Loss of RNA integrity precedes loss of viability in dry-stored seeds

How long can a seed survive? One way of determining seed longevity could be by identifying chemical changes inside the seed. These chemical changes could precede, and may be linked to, seed mortality. The standard assessment of seed viability is by germination proportion. This test reveals seed longevity by showing that germination proportion declines. However, it’s not possible to use this test to tell when is significantly compromised. Fleming and colleagues examine if assessing molecular integrity, such as RNA integrity, is more informative about changes in seed health that precede viability loss.

A graph showing declining germination — Proportion of seeds germinating in seed lots harvested in the indicated years and stored at 5 °C and 3–50 % RH. Image: Fleming *et al*.

The authors used a collection of seeds stored at 5°C and 35–50 % relative humidity for 1–30 years to test how germination proportion and RNA integrity are affected by storage time.
Similarly, a collection of seeds stored at temperatures from −12 to +32 °C for 59 years was used to manipulate ageing rate. The scientists calculated RNA integrity using total RNA extracted from one to five seeds per sample, analysed on an Agilent Bioanalyzer.

The authors found decreased RNA integrity before viability loss. Correlation of RNA integrity with storage time or storage temperature was negative and significant for most species tested. The exceptions were watermelon, for which germination proportion and storage time were poorly correlated, and tomato, which showed electropherogram anomalies that affected RNA integrity number calculation. Temperature dependencies of ageing reactions were not significantly different across species or mode of detection. The overall correlation between germination proportion and RNA integrity, across all experiments, was positive and significant.

The team conclude that changes in RNA integrity when ageing is asymptomatic can be used to predict onset of viability decline. RNA integrity appears to be a metric of seed ageing that is broadly applicable across species. Time and molecular mobility of the substrate affect both the progress of seed ageing and loss of RNA integrity.