The liverwort Marchantia polymorpha contains two isoforms of the plastid terminal oxidase (PTOX), an enzyme that catalyzes the reduction of oxygen to water using plastoquinol as substrate. Phylogenetic analyses showed that one isoform, here called MpPTOXa, is closely related to isoforms occurring in plants and some algae, while the other isoform, here called MpPTOXb, is closely related to the two isoforms occurring in Chlamydomonas reinhardtii. Mutants of each isoform were created in Marchantia polymorpha using CRISPR/Cas9 technology. While no obvious phenotype was found for these mutants, chlorophyll fluorescence analyses demonstrated that the plastoquinone pool was in a higher reduction state in both mutants. This was visible at the level of fluorescence measured in dark-adapted material and by post illumination fluorescence rise. These results suggest that both isoforms have a redundant function. However, when P700 oxidation and re-reduction was studied, differences between these two isoforms were observed. Furthermore, the mutant affected in MpPTOXb showed a slight alteration in the pigment composition, a higher non-photochemical quenching and a slightly lower electron transport rate through photosystem II. These differences may be explained either by differences in the enzymatic activities or by different activities attributed to preferential involvement of the two PTOX isoforms to either linear or cyclic electron flow.

地草多形马氏菌包含质体末端氧化酶（PTOX）的两种同工型，该酶以质体喹诺醇为底物催化将氧气还原为水。系统发育分析表明，一个同种型，这里称为MpPTOXa，与植物和某些藻类中存在的同种型密切相关，而另一个同种型，这里称为MpPTOXb，与莱茵衣藻中的两个同种型密切相关。每个同种型的突变体均在多形马氏酵母中产生使用CRISPR / Cas9技术。虽然没有发现这些突变体的明显表型，但叶绿素荧光分析表明，在两个突变体中，质体醌库均处于较高的还原状态。在暗适应材料中测得的荧光水平以及照明后的荧光上升情况下，这是可见的。这些结果表明两种同工型均具有冗余功能。然而，当研究P700的氧化和还原时，发现这两种同工型之间存在差异。此外，受MpPTOXb影响的突变体显示出颜料成分的轻微变化，更高的非光化学猝灭和通过光系统II的较低的电子传输速率。