Fig. 9: Overview of intracellular iron partitioning strategies behind −Fe’s role in heterotrophy and TAG accumulation.

The panel on the left summarizes predicted intracellular iron prioritizations in WT−Fe+Glc that underly the glucose-mediated switch-off of oxygenic photosynthesis during TAG accumulation. Yellow arrows indicate a potential iron-prioritization from a repressed iron-containing proteins (turquoise) that are spared when a HXK1-mediated GLC signaling pathway activates respiration and dnFAS. Prioritized and induced iron-containing proteins are labeled (orange). When photosynthetic ETC is repressed, the released iron is potentially trafficked from the plastid to the mitochondria via MFL1 which is upregulated in +Glc. Additionally, spared iron proteins likely provide Fe resources for the upregulated dnFAS pathway, including iron-rich players conducting redox transfers of lipid desaturation: FDX2 to SAD1 and CBR1 to FAD2 (See Discussion). These proteins exhibit upregulation in TAG even though most ferredoxins and desaturases are depleted, suggesting their specific prioritization to support dnFAS during TAG accumulation. Thylakoid membranes are likely depleted in WT−Fe+Glc due to loss of membrane synthesis supported by most desaturases. Fe may also be reallocated to iron-containing proteins THIC, THI4A-C, and PAO5 which are uniquely upregulated in the heterotrophic WT−Fe+Glc conditions. In the right panel, photosynthesis is maintained during TAG accumulation due to replete Fe. The green arrows represent how replete iron in WT+Fe+Glc allows enough iron resources to be distributed to the pathways that would have been depleted in iron deficiency. While +Glc still triggers a HXK1-dependent upregulation (orange) of dnFAS, +Fe is distributed to both TAG-supporting and thylakoid supporting desaturases, the latter of which are highly induced in the mixotrophic WT+Fe+Glc condition. Both thylakoids and photosynthetic complexes are maintained due to replete iron (green=maintained). While MFL1 is still upregulated by +Glc, putative thylakoid importers (TIC21B, ABCI12) are maintained. Lipid droplets (LD) accumulate in both conditions through the +Glc-HXK1 pathway. The organelles in the diagram are plastid (green), mitochondria (red), cytoplasm, (blue), and endoplasmic reticulum (yellow). Full names of protein abbreviations can be found in Supplementary Data 9.