**12. Conclusions**

Seed priming is an old empirical strategy used since centuries by farmers, and since decades by seed companies, to improve germination processes in cultivated plant species. The underlying mechanisms involved in this positive impact of pre-sowing treatments remained obscure for a long time. The present review aimed to summarize recent information provided by various tools allowing the identification of molecular cues conditioning priming efficiency.

Data obtained from molecular approaches applied to some well-known plant species (rice, rapeseed, tomato, etc.) are now available. The role of genes associated to metabolic and cell cycle events now starts to be deciphered, mainly those encoding for translational components such as ribosomal subunits, translational initiation factor, enzyme involved in carbon metab‐ olism, histones, and transcription factors. A putative epigenetic basis of priming effects should also be considered. Some authors identified enzyme activity changes in relation to priming while others also reported numerous changes in the seed storage protein. Priming has also been reported to increase proteins related to the cell cycle activities such as α- and δ-DNA polymerase. Protecting proteins like dehydrins or HSP is expected to assume protective functions during the dehydration step. Similarly, proteins involved in water transport, cell wall modification, cytoskeletal organization, and cell divisions, may be to some extent regulated during priming. Gene expression and enzyme activities involved in osmocompatible solute synthesis may be of primary importance to regulate tissue protection during the dehydration step and water fluxes during the final germination phase. Measurements of water uptake by primed seeds suggest a reduction in the lag time of imbibition. Water uptake and its subsequent cell-to-cell movement during germination might be controlled by aquaporins and expression of the corresponding genes constitutes a specific target of the priming treat‐ ment.

for PEG and sometimes requires the simultaneous use of pesticide, although the impact of these compounds on the priming efficiency remains unknown. After priming treatment, seeds are dried back to their initial moisture content, but this dehydration phase is usually performed rapidly and occurs faster than classical dehydration of maturing seeds. It has been hypothe‐

The major drawback of priming is that it reduces the longevity of primed seeds as compared with the nonprimed seeds [190–192]. Storability of primed seed material is consequently reduced, and this results in higher costs for material maintenance for farmers and seed companies. The loss of viability, however, appears quite variable depending on the species, cultivars, and seed lots. In extreme cases, priming-induced advantages may even disappear after only 14 d of storage and the obtained seedling may then perform worse than those issued from unprimed seeds [193]. One of the limitations of these studies, however, is that experi‐ ments are commonly performed on artificially aged seeds using short-term exposure to high temperature in a moist environment, which is not necessarily fulling relevant from a real aging process. Some studies, using classical long-term strategies, reported that longevity is not necessarily affected or may even be increased by priming treatments [194]. Hussain et al. [195] recently demonstrated that post-priming temperature plays a key role in maintenance of seed longevity, which is indeed rapidly compromised in rice seeds maintained at room temperature while it remains intact when the material is stored at 4°C. According to these authors, the deleterious effect observed at 25°C storage was related to hampered starch metabolism in primed seeds. It was also suggested that maintenance of the seed longevity at 4°C may be due to a high viscosity that strongly reduces molecular mobility in the cytoplasm and thus limits the impact of deteriorative process even in seeds exhibiting low water content [196]. Oxygen during storage may trigger metabolic processes in primed seeds, which did not re-establish a true quiescent stage after dehydration when stored at room temperature while oxygen had no

In some cases, repeat priming treatment after storage may partly remove the damaging effect on seed viability [193] while, in other cases, such a loss is permanent and not reversible [194]. Whatever, the fact that an additional treatment may be required to restore full germination

Seed priming is an old empirical strategy used since centuries by farmers, and since decades by seed companies, to improve germination processes in cultivated plant species. The underlying mechanisms involved in this positive impact of pre-sowing treatments remained obscure for a long time. The present review aimed to summarize recent information provided by various tools allowing the identification of molecular cues conditioning priming efficiency. Data obtained from molecular approaches applied to some well-known plant species (rice, rapeseed, tomato, etc.) are now available. The role of genes associated to metabolic and cell cycle events now starts to be deciphered, mainly those encoding for translational components

potential represents an additional cost and source of variability.

sized that this brutal desiccation procedure alters the beneficial effect of priming [2].

28 New Challenges in Seed Biology - Basic and Translational Research Driving Seed Technology

influence at low temperatures [195].

**12. Conclusions**

Among the different hypothesis proposed to explain the biochemical basis for germination improvement, DNA replication and cell cycle advancement during priming treatment as well as synchronization of the cell cycle at the G2 phase are supported by some experimental evidences. DNA synthesis is involved during priming treatment itself but also during postgerminative events. Changes are also observed with the modification of the membrane structure and reorganization of mitochondrial integrity. Activation of antioxidative properties by priming treatment may also explain the improved behavior of plant material, especially when final germination and/or growth occur under stress condition.

The priming-induced decrease of the storage capacity is a major limitation for the application of the priming technique by seed companies. Partial vacuum storage may be useful for extending the longevity of primed seeds. Improved longevity may be related to enhanced antioxidative activity that minimizes the accumulation of total peroxide during long-term storage. Another challenge for private seeds company is to identify appropriate treatments able to restore vigor of old dry seed lots in order to increase their mean percentage of germination to values compatible with commercial purposes.

If priming may undoubtedly be considered as a valuable strategy to improve stand establish‐ ment, its impact on final yield and crop production has not be always confirmed. Most studies devoted to "omics" approaches of seed priming are performed on young seedling cultivated under fully controlled environmental conditions. The link between seedling behavior in plant growth chambers and the adult plant performance in field conditions is far from being clear. As a consequence, there is an urgent need to focus on transcriptomic, proteomic, and metab‐ olomics of adult plants issued from primed seeds, especially at the reproductive stage, in order to assess the long-term impact of priming on cultivated plants throughout the plant cycle.

In cultivated plant species, a given priming treatment also has contrasting effects on various cultivars. It may be hypothesized that the ability to respond to priming treatment might be genetically controlled but, to the best of our knowledge, no data are available concerning this important aspect. Thus, further progresses are needed not only to identify the set of genes that are regulated by priming, but also the set of genes that putatively regulate pri‐ ming response and efficiency themselves.
