Final Report Summary - PLANTMTERF (Unveiling the functions of mTERF proteins in plant organelles)
The CIG funding enabled a young plant scientist who was hired on a tenure-track to conduct his research autonomously on the topic he has chosen at CNRS, Strasbourg (France) and to develop his career perspectives (http://www.ibmp.cnrs.fr/equipes/adaptation-genetique-du-chloroplaste/). Plants are the backbone of life on earth as they bring us oxygen as a byproduct of photosynthesis and crops constitute the world major food stock for humans. The functions of the many genes that are essential for plant fitness have not all been elucidated so far, even in the extensively studied plant model Arabidopsis. The aim of this project was to decipher the physiological functions of a family of plant nuclear genes that is essential for chloroplast photosynthesis and mitochondrial respiration, two cellular reactions whose efficiency seriously impact plant yield and survival. In fact, the perturbation of the activity of mTERF genes in plants has been shown to lead to seed formation arrest or to perturb plant growth but the reason underlying this was unknown. The mTERF gene family codes for proteins that are specific to animals and plants and that localize to mitochondria or chloroplasts where they influence gene expression. To understand their roles in chloroplasts, we knocked out selected mTERF genes in Arabidopsis and maize and took advantage of a combined approach of biochemistry and molecular biology to study the loss-of-function mutants. We isolated two maize mutants for mTERF genes that display a chlorotic and a seedling lethal phenotype and we characterized the mutant seedlings in details at the molecular level. Both of the mutants displayed defects in the accumulation of chloroplast encoded proteins. A closer look at the expression of chloroplast genes in the mutants revealed that these two mTERF proteins were essential for the removal of chloroplastic RNA introns and the biogenesis of chloroplastic ribosomal RNA, respectively (Hammani and Barkan, 2014; Fat et al., in preparation). These molecular RNA defects provoked a loss of chloroplast protein content in the plant mutants which in turn led to the maize plant death. Two mutants for mTERF genes in Arabidopsis were as well isolated but not studied due to lack of time. Another objective of the project was to identify the proteins in chloroplast that functionally interact with mTERF proteins and to understand their interplay. We characterized the function of one such protein (named PPR103) using an analogous strategy in maize and we assigned to this protein a role in the stabilization of a chloroplastic messenger RNA that encodes a ribosomal protein (Hammani et al, 2016). Our project improved our general understanding of the functional repertoire of a group of genes that is essential for important plant agronomical traits (survival and biomass production) through the regulation of the plant energy transduction machineries (chloroplasts in our case). Improving our knowledge on the regulation of the chloroplast gene expression apparatus will be highly relevant for strategies that engineer the chloroplast genome to use chloroplasts as bioreactors for the production of high-level foreign proteins and metabolites of commercial interest. Finally, organelles represent perfect sites for increased biomass yield, which has direct implication for bioenergy production.