76
S. Kato and T. Shinomura
comparable level of carotenoids in the cells grown at 30 °C under high-intensity
illumination at 2500 μmol photon m
−2 s
−1 , suggesting that the carotenoid level of
this alga was controlled depending on excitation pressure on PSII in response to
temperature and irradiance changes.
In E. gracilis, our previous study revealed that the gene expression of crtB,
phytoene desaturase genes (crtP1 and crtP2), and ζ-carotene desaturase gene (crtQ)
was increased by 1.3- to 1.8-fold by cultivation at 20 °C under 240 μmol photon
m
−2 s
−1 relative to the cells grown at 25 °C, whereas the cellular content of total
major carotenoids, β-carotene, neoxanthin, and diadinoxanthin was decreased by
more than half compared with the cells grown at 25 °C (Kato et al. 2019) (Fig. 4.1b).
On the other hand, E. gracilis cells grown at 20 °C contained a comparable level of
diatoxanthin to that in cells grown at 25°C and showed an increase in Dtx/Ddx ratio
and total carotenoid/chlorophyll a ratio (Kato et al. 2019). Our findings suggested
that cold stress enhanced light-induced stress to this alga and diatoxanthin might
participate in reducing the excitation pressure on PSII under cold and intense light
condition (Kato et al. 2019).
4.5 Perspectives
Molecular mechanisms of the regulation of carotenogenesis in microalgae have still
been elusive. In land plants, regulatory mechanisms of carotenoid biosynthesis have
gradually come to light: for example, direct regulation of phytoene synthase at transcriptional level by phytochrome interacting factors (PIFs) and long hypocotyl 5
(HY5) (Toledo-Ortiz et al. 2014) and at post-translational level by ORANGE (OR)
and Clp protease (Welsch et al. 2018) and redox-dependent ligand switching of 15cis-ζ-carotene isomerase (Z-ISO) (Beltrán et al. 2015). To respond environmental
stimuli at different time scale, microalgae would also have developed multiple regulatory systems at transcriptional and post-transcriptional levels to control the carotenoid
biosynthesis. Elucidation of those mechanisms will lead to a deeper understanding
of physiological functions of carotenoids in microalgae under various environmental
conditions and would contribute to an efficient production of valuable carotenoids
in the food and pharmaceutical industries.
Acknowledgements The authors thank the funding from Japan Society for the Promotion of
Science [grant number 17K07945] and Ministry of Education, Culture, Sports, Science and
Technology [grant number S1311014] to T. S.
S. Kato and T. Shinomura
comparable level of carotenoids in the cells grown at 30 °C under high-intensity
illumination at 2500 μmol photon m
−2 s
−1 , suggesting that the carotenoid level of
this alga was controlled depending on excitation pressure on PSII in response to
temperature and irradiance changes.
In E. gracilis, our previous study revealed that the gene expression of crtB,
phytoene desaturase genes (crtP1 and crtP2), and ζ-carotene desaturase gene (crtQ)
was increased by 1.3- to 1.8-fold by cultivation at 20 °C under 240 μmol photon
m
−2 s
−1 relative to the cells grown at 25 °C, whereas the cellular content of total
major carotenoids, β-carotene, neoxanthin, and diadinoxanthin was decreased by
more than half compared with the cells grown at 25 °C (Kato et al. 2019) (Fig. 4.1b).
On the other hand, E. gracilis cells grown at 20 °C contained a comparable level of
diatoxanthin to that in cells grown at 25°C and showed an increase in Dtx/Ddx ratio
and total carotenoid/chlorophyll a ratio (Kato et al. 2019). Our findings suggested
that cold stress enhanced light-induced stress to this alga and diatoxanthin might
participate in reducing the excitation pressure on PSII under cold and intense light
condition (Kato et al. 2019).
4.5 Perspectives
Molecular mechanisms of the regulation of carotenogenesis in microalgae have still
been elusive. In land plants, regulatory mechanisms of carotenoid biosynthesis have
gradually come to light: for example, direct regulation of phytoene synthase at transcriptional level by phytochrome interacting factors (PIFs) and long hypocotyl 5
(HY5) (Toledo-Ortiz et al. 2014) and at post-translational level by ORANGE (OR)
and Clp protease (Welsch et al. 2018) and redox-dependent ligand switching of 15cis-ζ-carotene isomerase (Z-ISO) (Beltrán et al. 2015). To respond environmental
stimuli at different time scale, microalgae would also have developed multiple regulatory systems at transcriptional and post-transcriptional levels to control the carotenoid
biosynthesis. Elucidation of those mechanisms will lead to a deeper understanding
of physiological functions of carotenoids in microalgae under various environmental
conditions and would contribute to an efficient production of valuable carotenoids
in the food and pharmaceutical industries.
Acknowledgements The authors thank the funding from Japan Society for the Promotion of
Science [grant number 17K07945] and Ministry of Education, Culture, Sports, Science and
Technology [grant number S1311014] to T. S.