**3. Hormesis and molecular mechanisms of the adaptive response**

The hormesis term comes from greek meaning "to excite". Exposure of sublethal doses of ionizing radiation can induce protective mechanisms against a subsequent higher dose irradiation. So, the hormesis is the excitement and stimulation by small doses of any agent on any system. Luckey (1980), in his book entitled "Hormesis", documented thousand experiments where fungi and other lower life forms were seen to prosper with doses of radiation exceeding their normal background exposures with ionizing radiation. In a second book entitled "Radiation Hormesis" (Luckey, 1991) examined hundreds of studies on animals and humans, showing that low levels of radiation were beneficial to health, longevity, and reproduction.

Many studies have been also indicated that pre-expose to low dose radiation (or some other genotoxic agent) can change radiosensitivity, reducing score of chromosomal aberrations, micronuclei and mutations. This phenomenon is called adaptation and could be related with defense mechanisms some of them have evolved to minimize genotoxic damage. One of these is induced radioresistance or adaptive response (AR). The term "adaptive response" usually means that a relatively small "conditioning" radiation dose induces increased radioresistance when the cells are irradiated with higher doses several hours later (Hillova & Drasil, 1967). Thus, radioadaptive response induction expresses the ability of low dose radiation to induce cellular changes that alter the level of subsequent radiation-induced or spontaneous damage (Amundson *et al*., 1999). The exposure to minimal stress inducing a very low level of damage can trigger an AR resulting in increased resistance to higher levels of the same or of other types of stress (Patra *et al*., 2003; Asad *et al*., 2004; Girigoswami & Ghosh, 2005; Yan *et al*., 2006). The AR could be considered a nonspecific phenomenon and have been confirmed but not explained by many studies. Adaptation after preexposure to chronic or prolonged exposure to low-level radiation doses was not often described.

Current Importance and Potential Use of Low Doses of Gamma Radiation in Forest Species 269

DNA repair underlies the AR induced by low radiation doses in human and plant cells (Lambin *et al*, 1994; Patra *et al.*, 2003) by increasing the amount and rate of DNA repair (Joiner *et al.*, 1996; 1999). It has been proposed that these effects could be related to the induction of an AR. A clue as to the nature of the underlying process was provided by results showing a dependence on de novo protein synthesis. The synthesis of DNAbinding proteins (MWs 50, 74 and 130 kdal) was found in radiation-conditioned cells of *C. reinhardtii* (Bryant, 1979). The induction of new protein synthesis by low doses could be caused by an effect of low doses on chromatin conformation near genes coding for DNA repair proteins (Belyaev *et al.*, 1996). For example, there are earlier observations that hydrogen peroxide induced a cross-adaptive response to cumene hydroperoxide in *E. coli* which did not require novel gene products but involved modification of the small subunit of Ahp, a protein involved in the protection against alkyl hydroperoxides (Asad *et al.*, 1998). On the other hand, Reactive Oxygen Species (ROS) could serve as signal transducers in plant and animal cells (Babu *et al.*, 2003; Matsumoto *et al.*, 2004). As signaling molecules, ROS might affect the development of AR through participation in the

damage-sensing process after conditioning dose exposure.

trees, one of the main agronomic traits for forest management.

**A case study** 

much easier in the herbaceous form.

**4. Current and potential use of low ionizing radiation in forestry:** 

At present there are virtually no studies of hormesis by ionizing radiation in forest species. Most of the work focused radiation treatment of species of agronomic interest since they have shorter lifetime and germination time, the tissues of this type of seeds have a greater amount of water, which maximizes the effect of radiation; and the generation of seedlings is

One of the few jobs that exist in tree species was conducted with *Araucaria angustifollia* (Bert) O. Kuntze (Ferreira *et al*., 1980). The study showed a hormetic effect on seed germination and seedling growth at low doses of gamma radiation (0.1 to 0.4 kR). This first study showed the effectiveness of ionizing radiation to improve seed germination in species of

*Abies religiosa* (fir) and *Pinus hartwegii* are two conifer species that develop on the National Park Cofre de Perote, Veracruz, Mexico. Both have great ecological (*P. harwegii* is the conifer species taking place at higher altitudes) and economical importance (in particular, the fir is valuable for its timber, trementine production and as an ornamental Christmas trees). These forests have protective functions to other resources to cushion the effects of environmental

In Veracruz, these populations develop principally in the National Park Cofre de Perote and Pico de Orizaba between 2 800 y 3 500 m.a.s.l. in 17°30' to 20°00' N and 97°104' W (Manzanilla 1974; Sánchez-Velásquez & Pineda-López 1993). Both species have been seriously affected by fire and logging clandestine, has resulted in a reduction of the effective size of the same low viability and high percentage of abortive seeds, and a significant decrease in reproductive rate, apparently due to manifestation of the phenomenon of inbreeding depression, common in coniferous species (Williams & Savolaienen, 1996). But,

pollution and contribute as a regulation of the hydrological cycle (Solís, 1994).

in both species, is common a low reproductive rate (Franklin, 1974).

