Understanding the longevity of rice seeds

How long can seed last in storage? It depends on the seed. Research by Jae-Sung Lee and colleagues has taken a closer look at the seeds. Rice seeds do not respond identically. By examining closely related seeds, and the variation in their viability, they hope to get a better understanding of the molecular basis of longevity.

Fiona Hay, now working at Aarhus University in Denmark, explained why the team worked on this research: “Seed longevity is a trait that is of particular importance for genebank managers. They want to ensure that species or crop varieties don’t disappear, as they may contain unique and/or vital genetic traits which can be used to solve current and future problems.

“Understanding the relative longevity of different seed lots will help improve genebank management procedures. However, seed longevity is also important for anyone involved in producing, storing, trading or using seeds.”

“Seeds that survive longer in storage will also maintain high levels of vigour for longer, ultimately resulting in improved seedling emergence, crop establishment and yield.”

Rice is a particular problem, because of the biology of the plant, said Dr Hay. “Since rice seeds are quite short-lived compared with seeds of other major crops, and because they are often produced and stored under warm, humid conditions (which accelerate the rate of loss of viability), knowing which genes may be responsible for high seed longevity opens up the possibility of being able to improve the longevity and vigour of commercial rice cultivars, and therefore improve the livelihood of rice farmers.”

Dr Hay said that seeds can work as a time capsule: “They preserve the genetic material of one generation through time – and perhaps for a very long time if they are stored under appropriate conditions (with low moisture and low temperature).” But while a plant produces a lot of seeds, not all of them survive into the future. “A fascinating feature of seed longevity is that the progress of loss in viability can be modelled in a very consistent way, using the normal distribution. Bringing these two aspects together makes seed longevity an appealing trait to study.”

“One of the things, we were particularly interested in doing, was to truly characterize the longevity of all the seed lots we produced for the study, by determining full seed survival curves,” said Hay. However, many different factors can change the longevity of seed.

“Seed longevity is a highly plastic trait – it can be influenced by factors such as the environment during seed development, how and when the seeds are harvested, and how the seeds are processed and dried. We tried to take one of these factors into account, by harvesting seeds at different times (days after heading), and in the genome-wide analysis, we also considered the moisture content of the seeds at harvest. However, there is still a lot to do to fully understand how seed longevity is determined by these various factors – and the underlying mechanisms.”

The International Rice Genebank at the International Rice Research Institute, where the research was carried out, holds the most extensive collection of rice germplasm in the world, most of which is made freely available to researchers under the International Treaty on Plant Genetic Resources for Food and Agriculture. The research by Lee and colleagues will help the management of the collection. Hay says that other sites could also benefit. “The candidate markers could be used by any rice genebank to potentially identify varieties with seeds that are particularly short- or long-lived in storage. Viability monitoring intervals could then be customized by variety.”

At the moment, the results are limited to one particular variety group of rice, Indica. Hay expects future research to increase knowledge of other kinds of rice. “We have already started similar studies on other variety groups. It will be interesting to see whether we find the same and/or perhaps, other genes. Validation studies are also needed to confirm that these candidate genes are important. We expect at least some of the underlying mechanisms conferring high seed longevity to be consistent across species, so those working on seed longevity and vigour in other crops should also find the paper interesting.”