determined, but may depend on the inherent sensitivity of specific mRNAs to oxidation. A decline in antioxidant capacity may facilitate the progression of programmed cell death signalling The down-regulation of several antioxidant genes is possibly due to mRNA degradation, and a similar decline was reported for catalase transcript levels during ageing of sunflower seeds. Expression of genes encoding antioxidants or proteins involved in repair of oxidative damage may be repressed as part of the PCD process, to allow cell death to proceed rapidly. He and Kermode reported that H2O2 production was required for activation of caspase-like proteases, and that antioxidants slowed the progression of cell death. Furthermore, inhibition of the degradation of catalase by the ubiquitin-proteasome pathway also slowed cell death, suggesting that selective degradation of antioxidant enzymes provides a means of regulating ROS production and PCD. Nitric oxide and protein nitrosylation may play a role in PCD by modulating antioxidant activity. Some oxidative stress-related genes were induced during seed ageing. For example ferritins, which bind iron and prevent ROS formation via the Fenton reaction, and thioredoxin h, which may be induced by oxidative stress, and is involved in the regulation of protein thiolation. The thioredoxin and glutaredoxin systems are reported 25617690 to be involved in apoptosis-like PCD. Reduced thioredoxin binds to apoptosis signal-regulating kinase 1 to inhibit downstream signalling pathways. Glutaredoxins and peroxiredoxins may also act in a similar way through interaction with the thioredoxin-binding domain of ASK1. Other negative regulators of ASK1 include 14-3-3 proteins. Two genes encoding 14-3-3-like proteins were IMR 1 web altered in expression during seed ageing, one was up-regulated and the other downregulated. ROS and oxidative stress cause dissociation of inhibitor proteins to activate ASK1 and initiate downstream MAP kinase signalling pathways leading to stress responses and PCD. and senescence. In this study BI-1 was upregulated after 8 days of seed ageing and is likely to play a role in suppressing PCD and maintaining seed viability. BI-1 is also up-regulated in response to water deficit, and suppresses ER stress-induced PCD. Conditions of oxidative stress disturb the protein folding environment of the ER lumen leading to accumulation of unfolded and misfolded proteins. This triggers the unfolded protein response, which mitigates ER stress by up-regulating the expression of chaperones and co-chaperones of the protein folding machinery. Binding protein is the most abundant chaperone of the ER and its up-regulation is considered to be a marker of ER stress. In this study BiP2 expression was induced after 12d, which indicates the occurrence of ER stress during seed ageing. Derlins may be involved in the targeting of misfolded proteins to the cytosol for ubiquitination. 24900262 Two derlin 2.2 genes were differentially expressed during seed ageing, indicating that ER-associated degradation is affected by ageing. The ubiquitin/26S proteasome system degrades misfolded proteins, 28 ubiquitin-proteasomal genes 
were altered in expression during ageing, indicating that this is a key ageing-responsive pathway. The ubiquitin-proteasomal system may play a vital role in the degradation of damaged proteins, such as those irreversibly oxidised during seed ageing. In addition, ubiquitination may promote PCD through the targeted degradation of negative regu
