Deletion of any of the core enzymes of the photorespiratory cycle, one of the major pathways of plant primary metabolism, results in severe air-sensitivity of the respective mutants. The peroxisomal enzyme hydroxypyruvate reductase (HPR1) represents the only exception to this rule. This indicates the presence of extraperoxisomal reactions of photorespiratory hydroxypyruvate metabolism. We have identified a second hydroxypyruvate reductase, HPR2, and present genetic and biochemical evidence that the enzyme provides a cytosolic bypass to the photorespiratory core cycle in Arabidopsis thaliana. Deletion of HPR2 results in elevated levels of hydroxypyruvate and other metabolites in leaves. Photosynthetic gas exchange is slightly altered, especially under long-day conditions. Otherwise, the mutant closely resembles wild-type plants. The combined deletion of both HPR1 and HPR2, however, results in distinct air-sensitivity and a dramatic reduction in photosynthetic performance. These results suggest that photorespiratory metabolism is not confined to chloroplasts, peroxisomes, and mitochondria but also extends to the cytosol. The extent to which cytosolic reactions contribute to the operation of the photorespiratory cycle in varying natural environments is not yet known, but it might be dynamically regulated by the availability of NADH in the context of peroxisomal redox homeostasis.
删除任何光呼吸循环的核心酶之一,这是植物主要代谢途径之一,会导致相应突变体的严重空气敏感性。过氧化物酶体酶羟丙酸还原酶(HPR1)是唯一的例外。这表明光呼吸羟丙酸代谢存在额外的过氧化物酶体外反应。我们已经鉴定出第二个羟丙酸还原酶HPR2,并提供遗传和生化证据表明该酶在阿拉伯芥细胞质中提供了一种绕过光呼吸核心循环的途径。删除HPR2会导致叶片中羟丙酸和其他代谢物的水平升高。光合气体交换略有改变,特别是在长日照条件下。否则,该突变体与野生型植物非常相似。然而,同时删除HPR1和HPR2会导致明显的空气敏感性和光合性能的显著降低。这些结果表明,光呼吸代谢不仅局限于叶绿体、过氧化物体和线粒体,还延伸到细胞质。细胞质反应对光呼吸循环在不同自然环境中的作用程度尚不清楚,但可能在过氧化物体氧化还原平衡的背景下动态调节NADH的可用性。