As seeds age, their ability to germinate declines even if stored under optimal conditions. This loss of production potential is an obstacle to agricultural production and to germplasm and species conservation. Understanding how this occurs and potentially preventing or slowing the loss of seed vigour is of great interest to plant researchers.
Reactive oxygen species (ROS), released during normal metabolism of oxygen, are known to have significant roles in the process of seed germination. However, the ROS levels must be closely regulated in a relatively narrow range for germination to proceed. If ROS levels are too low seeds will never leave dormancy, and if they are too high then seeds will suffer excessive oxidative damage during seed imbibition and will be non-viable. Research has shown that mitochondrial function and antioxidant scavenging systems are critical for maintaining the right balance of ROS.
In a 2017 study Li et al. examined the impact of ROS on the mitochondrial protein profile in elm seeds as they aged. Using a previously developed protocol to simulate deterioration due to aging, they examined a number of markers of oxidative stress and mitochondrial function in the presence of either MitoTEMPO (a mitochondria-specific ROS scavenger) or methyl violet (an inhibitor of electron transport that ultimately increases ROS production). Oxidative stress was assessed using a variety of techniques including: determination of hydrogen peroxide levels (using Arbor Assays Hydrogen Peroxide Fluorescent Detection Kit, K034-F1), confocal microscopy with hydrogen peroxide and mitochondrial function staining, mitochondrial respiration assay, analysis of mitochondrial proteins, MDH activity and NADH oxidation assays, and immunoblotting for carbonylated proteins. The control elm seeds showed a typical response to the simulated accelerated aging protocol, with germination rates declining progressively beginning on day 2 of treatment through to day 5 when viability was completely lost. Pretreatment of the seeds with MitoTEMPO significantly inhibited hydrogen peroxide production at the early stages of seed aging, protecting seed germination rates on day 2, but the effect was shortlived and germination rates declined on days 3-5. With methyl violet pretreatment hydrogen peroxide levels were increased over control initially and accelerated the loss of vigour by approximately 1.5 days.
Ultimately the Li et al. study demonstrated the increase in ROS during seed storage may ultimately be linked to a decrease in function of antioxidant enzymes, and once the imbalance forms, the excess ROS can cause damage through multiple different pathways. Finding means to prevent the accumulation of excess ROS in seeds stored long term is a promising area of research to extend viability and protect valuable stores.