146
7.3 Knowledge Gaps
7.3.1 The Function of Photoreceptors
Plants perceive changes in the light environment using a variety of photoreceptors,
e.g., red and far-red light-absorbing phytochromes and UV-A/blue light-absorbing
cryptochromes. The perception of light signals primes plants photoadaptation,
enabling avoidance of stress imposed by limiting and high light. Additionally, it also
plays a significant role in plant growth and development. These photoreceptors are
also found in some bacteria, fungi, and algae (Duanmu et al. 2014; Rensing et al.
2016). Phytochrome is the best-characterized class of photoreceptors. Multiple gene
duplication events and subfunctionalization have given rise to various phytochromes
with different functions in bryophytes, pteridophytes, and higher plants (Rensing
et al. 2016). Such diversity of phytochromes in these groups of plants might allow
them to cope with fluctuation of light conditions with greater plasticity (Li et al. 2011).
Light signals have found to trigger a profound alteration in transcriptomes of macroalgae and higher plants (Facella et al. 2008; Deng et al. 2012). Additionally, it has been
shown in various plant species that light can induce significant changes in expressions
of the genes involved in photosynthetic light reactions, photorespiratory pathway, and
photosynthetic carbon reactions (Casal and Yanovsky 2005; Jung et al. 2008; Monnier
et al. 2010; Ono et al. 2010; Lehmann et al. 2011). Interestingly, some of these responsive genes are also transcription factors (Tepperman et al. 2001; Zhang et al. 2008).
The regulatory mechanisms of these transcription factors in a putative transcriptional
cascade of photoreceptor-mediated responses are, however, not yet fully understood
(Casal and Yanovsky 2005; Duanmu et al. 2014).
There are many studies on photoreceptor-mediated physiological responses in
terrestrial plants. On the contrary, knowledge on how seagrasses sense the light
environment is still lacking although significant daily and seasonal fluctuations in
light quantity and quality have been observed in seagrass habitats (Kenworthy and
Haunert 1991; Gallegos and Kenworthy 1996; Kahn et al. 2013). Genes associated
with photoreceptors have been identified in the seagrass P. oceanica including phytochrome A (PoPHYA), phytochrome B (PoPHYB), phytochrome C (PoPHYC),
cryptochrome 1 (CRY1), and cryptochrome 2 (CRY2) (Greco et al. 2013; Dattolo
et al. 2014). Greco et al. (2013) reported changes in methylation status of the gene
PoPHYB as well as chlorophyll a/b-binding proteins and phosphoenolpyruvate carboxylase (PoPPC4) in response to a light limitation in P. oceanica. From these findings, it is suggested that PHYB methylation might play a regulatory role in
transcriptional cascade underlying photoacclimatory responses of this seagrass.
Dattolo et al. (2014b) have shown that the expression of photosynthetic genes follows circadian light cycle, indicating a possible regulatory role of the photoreceptors. Recently Dattolo et al. (2017) have conducted reciprocal light experiments and
revealed a downregulation of PHYA and CRY1 in P. oceanica upon exposure to
higher light intensity and an upregulation of the same genes upon exposure to lower
light intensity (Dattolo et al. 2017). As for Z. marina, the photoreceptors including
P. Buapet
7.3 Knowledge Gaps
7.3.1 The Function of Photoreceptors
Plants perceive changes in the light environment using a variety of photoreceptors,
e.g., red and far-red light-absorbing phytochromes and UV-A/blue light-absorbing
cryptochromes. The perception of light signals primes plants photoadaptation,
enabling avoidance of stress imposed by limiting and high light. Additionally, it also
plays a significant role in plant growth and development. These photoreceptors are
also found in some bacteria, fungi, and algae (Duanmu et al. 2014; Rensing et al.
2016). Phytochrome is the best-characterized class of photoreceptors. Multiple gene
duplication events and subfunctionalization have given rise to various phytochromes
with different functions in bryophytes, pteridophytes, and higher plants (Rensing
et al. 2016). Such diversity of phytochromes in these groups of plants might allow
them to cope with fluctuation of light conditions with greater plasticity (Li et al. 2011).
Light signals have found to trigger a profound alteration in transcriptomes of macroalgae and higher plants (Facella et al. 2008; Deng et al. 2012). Additionally, it has been
shown in various plant species that light can induce significant changes in expressions
of the genes involved in photosynthetic light reactions, photorespiratory pathway, and
photosynthetic carbon reactions (Casal and Yanovsky 2005; Jung et al. 2008; Monnier
et al. 2010; Ono et al. 2010; Lehmann et al. 2011). Interestingly, some of these responsive genes are also transcription factors (Tepperman et al. 2001; Zhang et al. 2008).
The regulatory mechanisms of these transcription factors in a putative transcriptional
cascade of photoreceptor-mediated responses are, however, not yet fully understood
(Casal and Yanovsky 2005; Duanmu et al. 2014).
There are many studies on photoreceptor-mediated physiological responses in
terrestrial plants. On the contrary, knowledge on how seagrasses sense the light
environment is still lacking although significant daily and seasonal fluctuations in
light quantity and quality have been observed in seagrass habitats (Kenworthy and
Haunert 1991; Gallegos and Kenworthy 1996; Kahn et al. 2013). Genes associated
with photoreceptors have been identified in the seagrass P. oceanica including phytochrome A (PoPHYA), phytochrome B (PoPHYB), phytochrome C (PoPHYC),
cryptochrome 1 (CRY1), and cryptochrome 2 (CRY2) (Greco et al. 2013; Dattolo
et al. 2014). Greco et al. (2013) reported changes in methylation status of the gene
PoPHYB as well as chlorophyll a/b-binding proteins and phosphoenolpyruvate carboxylase (PoPPC4) in response to a light limitation in P. oceanica. From these findings, it is suggested that PHYB methylation might play a regulatory role in
transcriptional cascade underlying photoacclimatory responses of this seagrass.
Dattolo et al. (2014b) have shown that the expression of photosynthetic genes follows circadian light cycle, indicating a possible regulatory role of the photoreceptors. Recently Dattolo et al. (2017) have conducted reciprocal light experiments and
revealed a downregulation of PHYA and CRY1 in P. oceanica upon exposure to
higher light intensity and an upregulation of the same genes upon exposure to lower
light intensity (Dattolo et al. 2017). As for Z. marina, the photoreceptors including
P. Buapet