Protein turnover in plants-cost of synthesis, repair, and crop engineering opportunities.

Protein lifespan is shaped by plant developmental stage and cellular requirements; both are fairly well understood processes. Beyond regulated degradation, chemical damage is another major cause of protein turnover, yet it is an understudied area in plant biology and crop engineering. The associated respiratory costs limit harvestable crop yield because the degradation and resynthesis of inactivated proteins consume carbon that then cannot be used for biomass production. Approximately half of the carbon fixed by plants is lost through respiration; of that, protein turnover consists of ∼25% to ∼40%. In this review, we examine the importance of spontaneous protein modifications arising under both optimal and environmental stress conditions; the latter can elevate protein turnover costs, negatively impacting crop yield. While homeostasis of most proteins following damage is likely maintained via de novo synthesis, some molecular mechanisms exist in cells to prevent and repair protein damage. They offer a substantial energetic advantage over resynthesis. We illustrate these benefits with specific examples and quantify their associated costs. Finally, understanding the causes, energetic consequences, and repair mechanisms of protein damage can inform strategies to improve crop performance. We suggest potential protein targets and synthetic biology approaches to be exploited for future crop engineering.