By Juan Alejandro Perdomo, Lancaster Photosynthesis group postdoc.
“The relative abundance of wheat Rubisco activase isoforms is post-transcriptionally regulated.”
Perdomo, Buchner & Carmo-Silva, Photosynthesis Research (Paper link.)
Rubisco, the most abundant enzyme in plants, and its chaperone Rubisco activase (Rca) are key enzymes for photosynthesis. We wanted to get a better understanding of how much the expression and amount of these photosynthetic proteins vary during the day.
To obtain data over a suitable timescale to show how Rubisco and Rca varies in accordance with the daily cycle, and I slept outside the growth chambers for two days. We’ve learnt a number of interesting things from these results.
We found that alongside the different regulatory properties of each of the three Rca isoforms in wheat, each of them also shows different daily patterns. TaRca1-β expression was almost null during the whole daily cycle. The redox-sensitive TaRca-α isoform was less abundant, representing 85% of the redox-insensitive TaRca2-β at the transcript level and 12.5% at the protein level. Therefore, our results show that in wheat plants TaRca2-β is the isoform with the highest expression and amount. These changes in Rca isoform abundance and ratio are imperative since the three wheat Rca isoforms activate Rubisco at different rates and differ in their sensitivity to temperature and changes in the ADP/ATP ratio.
We found that Rubisco amounts are pretty constant during the daytime. However, its abundance dropped by 20% by the middle of the night. Could this be due to a high Rubisco degradation during the night, which represents a big nitrogen sink for wheat plants? We don’t yet know, but our data is an interesting note to explore further in the future.
On the other hand, gene expression and protein amount did not correlate for Rubisco or any of the Rca isoforms during the daily cycle. This outcome suggests that the abundance of both enzymes and their isoforms is post-transcriptionally regulated, and that for these proteins transcript levels are insufficient to predict functional protein levels. Though we have seen evidence of temperature impacts on transcription and subsequent protein amounts for the more thermally stable TaRca1-β isoform.
This was an interesting experiment to do to better understand how the expression of the different isoforms of this key regulator of photosynthesis behaves during the day (even if it meant some long nights sampling!). As we gain more and more understanding of both the expression and biochemical variability of wheat Rubisco activase isoforms, it gives us further knowledge to apply to improving the efficiency of photosynthesis, and improving wheat production for more sustainable, secure food production.