Several types of cellular responses to ionizing radiation, such as the adaptive response, suggest that low-dose radiation may possess characteristics that distinguish it from its highdose counterpart. Accumulated evidence also implies that the biological effects of low-doses and high-doses of ionizing radiation are not linearly distributed. This is an important physiological effect of exposure to low doses of radiation. The radioadaptive was first documented in a convincing way to protect against chromosomal aberrations (Scott *et al.*, 2009).

The capability of forest tree species to adapt to the new environments not only will depend on their genetic background, but also rely on their phenotypic plasticity. Several reports have shown the involvement of epigenetic modifiers as the basis of the phenotypic plasticity, and in particular to the adaptation to abiotic stresses. DNA methylation is one the most important epigenetic modification in eukaryotes. It is involved in specific biological processes such as gene transcription regulation, gene silencing, mobile element control or genome imprinting. Therefore, there is a great interest in analyzing methylation levels and distribution within the genome. Epigenetic regulation of gene activity is widespread in the genome of eukaryotic cells and can lead to silencing or activation of gene expression. According to Scott *et al.* (2009), high doses of radiation can promote epigenetically silencing of adaptive response genes, for example via promoter associated DAN and /or histone methylation or deacetylation.

Adaptive-response genes can be stabilized and activated in response to cellular stress (e.g., low dose radiation) through post translational modifications that include acetylation (Ito *et al.,* 2002). This radiation, above a stochastic threshold stimulate intracellular and intercellular signaling that leads to activated natural protection (ANP) against cancer and other genomic-instability-associated diseases (Scott, 2005; Scott & Di Palma, 2006).

The AR has been observed in bacteria (Assis *et al.*, 2002; Sedgwick & Lindahl, 2002; Rohankhedkar *et al.*, 2006), yeast (Boreham & Mitchel, 1991; Gajendiran & Jeevanram, 2002), algae (Chankova & Bryant, 2001; Rubinelli *et al.*, 2002; Chankova *et al.*, 2007), insect cells (Savina *et al.*, 2003), mammalian cells (Wang & Cai, 2000; Tiku & Kale, 2001; Ulsh *et al.*, 2004; Zhou *et al.*, 2004), human cells (Schlade-Bartusiak *et al.*, 2002; Atanasova *et al.*, 2005; Coleman *et al.*, 2005; Friesner *et al.*, 2005; Lanza *et al.*, 2005; Seo *et al.*, 2006) and higher plants models (Rieger *et al.*, 1993; Panda *et al.*, 1997; Jovtchev & Stergios, 2003; Patra *et al.*, 2003). A study of the conditions essential for the induction of an adaptive response is of critical importance to understanding the novel biological defense mechanisms against the hazardous effects of radiation. The results statistically significant with microorganisms, plants, non vertebrates and other animals of experimentation, showed the existence of a radiogenic metabolism, in other words, a metabolism promoted by ionizing radiation.

Little is currently known about the precise mechanisms of AR. There is evidence that different stress conditions can activate similar defense mechanisms in biological systems (Joiner *et al.*, 1996; 1999; Babu *et al.*, 2003). The AR probably involves the transcription of many genes and the activation of numerous signaling pathways that trigger cell defenses more efficient detoxification of free radicals, DNA repair systems, induction of new proteins in irradiated cells with a conditioning dose, and enhanced antioxidant production (Wolff, 1998; Mendez-Alvarez *et al.*, 1999; Pajovic *et al.*, 2001; Assis *et al.*, 2002; Chankova & Bryant, 2001; Coleman *et al.*, 2005; Lanza *et al.*, 2005). There is evidence that

Several types of cellular responses to ionizing radiation, such as the adaptive response, suggest that low-dose radiation may possess characteristics that distinguish it from its highdose counterpart. Accumulated evidence also implies that the biological effects of low-doses and high-doses of ionizing radiation are not linearly distributed. This is an important physiological effect of exposure to low doses of radiation. The radioadaptive was first documented in a convincing way to protect against chromosomal aberrations (Scott *et al.*,

