Changes in plastid proteome and structure in arbuscular mycorrhizal roots display a nutrient starvation signature

During arbuscular mycorrhizal symbiosis, arbuscule-containing root cortex cells display a proliferation of plastids, a feature usually ascribed to an increased plant anabolism despite the lack of studies focusing on purified root plastids. In this study, we investigated mycorrhiza-induced changes in plastidicpathways by performing a label-free comparative subcellular quantitative proteomicanalysis targeted on plastid-enriched fractions isolated from Medicago truncatularoots, coupled to a cytological analysis of plastidstructure. We identified 490 root plastidprotein candidates, among which 79 changed in abundance upon mycorrhization, as inferred from spectral counting. According to cross-species sequence homology searches, the mycorrhiza-responsive proteome was enriched in proteins experimentally localized in thylakoids, whereas it was depleted of proteins ascribed predominantly to amyloplasts. Consistently, the analysis of plastidmorphology using transmission electron microscopy indicated that starch depletion associated with the proliferation of membrane-free and tubular membrane-containing plastidswas a feature specific to arbusculated cells. The loss of enzymes involved in carbon/ nitrogen assimilationand provision of reducing power, coupled to macromolecule degradation events in the plastid-enriched fraction of mycorrhizal roots that paralleled lack of starch accumulation in arbusculated cells, lead us to propose that arbuscule functioning elicits a nutrient starvation and an oxidative stress signature that may prime arbuscule breakdown.