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ABSTRACT

 

Reactive oxygen species (ROS) are continuously produced by the metabolically active cells of seeds, and apparently play important roles in biological processes such as germination and dormancy. Germination and ROS accumulation appear to be linked, and seed germination success may be closely associated with internal ROS contents and the activities of ROS-scavenging systems. Although ROS were long considered hazardous molecules, their functions as cell signaling compounds are now well established and widely studied in plants. In seeds, ROS have important roles in endosperm weakening, the mobilization of seed reserves, protection against pathogens, and programmed cell death.ROS may also function as messengers or transmitters of environmental cues during seed germination. Little is currently known, however, about ROS biochemistry or their functions or the signaling pathways during these processes, which are to be considered in the present article.

 

Cite this article:  Nithya, N (2022) Nano Urea: The signalling role of Reactive Oxygen Species during Seed Germination. Food and Scientific Reports, 3(8):24-26. 

1.    Introduction

                Reactive Oxygen Species (ROS) play a key role in various events of seed life. Reactive oxygen species (ROS) are produced in the seed as a result of metabolism and play a significant role in seed germination. Weakening of the endosperm is a prerequisite for the initiation of seed germination and is driven by various internal and external factors. The decay of the endosperm is directly linked to the production of ROS in response to the availability of external environmental signals. Reactive oxygen species are from the reduction of oxygen which gives rise to superoxide (O₂‑.), hydrogen peroxide (H₂O₂), hydroxyl radical (HO^.) and singlet oxygen (¹O₂).

In seeds, ROS production has been considered for a long time as being very detrimental, since the works dealing with ROS were mainly focused on seed ageing or seed desiccation, two stressful situations which often lead to oxidative stress. Numerous recent works have nevertheless brought new lines of evidence showing that the role of ROS in seeds is not as unfavourable as it was considered previously. Contradictory to earlier believes, it now appears that ROS would play a key signalling role in the achievement of major events of seed life, such as germination or dormancy release.

 

 

2.    Role of Reactive Oxygen Species in seed germination

Reactive oxygen species (ROS) have traditionally been viewed as destructive agents in plants; however, it has been recently explained that ROS also play a positive role in seed germination (Oracz and Karpinski, 2016). Germination is the process which leads to the elongation of the embryonic axis from a seed, allowing subsequent seedling emergence. It consists in hydration of the quiescent seed (imbibition, phase I of the full process), and in the achievement of many metabolic and molecular events during the so called germination phase which occurs at a constant seed MC (phase II). Completion of the seed germination is the critical step of germination because it requires the activation of a complex regulatory system which is controlled by intrinsic (i.e., dormancy) and extrinsic (i.e., environmental conditions, such as temperature, oxygen and water availability) factors. Many reports have shown that the transition from a quiescent seed to a metabolically active organism is associated with ROS generation, suggesting that it is a widespread phenomenon. Production of hydrogen peroxide has been demonstrated at the early imbibition period of seeds of soybean, radish, maize, sunflower, wheat, pea and tomato seeds (Puntarulo et al., 1991). Nitric oxide (NO), hydroxyl radicals and superoxide radicals, also accumulate during the germination of seeds of various species. However, the intracellular sources of ROS production are poorly documented. Presumably, most of the ROS produced should originate from mitochondria (Bailly, 2019), since resumption of respiration in imbibed seeds might lead to electron leakage and increased production of ROS. However, the putative role of NADPH oxidase during germination is not known yet but should require attention regarding the various roles of this enzyme in various developmental processes. 

3.    Endosperm weakening

In seeds of some species, such as tobacco, tomato, pepper, Lepidium or Arabidopsis, germination is a constraint by the micropylar endosperm, which covers the radicle tip. Germination can proceed if the mechanical resistance imposed by the endosperm decreases to such a level that radicle can protrude through the weakened tissues. This endosperm weakening is under the regulation of abscissic acid (ABA) and gibberellic acid (GA) and several hydrolases (mannanase, cellulase, glucanase) which to contribute to cell wall loosening. There is an increasing bundle of evidence suggesting that ROS would play a key role in this phenomenon and it has been proposed that they would be involved in cell wall loosening in growing tissues (Kranner et al.,2010).

