On the Edge of Dispensability, the Chloroplast ndh Genes.

Bartolomé Sabater

doi:10.3390/ijms222212505

Abstract

The polypeptides encoded by the chloroplast ndh genes and some nuclear genes form the thylakoid NADH dehydrogenase (Ndh) complex, homologous to the mitochondrial complex I. Except for Charophyceae (algae related to higher plants) and a few Prasinophyceae, all eukaryotic algae lack ndh genes. Among vascular plants, the ndh genes are absent in epiphytic and in some species scattered among different genera, families, and orders. The recent identification of many plants lacking plastid ndh genes allows comparison on phylogenetic trees and functional investigations of the ndh genes. The ndh genes protect Angiosperms under various terrestrial stresses, maintaining efficient photosynthesis. On the edge of dispensability, ndh genes provide a test for the natural selection of photosynthesis-related genes in evolution. Variable evolutionary environments place Angiosperms without ndh genes at risk of extinction and, probably, most extant ones may have lost ndh genes recently. Therefore, they are evolutionary endpoints in phylogenetic trees. The low number of sequenced plastid DNA and the long lifespan of some Gymnosperms lacking ndh genes challenge models about the role of ndh genes protecting against stress and promoting leaf senescence. Additional DNA sequencing in Gymnosperms and investigations into the molecular mechanisms of their response to stress will provide a unified model of the evolutionary and functional consequences of the lack of ndh genes.

Highlights

The ndh genes are homologous to those encoding components of mitochondrial and bacterial respiratory complex I (NADH dehydrogenase, EC 1.6.5.3)
Their identification in chloroplast DNA, by the 1980s [1,2], was a surprise because the respiratory electron transport chain and the photosynthetic electron transport chain are characteristic of mitochondria and chloroplast respectively, and there was no evidence for the presence of any complex I-like or respiratory-like process in chloroplasts
The involvement of the NADH dehydrogenase (Ndh) complex optimizing photosynthesis would explain the absence of the plastid ndh genes in parasitic plants and some carnivorous plants [68,69] that rely on low or no photosynthetic activity

Summary

Introduction

The ndh genes are homologous to those encoding components of mitochondrial and bacterial respiratory complex I (NADH dehydrogenase, EC 1.6.5.3). Sequencing of chloroplast DNA from many plants, and immunological and proteomic identification of chloroplast proteins and protein complexes, have demonstrated the presence of ndh genes and a complex I-like (the Ndh complex) in the chloroplast of most land plants. Parasitic angiosperms, such as Epifagus virginiana and Cuscuta reflexa, which have low or no photosynthetic activity, were soon found to lack ndh genes [5,6,7], which suggested that the protein products of the ndh genes play a role in photosynthesis. This review will seek correlations of biochemical, functional, and protective properties attributable to the Ndh complex with the estimated time and mode of evolutionary loss of ndh genes

Chloroplast ndh Genes

Functional Role of the Thylakoid Ndh Complex

Dispensing with the Role of the ndh Genes

Findings

Concluding Remarks and Future Perspectives