What if the seeds don’t grow?
Remember the millennium? Maybe you’re too young. (Or possibly you just went to a better party than I did.) Aside from wasting silly amounts of money building stupid domes, one of the better ideas that quite a few people had was the creation of millennium seedbanks and as way of ensuring the prosperity (and full bellies) of future generations by preserving the germplasm of plant species in long term storage. And for flowering plants, that means seeds. But what if, when we really need them at some point in the future, the seeds don’t grow?
Seeds stored for prolonged periods are subjected to severe oxidative damage, caused by the progressive accumulation of reactive oxygen species (ROS) and that loss of seed viability and reduced germination represent the undesired consequences of ageing. Significant factors in seed longevity are the level of DNA damage and the DNA repair response, the amount of non-enzymatic antioxidants and activity of ROS-scavenging enzymes. In order to preserve the high seed viability at the pre-emergence step, both the DNA repair functions and the overall antioxidant activities must be kept at an appropriate level in the embryo. Different DNA repair pathways are activated during the early phase of seed imbibition. The ability to carry out ROS scavenging, expressed as the seed antioxidant potential, is a critical requirement to withstand stress and improve germination. The cell antioxidant systems prevent ROS attack but when ROS production exceeds the capacity of the antioxidant machinery, oxidative injury takes place.
Factors such as temperature and humidity are positively correlated with seed ageing and they must be strictly controlled during seed manipulation for long-term conservation in seed banks. To date, germination tests represent the most reliable method to assess seed viability, although it is a time-consuming and labour-intensive operation. Novel low-cost and equally reliable methods are required, which might speed up the seed viability analysis. Molecular and biochemical markers of seed ageing might be used for these purposes. A deeper understanding of the complex network of molecular events which control seed longevity is, however, required in order to select appropriate markers providing information on deterioration and germination potential of seed stocks collected for bank storage.
A new paper in Annals of Botany investigates reliable markers of seed deterioration. The response to DNA damage induced by artificial ageing was compared in seeds of Silene vulgaris and S. acaulis inhabiting low- and high-altitude locations of Northern Italy. Previous investigations have demonstrated that these species differ in seed longevity, making them useful candidates to assess novel markers of seed deterioration. An in-depth investigation which included ROS accumulation profiles, antioxidant capacity and telomere length was carried out, focusing mainly on dry seeds and seeds subjected to rehydration. A positive impact of the reported results could be envisaged within a relatively short time, since specific suggestions can be derived for improving the rehydration protocol of seeds from a high-altitude location.
DNA profiling, telomere analysis and antioxidant properties as tools for monitoring ex situ seed longevity. (2013) Annals of Botany 111 (5): 987-998. doi: 10.1093/aob/mct058
The germination test currently represents the most used method to assess seed viability in germplasm banks, despite the difficulties caused by the occurrence of seed dormancy. Furthermore, seed longevity can vary considerably across species and populations from different environments, and studies related to the eco-physiological processes underlying such variations are still limited in their depth. The aim of the present work was the identification of reliable molecular markers that might help in monitoring seed deterioration. Dry seeds were subjected to artificial ageing and collected at different time points for molecular/biochemical analyses. DNA damage was measured using the RAPD (random amplified polymorphic DNA) approach while the seed antioxidant profile was obtained using both the DPPH (1,1-diphenyl, 2-picrylhydrazyl) assay and the Folin–Ciocalteu reagent method. Electron paramagnetic resonance (EPR) provided profiles of free radicals. Quantitative real-time polymerase chain reaction (QRT-PCR) was used to assess the expression profiles of the antioxidant genes MT2 (type 2 metallothionein) and SOD (superoxide dismutase). A modified QRT-PCR protocol was used to determine telomere length. The RAPD profiles highlighted different capacities of the two Silene species to overcome DNA damage induced by artificial ageing. The antioxidant profiles of dry and rehydrated seeds revealed that the high-altitude taxon Silene acaulis was characterized by a lower antioxidant specific activity. Significant upregulation of the MT2 and SOD genes was observed only in the rehydrated seeds of the low-altitude species. Rehydration resulted in telomere lengthening in both Silene species. Different seed viability markers have been selected for plant species showing inherent variation of seed longevity. RAPD analysis, quantification of redox activity of non-enzymatic antioxidant compounds and gene expression profiling provide deeper insights to study seed viability during storage. Telomere lengthening is a promising tool to discriminate between short- and long-lived species.