Lermontova Inna, Grimm Bernhard
Institute of Biology/Plant Physiology, Humboldt University, Philippstr. 13, Building 12, 10115 Berlin, Germany.
Plant J. 2006 Nov;48(4):499-510. doi: 10.1111/j.1365-313X.2006.02894.x. Epub 2006 Oct 19.
Protoporphyrinogen oxidase (EC 1.3.3.4, PPOX) is the last enzyme in the branched tetrapyrrole biosynthetic pathway, before its substrate protoporphyrin is directed to the Mg and Fe branches for chlorophyll and haem biosynthesis, respectively. The enzyme exists in many plants in two similar isoforms, which are either exclusively located in plastids (PPOX I) or in mitochondria and plastids (PPOX II). Antisense RNA expression inhibited the formation of PPOX I in transgenic tobacco plants, which showed reduced growth rate and necrotic leaf damage. The cytotoxic effect is attributed to accumulation of photodynamically acting protoporphyrin. The expression levels of PPOX I mRNA and protein and the cellular enzyme activities were reduced to similar extents in transgenic plants grown under low- or high-light conditions (70 and 530 mumol photons m(-2) sec(-1)). More necrotic leaf lesions were surprisingly generated under low- than under high-light exposure. Several reasons were explored to explain this paradox and the intriguing necrotic phenotype of PPOX-deficient plants under both light intensity growth conditions. The same reduction of PPOX expression and activity under both light conditions led to similar initial protoporphyrin, but to faster decrease in protoporphyrin content during high light. It is likely that a light intensity-dependent degradation of reduced and oxidized porphyrins prevents severe photodynamic leaf damage. Moreover, under high-light conditions, elevated contents of reduced and total low-molecular-weight antioxidants contribute to the protection against photosensitizing porphyrins. These reducing conditions stabilize protoporphyrinogen in plastids and allow their redirection into the metabolic pathway.
原卟啉原氧化酶（EC 1.3.3.4，PPOX）是分支四吡咯生物合成途径中的最后一种酶，在此之前，其底物原卟啉分别被导向镁分支和铁分支，用于叶绿素和血红素的生物合成。该酶在许多植物中以两种相似的同工型存在，它们要么仅位于质体中（PPOX I），要么位于线粒体和质体中（PPOX II）。反义RNA表达抑制了转基因烟草植株中PPOX I的形成，这些植株表现出生长速率降低和叶片坏死损伤。细胞毒性作用归因于具有光动力作用的原卟啉的积累。在低光照或高光照条件（70和530 μmol光子·m⁻²·s⁻¹）下生长的转基因植株中，PPOX I mRNA和蛋白质的表达水平以及细胞酶活性均降低到相似程度。令人惊讶的是，低光照下产生的坏死叶斑比高光照下更多。人们探究了几个原因来解释这一矛盾现象以及PPOX缺陷型植株在两种光照强度生长条件下有趣的坏死表型。在两种光照条件下PPOX表达和活性的相同降低导致初始原卟啉相似，但在高光下原卟啉含量下降更快。还原型和氧化型卟啉可能存在光强依赖性降解，从而防止叶片受到严重的光动力损伤。此外，在高光条件下，还原型和总低分子量抗氧化剂含量的升高有助于抵御光敏卟啉。这些还原条件使质体中的原卟啉原稳定，并使其重新导向代谢途径。
