Minimal influence of G-protein null mutations on ozone-induced changes in gene expression, foliar injury, gas-exchange and peroxidase activity in Arabidopsis thaliana L

Authors: Booker, Fitzgerald; Burkey, Kent; Morgan, Patrick; Fiscus, Edwin; JONES, ALAN - University Of North Carolina

Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/2011
Publication Date: 4/1/2012
Citation: Booker, F.L., Burkey, K.O., Morgan, P.B., Fiscus, E.L., Jones, A. 2012. Minimal influence of G-protein null mutations on ozone-induced changes in gene expression, foliar injury, gas-exchange and peroxidase activity in Arabidopsis thaliana L. Plant Cell and Environment. 35:668-681.

Interpretive Summary: Plant biochemical and physiological responses to the air pollutant ozone are influenced in part by intracellular signaling processes that lead to changes in genetic and metabolic regulation. The heterotrimeric G-protein complex is known to be involved in signaling processes and was thus considered a potential target in understanding ozone response mechanisms. To investigate this, mutant lines of the model plant Arabidopsis thaliana L. lacking the alpha and beta subunits of the G-protein complex and Col-0 wild-type plants were evaluated with a suite of assays indicative of detrimental ozone effects on plants. Plants were treated with a range of ozone concentrations (5, 125, 175 and 300 ppb) for one and two days in controlled environment chambers. Transcript levels of the beta subunit, but not the alpha subunit, increased in Col-0 in the 125 ppb ozone treatment. However, differences between null mutant and Col-0 plants were minor for ozone-induced changes in gene expression, visible injury, relative ion leakage, net photosynthesis, stomatal conductance, peroxidase activity and ascorbic acid concentrations. Stomatal conductance rates in Arabidopsis leaves were relatively low compared with a number of other crop species. Calculations based on conductance rates of leaves in the 125 and 175 ppb ozone treatments indicated that ozone uptake in these treatments was comparable to that in many plants exposed to lower ozone concentrations, thus extending the applicability of these experiments to crops. While G-proteins are involved in ozone response mechanisms, their influence on processes leading to ozone injury and growth suppression in our experiment was minimal. They appear to have limited potential for genetic modification of plant tolerance to ozone. In addition, measurements of peroxidase enzymes and ascorbic acid located inside and outside leaf cells suggest that our current understanding of ozone response mechanisms warrants reassessment.

Technical Abstract: Ozone uptake by plants leads to an increase in reactive oxygen species (ROS) in the intercellular space of leaves and induces signalling processes reported to involve the membrane-bound heterotrimeric G-protein complex. Therefore, potential G-protein-mediated response mechanisms to ozone were compared between Arabidopsis thaliana L. lines with null mutations in the alpha- and beta-subunits (gpa1-4, agb1-2 and gpa1-4/agb1-2) and Col-0 wild-type plants. Plants were treated with a range of ozone concentrations(5, 125, 175 and 300 nL L-1) for 1 and 2 d in controlled environment chambers. Transcript levels of GPA1, AGB1 and RGS1 transiently increased in Col-0 exposed to 125 nL L^-1 ozone compared with the 5 nL L^-1 control treatment. However, silencing of alpha and beta G-protein genes resulted in little alteration of many processes associated with ozone injury, including the induction of ROS-signalling genes, increased leaf tissue ion leakage, decreased net photosynthesis and stomatal conductance, and increased peroxidase activity, especially in the leaf apoplast. These results indicated that many responses to ozone stress at physiological levels were not detectably influenced by alpha and beta G-proteins.