**7. ROS and seed development**

Seed development consists of embryogenesis, reserve accumulation and maturation/drying on the mother plant, leading from a zygotic embryo to a mature, quiescent seed. During maturation seeds undergo a period of desiccation where water content is reduced and the embryo is at a state of quiescence [10]. ROS are involved in final stage of seed development, in desiccation in tolerance. A dramatic loss of water becomes during desiccation or maturation phase which requires cellular adaptative mechanisms, at this stage ROS scavenging plays a key role, for allowing seed survival [10]. Recently, LEA (late embryogenesis abundant)-related proteins which are cited as accumulating proteins during drought conditions are correlated with desiccation tolerance, but their biological functions remain unclear [90]. A group-2 LEA class of proteins has been suggested to act as free-radical scavengers [91], emphasizing the importance of ROS scavenging in dehydration tolerance mechanisms. In developing or germinating seeds, the active mitochondria are probably one of the major sources of ROS, generating superoxide, and subsequently H<sup>2</sup> O2 [14, 32]. ROS is also generated in chloroplasts in the beginning of seed development, but they rapidly become nonfunctional [15, 63]. O2 •− and H2 O2 are produced in peroxisomes, and in seeds, glyoxysomes, which is a particular type of peroxisomes involving in mobilization of lipid reserves [15, 63]. High amounts of H2 O2 are produced in glyoxysomes resulting from the activity of enzymes such as glycolate oxidase. H2 O2 is known to promote seed germination of cereal plants, and exogenously applied H2 O2 is shown to ameliorate seed germination in many plants [7, 92]. Ascorbic acid is the most important reducing substrate for removal of H2 O2 , acting as an antioxidant, in plant cells. It is reported that ascorbic acid suppresses the germination of wheat seeds, recently [93]. In plant cells, ascorbate peroxidase (APX) and catalase (CAT) that are involved in scavenging H2 O2 are localized at the site of H2 O2 generation [93] (**Figure 2**). H2 O2 is mentioned to induce expression of many genes, coding defense-related proteins, transcription factors, phosphatases, kinases and enzymes involving in ROS synthesis or degradation [37, 54, 56, 79, 80] (**Figure 2**).

Seed filling is also associated with the high potential of the H<sup>2</sup> O2 detoxification machinery, mainly due to APX and CAT activities [94]. It is suggested that cellular membranes in germinating tissues are vulnerable to damage from desiccation [10, 69]. After the loss of desiccation tolerance several products of peroxidized lipids are accumulated [45], and activated forms of oxygen are generated through xanthine oxidase [35, 48, 50]. Some studies have also suggested

**Figure 2.** Production and functions of H2 O2 in seed biology [37].

important water-soluble antioxidant and is synthesized from the amino acids glycine, glutamate, and cysteine, which directly scavenges ROS such as lipid peroxides, and also plays a vital role in xenobiotic detoxification [85–88]. Research suggests that glutathione and vitamin C work interactively to quench free radicals and that they have a sparing effect upon each other [85–87]. Glutathione peroxidases (GPX) may also catalyze the reduction of H2

**Figure 1.** Main detoxifying mechanisms in plants. CAT, catalase; SOD, superoxide dismutase; APX, ascorbate peroxidase; MDHAR, monodehydroascorbate reductase; DHAR, dehydroascorbate reductase; GR, glutathione reductase; ASA, ascorbate; MDHA, monodehydroascorbate; DHA, dehydroascorbate; GSSG, oxidized glutathione; GSH, reduced glutathione; α-tocH, α-tocopherol; α-toc, α-tocopheryl; LOOH, lipid peroxide; LOO, lipid radical (Halliwell-Asada

and hydroperoxides [85–88]. Polyphenol oxidase (PPO), the function of this antioxidant system is to scavenge the toxic radicals produced during oxidative stress and thus help the plants to survive through such conditions. Various compounds, such as polyphenols,

Seed development consists of embryogenesis, reserve accumulation and maturation/drying on the mother plant, leading from a zygotic embryo to a mature, quiescent seed. During maturation

flavonoids and peroxiredoxins [89] also have a strong antioxidant function.

**7. ROS and seed development**

cycle).

172 Advances in Seed Biology

O2

that ROS metabolism might also be important during initial embryogenesis [17, 95]. During embriogenesis, metabolic activity and mitochondrial respiration are increased, suggesting that developing embryos have the potential to generate significant amounts of ROS [17, 95]. The antioxidant ascorbate system reported to play an important role in embryogenesis and cell growth [41, 85]. Ascorbate content proposed to influence cell growth by modulating the expression of genes involved in hormonal signaling pathways [96]. Totipotency also related to antioxidant system, because of high ROS content and repressed expression of totipotency [97]. Conversely, ROS have beneficial effects in growth and development of plants. Seed germination requires release from dormancy. Treatment of dormant seeds with methylviologen (as a generator of ROS including OH•) is reported to break dormancy [98]. Hydroxyl radicals are also postulated to be involved in cell wall extension during cell growth, and auxin-induced increases in OH• production is speculated to be involved in cell wall elongation, stiffening, and lignification depending on the concentration of auxin [55, 99]. Hydrogen peroxide is suggested to participate in lignin deposition in the cell walls in a peroxidase-catalyzed reaction [100]. The involvement of a diamine oxidase in H2 O2 production has been demonstrated along with lignin deposition in the chalazal cells, in developing barley grains, in developing barley grains [100]. Production of ROS and their release in the surrounding medium are supposed to play a part in protecting the embryo against pathogens during seed imbibition [99]. Some of the selected published reviews on the dual roles of ROS in seed biology are listed in **Table 3**.

As shown above, the effects of ROS, and more particularly H<sup>2</sup> O2 on transcriptome have been widely studied [56]. However, up to date, there is no information available establishing a direct link between the changes in ROS content and gene expression during seed germination and development. Further experiments in this area, will be highly informative for getting a comprehensive view of ROS in seed biology.


**Table 3.** Published reviews on the dual role of ROS in seed physiology [34].
