Proline suppresses Rubisco activity in higher plants.

Seedlings of Brassica juncea, Sesbania sesban, and Oryza sativa exposed to salt stress accumulated proline to levels as high as 4- to 20-fold over those of controls. Because chloroplasts are the major site for synthesis of stress induced proline accumulation, in vitro studies were conducted to see how the over-accumulation of this solute influences the activity of the major chloroplastic enzyme Ribulose 1,5-bis-phosphate carboxylase (Rubisco) purified from the above plant species. Surprisingly, proline (believed to be a compatible solute) suppressed the activity of Rubisco significantly even when present at a concentration as low as 100 mM. The extent of this inhibition in Rubisco activity increased with an increase in the concentration of proline. Irrespective of the plant species from which Rubisco was purified, its activity declined by about 50% in the presence of 1 M proline. Rubisco from all three plant species was sensitive to NaCl and proline accelerated salt induced suppression in its activity. To the best of our knowledge this is the first report wherein a negative effect of proline (which is believed to protect enzymes under stress) has been clearly demonstrated. This perturbing effect of proline on Rubisco from higher plant species belonging to three distinct families cautions targeting of gene(s)/gene products for over-production of proline into chloroplasts.