The relative abundance of wheat Rubisco activase isoforms is post-transcriptionally regulated

Diurnal rhythms and light availability affect transcription-translation feedback loops that regulate the synthesis of photosynthetic proteins. The CO2-fixing enzyme Rubisco is the most abundant protein in the leaves of major crop species and its activity depends on interaction with the molecular chaperone Rubisco activase (Rca). In Triticum aestivum L. (wheat), three Rca isoforms are present that differ in their regulatory properties. Here, we tested the hypothesis that the relative abundance of the redox-sensitive and redox-insensitive Rca isoforms could be differentially regulated throughout light-dark diel cycle in wheat. While TaRca1-beta expression was consistently negligible throughout the day, transcript levels of both TaRca2-beta and TaRca2-alpha were higher and increased at the start of the day, with peak levels occurring at the middle of the photoperiod. Abundance of TaRca-beta protein was maximal 1.5 h after the peak in TaRca2-beta expression, but the abundance of TaRca-alpha remained constant during the entire photoperiod. The redox-sensitive TaRca-alpha isoform was less abundant, representing 85% of the redox-insensitive TaRca-beta at the transcript level and 12.5% at the protein level. Expression of Rubisco large and small subunit genes did not show a consistent pattern throughout the diel cycle, but the abundance of Rubisco decreased by up to 20% during the dark period in fully expanded wheat leaves. These results, combined with a lack of correlation between transcript and protein abundance for both Rca isoforms and Rubisco throughout the entire diel cycle, suggest that the abundance of these photosynthetic enzymes is post-transcriptionally regulated.

Automated summary

The relative abundance of redox-sensitive and redox-insensitive Rca isoforms is differentially regulated throughout the light-dark diel cycle in wheat, with TaRca2-beta and TaRca2-alpha transcript levels higher at the start of the day. Meanwhile, the abundance of Rubisco decreases by up to 20% during the dark period in fully expanded wheat leaves, suggesting post-transcriptional regulation of these photosynthetic enzymes.