LRajjou.I.Debeaujon/C.R.Biologies 331(2008)796-805 80 their metabolically quiescent state,dry seeds can endure Arabidopsis seed vigor in relation with an increased an auto-oxidation reactions leading to a progressive accu- tioxidant capacity [47]. mulation of ROS 3.2.Removal of toxic compounds (cyanide ROS are highly A.and an unds accumulat ysfunct nd s ndnceinbbogy the fate of the other toxic co documented in seeds A recent proteomic in for ro action [50].Previous d the tion highlighted that the abundance in Arabidopsis dry ence of a large number rofproeinsiavoledinoidB stress response in dry mature seeds and in germinat ed aging [57].Indeed ing seeds.For example,it is worth noting that several protein high undant shly har studies have documented that the production of ros nvigor (1.e during after-ripening,aging and germination entails var ge, ious but specific seed protein damages [39.44.46.51.521 45%).th ulation level of an However,oxidative metabolism in seeds is not neces sarily a deleterious proc topyruvate to sulfur ac s as it eems closely associ of th ceptors such as thiols or cyanide,presumably contribut. fr ing to cyanide detoxification [58.59).In plants,cyanide This deto d oped can be produced by varous ways such as hydrolysi on mec of cyanogen npounds (e.g.,cyanogen (APX) it can 6 ugh the rigin of lati and glutathione reductase (GSSGR)[39.481. Recen during dry storage and ge nation remains unknown data indicated that a large number of these enzymes in the control of cyanide content seems to play an impor volved in ROS detoxification are present in dry mature tant role in seed physiology.Thus,despite the fact that low concentrations of cyanide are beneficial for releas seeds and in germinating seeds [52].It is also worth not- ing that many of the oxidized (carbonylated)proteins ing seed dormancy and improving germination [63].its produc n is often ciated with found in Arabidopsis dry mature seeds and germinating mp ds 53 651 thi RO and support exist ochondrial as ht [5 ory eve bate (vitamin c)synthesis in plants 1671 Thus evanide Th accumulation during seed aging could reduce the effi d if ciency of plant cells to scavenge ROS generated during age,leading to a reduction of seed seed storage.Our results reve for the first time that vigor Inde s ass iated with rea conditions whe such damages do not reach a crit lev ctopoteacelni ntain cal level the detoxification potential of the seed can be det n a high structures [57] restored by a priming treatment,an invigoration treat. ment of seeds based upon their controlled imbibition 4.Cell repair and turnover and ultimately improving their vigor [55].On the other hand,when these damages accumulate to harmful lev. 4.1.DNA els,seeds lose their ability to control ROS and cannot endure the resta Accumulation of macromolecular damage,including a pr DNA damage and genomic instability,is considered as rce ng pro s68 the fra L. Rajjou, I. Debeaujon / C. R. Biologies 331 (2008) 796–805 801 their metabolically quiescent state, dry seeds can endure auto-oxidation reactions leading to a progressive accu￾mulation of ROS during storage. Oxidative stress can occur due to an imbalance in prooxidant and antioxidant levels. ROS are highly reactive and may modify and inactivate proteins, lipids, DNA, and RNA and induce cellular dysfunctions [49]. Proteins are major targets for oxidants as a result of their abundance in biological sys￾tems (particularly in seeds), and their high rate constants for reaction [50]. Previous studies indicated the pres￾ence of a large number of proteins involved in oxidative stress response in dry mature seeds and in germinat￾ing seeds. For example, it is worth noting that several studies have documented that the production of ROS during after-ripening, aging and germination entails var￾ious but specific seed protein damages [39,44,46,51,52]. However, oxidative metabolism in seeds is not neces￾sarily a deleterious process as it seems closely associ￾ated with completion of the germination process [48]. In order to control free radical-induced cellular dam￾age, seeds have developed a detoxification mechanism. This detoxification system includes a number of antiox￾idant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), monode￾hydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione peroxidase (GSHPx), and glutathione reductase (GSSGR) [39,48]. Recent data indicated that a large number of these enzymes in￾volved in ROS detoxification are present in dry mature seeds and in germinating seeds [52]. It is also worth not￾ing that many of the oxidized (carbonylated) proteins found in Arabidopsis dry mature seeds and germinating seeds [44] have previously been identified as thiore￾doxin targets in wheat (Triticum aestivum) seeds [53]. The results lend further support for the existence of a link between ROS and redox regulatory events cat￾alyzed by thioredoxin in seeds [54]. The detoxification potential of seeds might be strongly altered if these enzymes were to undergo some damage during seed storage, leading to a reduction of seed vigor. Under conditions where such damages do not reach a criti￾cal level, the detoxification potential of the seed can be restored by a priming treatment, an invigoration treat￾ment of seeds based upon their controlled imbibition and ultimately improving their vigor [55]. On the other hand, when these damages accumulate to harmful lev￾els, seeds lose their ability to control ROS and cannot endure the restart of metabolism that occurs during seed germination. This behavior is in agreement with a pre￾vious report showing that salicylic acid (an elicitor of plant defence [56]) treatment leads an improvement of Arabidopsis seed vigor in relation with an increased an￾tioxidant capacity [47]. 3.2. Removal of toxic compounds (cyanide) ROS are not the only toxic compounds accumulat￾ing during dry seed storage and germination. However, the fate of the other toxic compounds is very poorly documented in seeds. A recent proteomic investiga￾tion highlighted that the abundance in Arabidopsis dry seeds of the β-mercaptopyruvate sulfurtransferase en￾zyme (MST) is correlated with seed aging [57]. Indeed, this protein is abundant in freshly harvested seeds of high vigor (i.e., characterized by a maximum germina￾tion percentage, Gmax, of 100%). However, in 7-year￾old seeds (Gmax = 45%), the accumulation level of this protein showed an important decline. MST catalyzes the transfer of sulfur from mercaptopyruvate to sulfur ac￾ceptors such as thiols or cyanide, presumably contribut￾ing to cyanide detoxification [58,59]. In plants, cyanide can be produced by various ways such as hydrolysis of cyanogenic compounds (e.g., cyanogenic glycosides and cyanolipids), decomposition of glucosinolates [60], and it can also be released as a by-product of ethylene (a gaseous plant hormone [61]) biosynthesis [62]. Al￾though the exact origin of cyanide accumulation in seed during dry storage and germination remains unknown, the control of cyanide content seems to play an impor￾tant role in seed physiology. Thus, despite the fact that low concentrations of cyanide are beneficial for releas￾ing seed dormancy and improving germination [63], its production is often associated with deleterious mecha￾nisms and must therefore be controlled. For example, cyanide can inhibit the activity of heme proteins as peroxidases [64,65] and catalases [66]. Moreover, this molecule is a potent inhibitor of mitochondrial ascor￾bate (vitamin C) synthesis in plants [67]. Thus, cyanide accumulation during seed aging could reduce the effi- ciency of plant cells to scavenge ROS generated during seed storage. Our results revealed for the first time that a loss in seed vigor is associated with a decreased level of MST, indicating that seeds must maintain a high ability to detoxify cyanide to protect cellular structures [57]. 4. Cell repair and turnover 4.1. DNA Accumulation of macromolecular damage, including DNA damage and genomic instability, is considered as a driving force for the aging process [68]. It is worth noting that in the framework of seed germination, cell