           Hydroxyl radicals have been shown to be present in the cell walls of growing organs and they can break down polysaccharides by an oxidative scission of backbone bonds. Their production could result from NADPH oxidase activity and /or Fenton reaction. Furthermore, it has also been proposed that ROS might control polar growth through their effect on calcium channels and auxin might promote cell growth through O₂^·– production and the subsequent generation of hydroxyl radical. Based on these properties, and on the role of ROS in some physiological processes, such as fruit softening, for example peroxidase activity developed in the tomato endosperm cap prior to radicle emergence. Some scientists also proposed that germination of Lepidium seeds, which requires endosperm rupture, involves the cleavage of cell wall polymers by ROS. They showed that H₂O₂ reversed the inhibitory effect of ABA on endosperm rupture, underlining the cross talk between these two compounds.

Many hypotheses have been put forward for attributing a function to these compounds in the germination process, including their contribution to cell wall loosening during endosperm weakening, programmed cell death of aleurone layer of cereal grains or protection of the emerging seedling against pathogens. At the cellular level, ROS are also known to regulate the cellular redox status, to cause the oxidation of proteins and to trigger specific gene expression. We propose here that ROS also play a key role in the completion of germination and that they should be considered as messengers of environmental cues during seed germination. The success of germination tightly depends on external factors such as temperature, light, oxygen and water availability (Krystyna and Stanislaw,2016). This implies that seeds must be endowed with internal sensors able to translate the environmental cues into the cellular mechanisms leading to germination. In whole plants it has been widely demonstrated that abiotic stresses such as heat and cold stress, UV or hypoxia cause the production of ROS. We assume that germination in non optimal conditions is a stressful situation, associated with ROS generation that in turn would prevent radicle emergence. Such a role of ROS would therefore be at the interface between signalling and deleterious effect.

 

4.    Conclusion

The Oxidative Window defines a critical level of ROS not to overcome(Figure.2), which would otherwise prevent germination, and a ROS threshold level, below which germination (radicle protrusion) cannot occur. Within this oxidative window ROS may play a role in cell signalling by interplaying with the hormone signalling pathways or by triggering the cellular events associated to ROS signalization, such as gene expression, calcium movements or control of redox status. In adverse environmental conditions or consequently to seed aging the amount of ROS in seed tissues exceeds the upper limit of the oxidative window, which leads to oxidative damage and inhibits germination or leads to abnormal seedlings. At the opposite, cellular signalling associated to the mechanism of seed germination is not triggered if the amount of ROS is too low, i.e., as in dormant seeds.

Future challenge of studies dealing with ROS in seeds will have to take into account the diversity of the roles of these compounds. It will also be of particular interest to properly document the putative sources of these compounds, to identify their cellular targets and to determine if they are the signal linking environmental cues to hormone signalization.

References

Bailly,C. (2019). The signalling role of ROS in the regulation of seed germination and dormancy. Biochemical Journal, 476(20):3019-3032.

Krystyna,O. and Stanislaw,K. (2016).Phytohormones signalling pathways and ROS involvement in

           seed germination. Frontiers in Plant Science,7:864.

Kranner, T. R., Richard, P. Beckett, C. W. and Farida, V. M. (2010). Extracellular production of reactive oxygen species during seed germination and early seedling growth in Pisum sativum. Journal of Plant Physiology, 167: 805–811. 

Oracz, K. and Karpinski,S.2016. Phytohormones signaling pathways and ROS involvement in seed germination. Front. Plant Sci,7:864.

Puntarulo,S., Galleano,M., Sanchez,R.A. and Boveris,A.1991.Superoxide anion and  hydrogen peroxide metabolism in soybean embryonic axes during germination. Biochim Biophys Acta,1074:277–283. 

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