On the role of the enzyme peptide methionine sulfoxide reductase in the response of Arabidopsis plants to oxidative stress

Reactive oxygen species (ROS) are produced during cellular metabolism. Low steady state levels of ROS are maintained by enzymatic and non-enzymatic mechanisms. Under extreme environmental conditions, ROS production can overwhelm the detoxification systems and oxidative stress results. ROS can oxidize Met residues to methionine sulfoxide (MetSO) thereby inactivating enzymes or tagging proteins for hydrolysis. The enzyme peptide methionine sulfoxide reductase (PMSR) reduces MetSO to Met, repairing damaged proteins. The MetSO-PMSR-Met system has been referred to as the "last chance" defense mechanism against ROS. The role of PMSR in protection against oxidative stress is well recognized in bacteria, yeast and animal cells; however the function of PMSR in plants is yet to be fully established. The objective of this research was to determine the role of PMSR in the response of plants to oxidative stress. One family of PMSR proteins, PMSRA of Arabidopsis was used as a model system. Arabidopsis has five PMSRA genes being expressed in different organs. PMSRA1 to PMSRA3 are cytosolic proteins, PMSRA4 is plastidic and PMSR5 is secreted. PMSRA4 transcript is the most abundant; PMSRA2, PMSRA3 and PMSRA5 are highly expressed in roots and stems; PMSRA1 is abundant in flower buds; and PMSRA4 is up-regulated by ozone, methyl viologen, cercosporin and high light. Kinetic analysis of recombinant enzyme showed that PMSRA4 had twice the kcat and was four times more efficient (kcat/Km) than PMSR3. Transgenic plants over-expressing PMSRA3 and PMSRA4 were more resistant to methyl viologen and ozone. PMSRA3 over-expressing plants were more resistant to cercosporin and PMSRA4 over-expressing plants more resistant to high light. Transgenic plants with low levels of PMSRA4 were susceptible to oxidative stress in the chloroplast. Treatments that lower PMSRA4 levels in wild-type plants, such as Mn2+ excess, increased the susceptibility of plants to oxidative stress damage (e.g. a modest 2-day water deprivation in plants grown under Mn2+ excess induces toxicity). PMSRA activity is essential for plant survival to oxidative stress. It appears that specific isozymes protect discrete cellular compartments. PMSRA4 is the most abundant and most efficient PMSRA in Arabidopsis and protects against oxidative stress derived from different cell compartments.