Lignins result from the oxidative polymerization of three hydroxycinnamyl (p-coumaryl, coniferyl, and sinapyl) alcohols in a reaction mediated by peroxidases. The most important of these is the cationic peroxidase from Zinnia elegans (ZePrx), an enzyme considered to be responsible for the last step of lignification in this plant. Bibliographical evidence indicates that the arabidopsis peroxidase 72 (AtPrx72), which is homolog to ZePrx, could have an important role in lignification. For this reason, we performed a bioinformatic, histochemical, photosynthetic, and phenotypical and lignin composition analysis of an arabidopsis knock-out mutant of AtPrx72 with the aim of characterizing the effects that occurred due to the absence of expression of this peroxidase from the aspects of plant physiology such as vascular development, lignification, and photosynthesis. In silico analyses indicated a high homology between AtPrx72 and ZePrx, cell wall localization and probably optimal levels of translation of AtPrx72. The histochemical study revealed a low content in syringyl units and a decrease in the amount of lignin in the atprx72 mutant plants compared to WT. The atprx72 mutant plants grew more slowly than WT plants, with both smaller rosette and principal stem, and with fewer branches and siliques than the WT plants. Lastly, chlorophyll a fluorescence revealed a significant decrease in ΦPSII and q L in atprx72 mutant plants that could be related to changes in carbon partitioning and/or utilization of redox equivalents in arabidopsis metabolism. The results suggest an important role of AtPrx72 in lignin biosynthesis. In addition, knock-out plants were able to respond and adapt to an insufficiency of lignification.

No abstract

Mitochondrial involvement has not been identified in the programmed cell death (PCD) of leaf senescence which suggests that processes such as those involving reactive oxygen species (ROS) are controlled by chloroplasts. We report that transgenic tobacco (DndhF), with the plastid ndhF gene knocked-out, shows low levels of the plastid Ndh complex, homologous to mitochondrial complex I, and more than a 30-day-delay in leaf senescence with respect to wt. The comparison of activities and protein levels and analyses of genetic and phenotypic traits of wtxDndhF crosses indicate that regulatory roles of mitochondria in animal PCD are assumed by chloroplasts in leaf senescence. The Ndh complex would increase the reduction level of electron transporters and the generation of ROS. Chloroplastic control of leaf senescence provides a nonclassical model of PCD and reveals an unexpected role of the plastid ndh genes that are present in most higher plants.

An Ndh‐deficient mutant of tobacco (Nicotiana tabacum cv. Petit Havana) was prepared by disrupting the ndhF gene in a transplastomic approach. The mutant (ΔndhF) showed 10% of the Ndh complex activity (EC 1.6.5.3) and 8% of the NDH‐F polypeptide of that of non‐transformed plants. However, in ΔndhF, NDH‐A, another Ndh polypeptide, was still present at 50% of the level in non‐transformed plants. ΔndhF tobacco showed higher sensitivity than non‐transformed plants to photo‐oxidative stress (as judged by chlorophyll bleaching) caused by increased light intensity and paraquat applications. These photo‐oxidative treatments increased the amount and activity of the Ndh complex, thylakoid peroxidase, post‐illumination chlorophyll fluorescence and non‐photochemical quenching (NPQ) of chlorophyll fluorescence in non‐transformed but not in ΔndhF tobacco. Highly stressed non‐transformed plants showed a rapid post‐rise decline of chlorophyll fluorescence, probably indicating a re‐oxidation of reduced plastoquinone. The results indicate that, in normal plants, the Ndh complex and thylakoid peroxidase (EC 1.11.1.7) provide and remove electrons, respectively, to balance the redox level of the intermediates of cyclic electron transport. In this way, they optimize the generation of the transmembrane H+ gradient of thylakoids and, as a consequence, increase the NPQ and the protection against photo‐oxidative stress.

No abstract