Disruptive effects of exogenous proline on chloroplast and mitochondrial ultrastructure in Arabidopsis leaves

A dose-dependent damage to chloroplastic and mitochondrial ultrastructure was observed in 21-day-old Arabidopsis thaliana seedlings grown in the presence of millimolar concentrations of exogenous proline. These findings draw into question the widely accepted, although poorly investigated, hypothesis that proline is an inert compatible solute that can be accumulated to high levels with minimal effects on cellular function. The destructive effects of proline on chloroplast ultrastructure may result from feedback inhibition of proline synthesis which causes over-reduction of the photosynthetic electron acceptor pools. Since proline degradation occurs in mitochondria, damage to cristae in plants grown on exogenous proline is proposed to result from elevated rates of mitochondrial electron transport. The resultant increase in levels of reactive oxygen intermediates in these organelles could account for the loss of chloroplast and mitochondrial integrity in plants exposed to exogenous proline.