were cloned and sequenced from H. pluvialis and were designated as ptox1 and ptox2. Genome sequence analysis and database searching revealed that duplication of PTOX gene occurred in certain eukaryotic algae, but not in cyanobacteria and higher plants. The physiological and biochemical evidence indicated that PTOX is involved in astaxanthin synthesis and plays a critical protective role against stress. Analysis of the transcriptional expression of the PTOXs and phytoene desaturase gene further suggests that it may be PTOX1 rather than PTOX2 that is co-regulated with astaxanthin synthesis. The fact that the changes in transcripts of ptoxs in response to high light and other stressors and the differential expression of ptox1 and ptox2, suggests that PTOX, coupled with astaxanthin synthesis pathway, exerts broad, yet undefined functions in addition to those identified in higher plants.
质体末端氧化酶（PTOX）是一种参与高等植物类胡萝卜素生物合成的质体醌氧化酶，也可能是氯呼吸中难以捉摸的氧化酶。雨生红球藻是一种绿藻，能够在各种胁迫条件下合成并积累大量红色类胡萝卜素虾青素（约占干重的2%）。然而，PTOX在该生物体虾青素合成和胁迫响应中的存在及功能尚不清楚。在本研究中，从雨生红球藻中克隆并测序了两个ptox cDNA，分别命名为ptox1和ptox2。基因组序列分析和数据库搜索显示，PTOX基因在某些真核藻类中发生了复制，但在蓝细菌和高等植物中未发生。生理生化证据表明，PTOX参与虾青素合成，并在胁迫中发挥关键的保护作用。对PTOXs和八氢番茄红素去饱和酶基因转录表达的分析进一步表明，与虾青素合成共同调控的可能是PTOX1而非PTOX2。ptoxs转录本对高光和其他应激源的响应变化以及ptox1和ptox2的差异表达，表明PTOX除了在高等植物中已确定的功能外，还与虾青素合成途径一起发挥广泛但尚未明确的功能。
