Proline suppresses Rubisco activity in higher plants

Area of Science:

• Plant Physiology
• Biochemistry
• Molecular Biology

Background:

• Plants accumulate proline, an amino acid, under salt stress to cope with osmotic imbalance.
• Chloroplasts are identified as the primary sites for stress-induced proline synthesis.
• Proline is generally considered a compatible solute, aiding enzyme function under stress.

Purpose of the Study:

• To investigate the effect of accumulated proline on the activity of Ribulose 1,5-bis-phosphate carboxylase (Rubisco).
• To determine if proline, at high concentrations, negatively impacts Rubisco function in Brassica juncea, Sesbania sesban, and Oryza sativa.

Main Methods:

• Purification of Rubisco enzyme from three different plant species: Brassica juncea, Sesbania sesban, and Oryza sativa.
• In vitro assays to measure Rubisco activity in the presence of varying concentrations of proline (up to 1 M) and NaCl.

Main Results:

• Proline significantly suppressed Rubisco activity, even at a concentration of 100 mM.
• The inhibitory effect of proline on Rubisco activity increased with higher proline concentrations.
• Rubisco activity decreased by approximately 50% in the presence of 1 M proline, irrespective of the plant species.
• Proline accelerated the salt-induced suppression of Rubisco activity.

Conclusions:

• This study demonstrates a novel negative effect of proline on Rubisco activity in higher plants.
• The findings challenge the long-held belief that proline solely acts as a protective osmolyte for enzymes under stress.
• Engineering plants for increased proline synthesis in chloroplasts warrants caution due to potential detrimental effects on Rubisco function.