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Magnetoreception in Plants
blue light (Ahmad and Cashmore, 1997). Phytochrome phyAphyB defcient mutants showed a visibly
enhanced response to applied MFs (Pooam et al., 2019). Terefore, plants can use diferent cues to per
ceive variations in the MF and trigger signal transduction events eventually leading to biochemical and
developmental changes.
Te photocycle underlying magnetoreception has not yet been unequivocally identifed. MFE studies
on isolated cryptochromes and closely related photolyases have ascribed the magnetosensitivity to the
RP [FAD•−/W•+], produced through a sequence of swif electron transfer steps involving the tryptophan
triad (Maeda et al., 2012). In animals, the transferability of this fnding to in vivo conditions has been
questioned by several behavioral and histochemical studies suggesting that the key step involves the
reoxidation of the fully reduced FADH−, likely by molecular oxygen (Muller and Ahmad, 2011). Support
for this hypothesis is predominantly drawn from the light dependence of the compass sense. For
instance, in birds, pre-exposure to white light could generate orientation under green light (~560 nm)
even though oxidized FAD is not excitable below ~500 nm (Wiltschko et al., 2010). Tis fnding has been
attributed to the secondary photoreduction of the semiquinone radical FADH• to FADH−, which can
indeed be facilitated by green light. Along the same lines, histochemical studies suggested that Cry1a in
the retina of chickens could be photoactivated by green light, and that structural changes connected to
the C-terminal region of the cryptochrome, which could be relevant in signaling, are triggered in the
fully reduced form (Nießner et al., 2014). It was also argued that the efcient charge separation in animal
cryptochromes and closely related animal photolyases containing a tryptophan tetrad would preclude
magnetosensitivity in the photoinduced favin-tryptophan RPs because the rate of spin-selective recom
bination was too low (Cailliez et al., 2016). Te only currently hypothesized RP that may potentially
explain these results is FADH• / O•
2
− , generated in the reoxidation of the fully (photo)reduced FADH−.
Most of the knowledge on MFE in plants comes from the work of Hore, Ahmad, and Solov’yov and
their co-workers.
Te activity of cryptochrome-1 in A. thaliana is enhanced by the presence of a weak external MF,
confrming the ability of cryptochrome to mediate MF responses. As noticed, cryptochrome’s signaling
is tied to the photoreduction of FAD. Te spin chemistry of this photoreduction process, which involves
electron transfer from a chain of three tryptophans, can be modulated by the presence of an MF with
the RPM. In Arabidopsis, the RPM in cryptochrome can produce an increase in the protein’s signaling
activity of about 10% for MF on the order of 5 G (500 μT), which is consistent with experimental results
(Solov’yov et al., 2007).
Despite a variety of supporting evidence, it is still not clear whether cryptochromes have the proper
ties required to respond to magnetic interactions orders of magnitude weaker than the thermal energy,
kBT. It has been shown that the kinetics and quantum yields of photo-induced favin-tryptophan RPs
in cryptochrome are indeed magnetically sensitive. Te mechanistic origin of the MFE has been sug
gested, its dependence on the strength of the MF measured, and the rates of relevant spin-dependent,
spin-independent, and spin-decoherence processes determined. Terefore, cryptochrome appears to ft
for purpose as a chemical magnetoreceptor (Maeda et al., 2012).
One of the most stimulating observations in plant evolution is a correlation between the occurrence of
GMF reversals (or excursions) and the moment of the radiation of angiosperms (Figure 5.5) (Occhipinti
et al., 2014). Tis led to the hypothesis that alterations in GMF polarity may play a role in plant evolution.
A. thaliana exposed to artifcially reversed GMF conditions in the presence of light showed signifcant
efects on plant growth and gene expression, supporting the hypothesis that the GMF reversal contrib
utes to inducing changes in plant development that might justify a higher selective pressure, eventually
leading to plant evolution (Bertea et al., 2015).
In A. thaliana seedlings grown under NNMF in the presence of light, fowering time was found to
be delayed compared with seedlings grown in normal GMF (Xu et al., 2013, 2015, 2017, 2018). Moreover,
the transcription level of a few fowering-related genes also changed (Xu et al., 2012). Furthermore, the
biomass accumulation of plants in NNMF was signifcantly suppressed at the time when plants were
switching from vegetative growth to reproductive growth compared to that of plants grown in normal