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Bioelectromagnetism

 

FIGURE 5.4 Scheme of magnetoreception extended to comprise a radical scavenging reaction of the primary RP.

A^•− is generally thought to derive from a cryptochrome-bound FAD, implying the semiquinoid radical FAD^•− or its

protonated form, FADH^•. B^•+ appears to be more elusive; diferent schemes and radicals are discussed in the litera­

ture. C^• is the scavenger radical. (Adapted from Kattnig (2017).)

Tis means that tryptophan tetrads (rather than triads) or systems involving freely difusing radicals

can also give rise to sizable MF efects and that the detrimental efects of inter-radical exchange and

dipolar interactions can be minimized. Te spin dynamics in these three-radical systems are character­

ized by doublet-quartet conversions (instead of the conventional singlet−triplet interconversions char­

acteristic of the classical RP mechanism). Te doublet state can be formulated as singlet in the A/B- or

A/C-manifold in combination with a doublet radical, as shown in Figure 5.4. Tese singlet substates are,

however, not mutually exclusive, i.e., orthogonal.

Te remarkable resilience to fast relaxation in the second radical, B^•+, which, for kb > 0 even afords

relaxation-enhancement, widens the scope of the RPM to include constituent radicals with spin relax­

ation rates that are too fast to elicit a signifcant MFE via the conventional RPM. In the context of

plant magnetoreception, this prospect appears particularly relevant to reaction schemes involving the

superoxide radical, which has been advocated by several studies but sufers from fast spin relaxation.

An increase in the intensity of the ambient MF from 33–44 to 500 μT enhanced growth inhibition in

Arabidopsis thaliana under blue light, when cryptochromes are the mediating photoreceptor, but not

under red light when the mediating receptors are phytochromes showing that higher plants are sensitive

to the MF in responses that might be linked to cryptochrome-dependent signaling pathways (Ahmad

et al., 2007). A number of recent theoretical works suggest that cryptochromes might also be the recep­

tors responsible for the sensing of the GMF (e.g., in insects, migratory birds, or migratory fsh).

5.3 Light-Dependent Magnetoreception

Plants show both light-dependent (Xu et al., 2015; Vanderstraeten et al., 2018; Pooam et al., 2019) and

light-independent (Dhiman and Galland, 2018; Agliassa and Mafei, 2019) magnetoreception, which

may refect a diferential ability of plant organs to interact with light and the GMF. For instance, leaves

are constantly exposed to both light and GMF, whereas roots perceive the GMF but only a low light fu­

ence when close to the soil surface and no light when they grow deep in the ground.

Cryptochromes, which are blue-light photoreceptors, are involved in magnetoreception and the

efect of MFs was reported in cryptochrome phosphorylation (Xu et al., 2014; Agliassa et al., 2018b;

Pooam et al., 2019; Hammad et al., 2020). Phytochromes, the red-light photoreceptors, are known to

interact with cryptochrome at many levels and enhance and maintain plant downstream responses to