**8. Conclusion**

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

O2

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**.

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

) Alleviation of seed dormancy Wang et al. [104]

) Breakdown of polysaccharides Schweikert et al. [106]

) Plant defense response Wisniewski et al. [24]

O2 Response to wounding Oroczo-Cardenas et al. [74]

O2 Seed germination-ABA levels Barba-Espin et al. [102]

<sup>−</sup> Cell growth by auxin Schopfer et al. [55] OH<sup>−</sup> Cell wall loosening Müller et al. [107]

O2 Lateral root formation Chen et al. [101]

O2 Programmed cell death de Jong et al. [8]

<sup>−</sup> Cell death Doke et al. [22]

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

O2 Seed germination via pentose phosphate pathway Barba-Espin et al. [103]

production has been demonstrated along

on transcriptome have been

O2

[100]. The involvement of a diamine oxidase in H2

comprehensive view of ROS in seed biology.

O2

Hydrogen peroxide (H2

174 Advances in Seed Biology

Hydroxyl radical (OH<sup>−</sup>

−

Superoxide (O2

H2

O2

H2

H2

O2

H2

H2

H2

O2

As shown above, the effects of ROS, and more particularly H<sup>2</sup>

**ROS molecule Physiological trait Reference**

O2 Somatic embryogenesis Cui et al. [17]

<sup>−</sup> Survival and germination seeds Roach et al. [105]

ROS and antioxidants play important roles in seed biology. In seed life, ROS are involved in all the stages of seed development, from embryogenesis to germination. ROS can react with the majority of biomolecules, resulting in cellular damage. In developing or germinating seeds, major amounts of ROS are generated, which are highly toxic and thus generate oxidative stress in seed cells. Plants have developed an array of defense strategies (antioxidant system) to cope up with oxidative stress. Conversely, ROS are suggested to have beneficial effects in growth and development of seeds, and are considered as part of a signaling network involving in numerous regulatory components of seed development. For example, H2 O2 is known to promote seed germination of cereal plants. The antioxidant system reported to play an important role in embryogenesis and cell growth. Ascorbate content is proposed to influence cell growth by modulating the expression of genes involved in hormonal signaling pathways. The above findings show that, these dual effects of ROS in seed biology are very interesting subjects and need further examinations for determination of the roles of ROS in seed physiology. Depending on the progress that has been required in seed tissue physiology, cellular production sites of ROS and their diffusion within the cell are established. Investigations in this field encourage to enlighten the cellular mechanisms involved in acquisition of the desiccation tolerance, germination and alleviation of dormancy. Finally, ROS signaling transduction pathway in seeds, from sensing to changes in gene expression, is not fully understood yet. Therefore, there is still a domain to be examined in future studies dealing with seed biology and ROS, which concerns the direct effects of these compounds on gene expression. Analyses of gene expression using the novel methods will be of help in elucidating the mechanisms underlying the interplay of ROS with hormones and their cross-talk in seed germination and development, providing a challenge for future research in this area.
