The maintenance of ion gradients across the chloroplast envelope plays a key role in the bioenergetics of photosynthesis by moderating proton motive force and membrane dynamics. However, this research supports the novel finding that plastid ion transporters are additionally involved in maintaining plastid gene expression (PGE), likely by impacting the stromal buffer conditions required for function of plastid RNA binding proteins. We discovered this by investigating mutant Arabidopsis thaliana lines with loss-of-function of two inner envelope membrane potassium-proton (K+/H+) antiporters KEA1 and KEA2. Simultaneous loss of both transporters results in a unique “virescent” phenotype in which young leaves have disproportionately lower photosynthetic efficiency, chlorophyll production, and underdeveloped chloroplasts compared to older leaves. The goal of this research was to determine how the loss of KEA1/2 transporters results in this peculiar chloroplast developmental phenotype. Preliminary experiments using Total-reflection X-ray Fluorescence (TXRF) revealed that loss of KEA1/2 perturbs overall plastid ion homeostasis. An analysis of nuclear and plastome gene expression revealed significant defects in plastid ribosomal RNA processing in kea1kea2 mutant lines. This likely results from altered stromal ion concentrations inhibiting the function of nuclear-encoded chloroplast RNA binding proteins involved in plastid gene expression. This defect coincided with decreased steady-state levels of photosynthesis-related proteins, and lower translation rates in the stroma. We also discovered that plastid-to-nucleus retrograde signaling protein Genomes Uncoupled 1 (GUN1) was essential to seedling survival in kea1kea2. Loss of GUN1 in the kea1kea2 background resulted in higher expression of many nuclear-encoded photosynthesis-associated genes which are normally suppressed in response to disruption of PGE. In summary, ionome-induced impairment of plastid gene expression and subsequent retrograde signaling to suppress nuclear gene expression culminates in the virescent phenotype displayed by the kea1kea2 mutants. These findings underscore the importance of ion transporters in chloroplast development
