Perceival isoforms CRY1 and CRY2, showing similarity with

Perceival of the light of different wavelength by plant
is mainly due to presence of receptors that transduce the information via signalling pathway to the target
molecule (Talaat, 2013). Plants have three types of photoreceptors: phytochrome
(red light receptor), cryptochrome (blue/UV-A light receptor) and UVR8 (UV-B
receptor). Phytochrome is a well characterised receptor and its five isoforms viz.,
phyA, phyB, phyC, phyD and phyE have been described in Arabidopsis thaliana.
Cryptochrome, a blue/UV-A receptor has been reported to present in two isoforms
CRY1 and CRY2, showing similarity with bacterial class I DNA photolyase and two
membrane-associated phototropins (Lin, 2000; Kagwa et al., 2001). UVR8, a newly
characterised receptor has been found to function at low fluence rate of UV-B
as suggested by several workers (Singh et al., 2012). Although the ongoing
studies have revealed the structure, physiology, localisation and mechanism of
action of this receptor, however, no evolutionary history has been traced out
yet. The study performed in this direction could help in exploring the ongoing
changes starting from beginning to till date, regarding the UVR8 interaction
with other receptors and might fill several missing links of ancient evolutionary
mechanism.

 

Commonalities and differences
to visible light and UV-B mediated signalling

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The ongoing
research on UVR8 has clearly revealed it as the only receptor for UV-B but
still it has been not traced out whether UVR8 has evolved from other
photoreceptors as a need of environmental changes and is now towards the
degrading or evolutionary phase. Several photoreceptors like cryptochromes
(cry1 and cry2), phototropins (phot1 and phot 2) and Zeitlupe family proteins,
mediating responses under UV-A/blue light, and phytochromes (phyA, B, C, D, and
E in Arabidopsis), that are majorly involved in mediating responses
principally to red and far-red light as well as UVR8, mediating responses under
UV-B are initiated by light. Thus there exists some integration in pathways of
these photoreceptors but is unknown. Liu et al. (2015) have observed regulation
of several genes associated with the chloroplast and UV-B protection, and
suggested UV-B signalling with the cellular components. Wade et al. (2001) have
suggested that induction of CHS expression, which is mainly UV-B mediated is
negatively regulated by phyB, and contrary to this it was synergistically mediated
by UV-A and blue light. Furthermore, a study by Morales et al. (2013) with
context to transcriptome and metabolite profiles in wild and uvr8 mutant plants grown in filed
condition, suggests that there is integration in pathways of UVR8 and
cryptochrome. As the expression of gene associated with UV protection was
regulated by UVR8 as well as there was positive and negative effect on
expression of genes and metabolites when supplemented with UV-A, similar gene
expression were mediated by cryptochrome under UV-A, thus there exists some
possibility of integration of UVR8 pathway with that of cryptochrome. In
addition, Feher et al. (2011) reported that low fluence rate UV-B entrains
circadian clock, which is mediated by UVR8 by enhancing transcription of genes
encoding components of circadian clock. Likewise, phytochrome and cryptochrome
photoreceptors have also been reported to mediate light signals involved in
entraining circadian clock (Jiao et al. 2007). However, responses are gated by
clock, whether mediated by UV-B or other photoreceptors, in order to maximise
the number of transcripts to their maximum during particular time in circadian
cycle. Another point of similarity that exists between the UVR8 and
cryptochrome is having the same action spectra (between 290 and 300 nn) for
biosynthesis of phenolic like flavonoids and anthocyanin, while receptors for
both the photoreceptors are different, as in case of UVR8 is tryptophan while
pterin or Flavin in case of blue light cryptochrome photoreceptors. Thus, UVR8 could
be suggested to have evolved with several modifications from the other
photoreceptors.

Similar to other photoreceptors
(in response to their wavebands), UVR8 also accumulate rapidly upon UV-B
exposures, interacts with COP1, HY5 acts a downstream effector and regulated by
negative feedback pathway. Favory et al. (2009) hypothesised that COP1 might
have been taken out from phytochrome and cryptochrome during UVR8 interaction
with COP1 and this hypothesis was supported by overexpressing line cop1 phenotype of UVR8 under UV-B
exposure. However, it has been noticed that visible light causes nuclear
exclusion of COP1 while UV-B exposure results in nuclear accumulation and
stabilization (Favory et al., 2009; Oravecz et al., 2006). Further, regulatory
action of COP1 as a repressor of photomorphogenesis depends on SPA protein
(Laubinger et al., 2004) and SPA proteins are not involved in regulatory action
by COP1 under UV-B supplementation (Oravecz et al. 2006). It becomes quit
interesting that there is similarity in phylogeny of SPA and RUP genes in
providing negative feedback, by interacting with COP1 (Gruber et al., 2010;
Fittinghoff et al., 2006). In addition to this similarity, the WD40 repeat
domain of COP1 is also interaction domain for transcription factors like HY5,
HYH, HFR1 and also for photoreceptors CRY1, CRY2, phyA and phyB. All these
similarities suggest towards the evolution of complex photoreceptor UVR8 from
the other photoreceptors and thus, there is need of finding the missing link.

Whether UVR8 is towards evolution or degradation phase?

 

Another important
question to be answered is whether the UVR8 that we have been studying in Arabidopsis
is towards its evolution or degradation. Orthologs of UVR8 have been identified
in plants from their sequence information and more importantly from
single-celled green algae and mosses (Rizzini et al., 2011). As the
cyanobacteria were first to evolve on earth when the UV-B was highest and no
ozone layer existed and presence of UVR8 orthologs under such high UV-B,
radiation during early evolution of photosynthetic organism might have
protected organisms from UV and might have also helped in transition to
terrestrial life (Rozema et al., 1997). 
However, there are some points that cannot be ignored (i) when UVR8 like
proteins were present in cyanobacteria during high UV-B exposure than why UVR8
in Arabidopsis have been suggested to activate UV-B protective responses
under low fluence rate of UV-B, this point suggests towards their degrading
phase which could be due to development of stratospheric ozone layer and if it
is so than with increasing intensity of UV-B on earth due to ozone layer
depletion, might lead to evolutionary phase as well from simplified to more
complex form, (ii) Functioning of UVR8 at low fluence rate also suggests that
there is requirement of other regulator for UVR8 functioning without which it
would be useless, thus inheriting it would be a wastage of energy and it
indicates that it might be degraded or replaced by some other more complexed
receptor that would function without involving so many regulators. 

 

Photoreceptors
regulating several processes of growth and development have been identified and
much work has been conducted on newly characterised UV-B receptor: UVR8.
However, studies have revealed the role of UVR8 starting from their structure
to the mechanism of action, but focus on the evolution of this UVR8 molecule
has not been done. We have here tried giving a new point for finding the
missing link between UVR8 and other photoreceptors by showing several
commonalities between the functioning of UVR8 and other photoreceptor. We have
also tried exploring the fact whether UVR8 is towards evolutionary or
degradation phase (Fig 1). This opinion could be a new initiative in study of
UVR8 associated studies.

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