The capability of forest tree species to adapt to the new environments not only will depend on their genetic background, but also rely on their phenotypic plasticity. Several reports have shown the involvement of epigenetic modifiers as the basis of the phenotypic plasticity, and in particular to the adaptation to abiotic stresses. DNA methylation is one the most important epigenetic modification in eukaryotes. It is involved in specific biological processes such as gene transcription regulation, gene silencing, mobile element control or genome imprinting. Therefore, there is a great interest in analyzing methylation levels and distribution within the genome. Epigenetic regulation of gene activity is widespread in the genome of eukaryotic cells and can lead to silencing or activation of gene expression. According to Scott *et al.* (2009), high doses of radiation can promote epigenetically silencing of adaptive response genes, for example via promoter associated DAN and /or histone

Adaptive-response genes can be stabilized and activated in response to cellular stress (e.g., low dose radiation) through post translational modifications that include acetylation (Ito *et al.,* 2002). This radiation, above a stochastic threshold stimulate intracellular and intercellular signaling that leads to activated natural protection (ANP) against cancer and

The AR has been observed in bacteria (Assis *et al.*, 2002; Sedgwick & Lindahl, 2002; Rohankhedkar *et al.*, 2006), yeast (Boreham & Mitchel, 1991; Gajendiran & Jeevanram, 2002), algae (Chankova & Bryant, 2001; Rubinelli *et al.*, 2002; Chankova *et al.*, 2007), insect cells (Savina *et al.*, 2003), mammalian cells (Wang & Cai, 2000; Tiku & Kale, 2001; Ulsh *et al.*, 2004; Zhou *et al.*, 2004), human cells (Schlade-Bartusiak *et al.*, 2002; Atanasova *et al.*, 2005; Coleman *et al.*, 2005; Friesner *et al.*, 2005; Lanza *et al.*, 2005; Seo *et al.*, 2006) and higher plants models (Rieger *et al.*, 1993; Panda *et al.*, 1997; Jovtchev & Stergios, 2003; Patra *et al.*, 2003). A study of the conditions essential for the induction of an adaptive response is of critical importance to understanding the novel biological defense mechanisms against the hazardous effects of radiation. The results statistically significant with microorganisms, plants, non vertebrates and other animals of experimentation, showed the existence of a radiogenic metabolism, in

Little is currently known about the precise mechanisms of AR. There is evidence that different stress conditions can activate similar defense mechanisms in biological systems (Joiner *et al.*, 1996; 1999; Babu *et al.*, 2003). The AR probably involves the transcription of many genes and the activation of numerous signaling pathways that trigger cell defenses more efficient detoxification of free radicals, DNA repair systems, induction of new proteins in irradiated cells with a conditioning dose, and enhanced antioxidant production (Wolff, 1998; Mendez-Alvarez *et al.*, 1999; Pajovic *et al.*, 2001; Assis *et al.*, 2002; Chankova & Bryant, 2001; Coleman *et al.*, 2005; Lanza *et al.*, 2005). There is evidence that

other genomic-instability-associated diseases (Scott, 2005; Scott & Di Palma, 2006).

other words, a metabolism promoted by ionizing radiation.

2009).

methylation or deacetylation.

DNA repair underlies the AR induced by low radiation doses in human and plant cells (Lambin *et al*, 1994; Patra *et al.*, 2003) by increasing the amount and rate of DNA repair (Joiner *et al.*, 1996; 1999). It has been proposed that these effects could be related to the induction of an AR. A clue as to the nature of the underlying process was provided by results showing a dependence on de novo protein synthesis. The synthesis of DNAbinding proteins (MWs 50, 74 and 130 kdal) was found in radiation-conditioned cells of *C. reinhardtii* (Bryant, 1979). The induction of new protein synthesis by low doses could be caused by an effect of low doses on chromatin conformation near genes coding for DNA repair proteins (Belyaev *et al.*, 1996). For example, there are earlier observations that hydrogen peroxide induced a cross-adaptive response to cumene hydroperoxide in *E. coli* which did not require novel gene products but involved modification of the small subunit of Ahp, a protein involved in the protection against alkyl hydroperoxides (Asad *et al.*, 1998). On the other hand, Reactive Oxygen Species (ROS) could serve as signal transducers in plant and animal cells (Babu *et al.*, 2003; Matsumoto *et al.*, 2004). As signaling molecules, ROS might affect the development of AR through participation in the damage-sensing process after conditioning dose exposure.
