108
M. Scarsini et al.
contradictory (Table 5.10), suggesting that the effects are taxon dependent. According
to Chen et al. (2020) indole-3-propionic acid (IPA) was the most effective phytohormone to stimulate the accumulation of astaxanthin in Chromochloris zofigiensis
(49% increase). Other phytohormones such as IPA and abscisic acid (ABA) had a
similar effect but accompanied by a depressed biomass production. The fact that
phytohormones such as gibberellic acid, indole-3-acetic acid (IAA) and indole-3butyric acid (IBA) promoted astaxanthin but not lipids suggests that these phytohormones may be used for enhancing free astaxanthin accumulation in the absence
of the usually synergetically biosynthesis of lipids (Schoefs et al. 2001; Chen et al.
2020). To summarize, the real role of phytohormones in carotenoid accumulation
would be more in coordinating the efficient production of astaxanthin and lipids in
Chromochloris zofigiensis (Chen et al. 2020).
5.4.1.7 Elements of Regulation
A few data are available on the regulation mechanisms of the carotenoid biosynthesis. Classically, transcriptional, post-transcriptional and biochemical factors are
all involved in the regulatory network. For instance, high levels of white light
(1000 μmol m
−2 s
−1 ) induced a contrasting modifications of the mRNA level corresponding to the carotenoid hydroxylase Cyp97a1 of Chlorella kessleri; it decreased
at the early stage of the treatment before a strong increase when the treatment was
continued after 10 h (Yu et al. 2014). The amount of carotenoids varied concomitantly
with the mRNA level (Yu et al. 2014).
Red light lowers the concentration of PIF1 transcription factor, a repressor of
carotenoid biosynthesis (Salguero et al. 2003).
The rate limiting steps of several sections of the carotenoid biosynthetic pathways
have been identified (Table 5.8). For instance, in the cyanobacterium Synechocystis
sp. PCC6803, β-carotene hydroxylase is the rate limiting step of the conversion of βcarotene to zeaxanthin, the regulation being exerted by the availability in the enzyme
itself (Lagarde et al. 2000). Despite the fact that it was reported that phytoene desaturase constitutes the rate limiting step in Synechococcus sp PCC 7942 (Chamovitz
et al. 1993), the heterologous overexpression of crtP in the cyanobacterium Synechocystis sp. PCC 6803 did not yield to an increase of the carotenoid cellular quota
(Lagarde et al. 2000). However, the overexpression of both crtP and crtB resulted
in an increase of the carotenoid content by 50% (Lagarde et al. 2000), suggesting
that the two enzymes form a complex in which the product of the first reaction is
funnelled to the second enzyme. The limiting steps of astaxanthin biosynthesis are
phytoene, lycopene and canthaxanthin synthesis (Su et al. 2014; Gao et al. 2015).
Carotenoid accumulation under stress conditions may require de novo enzyme
biosynthesis as in the case of astaxanthin accumulation in Haematococcus pluvialis
(Gwak et al. 2014; Hu et al. 2019; Schoefs et al. 2001).
M. Scarsini et al.
contradictory (Table 5.10), suggesting that the effects are taxon dependent. According
to Chen et al. (2020) indole-3-propionic acid (IPA) was the most effective phytohormone to stimulate the accumulation of astaxanthin in Chromochloris zofigiensis
(49% increase). Other phytohormones such as IPA and abscisic acid (ABA) had a
similar effect but accompanied by a depressed biomass production. The fact that
phytohormones such as gibberellic acid, indole-3-acetic acid (IAA) and indole-3butyric acid (IBA) promoted astaxanthin but not lipids suggests that these phytohormones may be used for enhancing free astaxanthin accumulation in the absence
of the usually synergetically biosynthesis of lipids (Schoefs et al. 2001; Chen et al.
2020). To summarize, the real role of phytohormones in carotenoid accumulation
would be more in coordinating the efficient production of astaxanthin and lipids in
Chromochloris zofigiensis (Chen et al. 2020).
5.4.1.7 Elements of Regulation
A few data are available on the regulation mechanisms of the carotenoid biosynthesis. Classically, transcriptional, post-transcriptional and biochemical factors are
all involved in the regulatory network. For instance, high levels of white light
(1000 μmol m
−2 s
−1 ) induced a contrasting modifications of the mRNA level corresponding to the carotenoid hydroxylase Cyp97a1 of Chlorella kessleri; it decreased
at the early stage of the treatment before a strong increase when the treatment was
continued after 10 h (Yu et al. 2014). The amount of carotenoids varied concomitantly
with the mRNA level (Yu et al. 2014).
Red light lowers the concentration of PIF1 transcription factor, a repressor of
carotenoid biosynthesis (Salguero et al. 2003).
The rate limiting steps of several sections of the carotenoid biosynthetic pathways
have been identified (Table 5.8). For instance, in the cyanobacterium Synechocystis
sp. PCC6803, β-carotene hydroxylase is the rate limiting step of the conversion of βcarotene to zeaxanthin, the regulation being exerted by the availability in the enzyme
itself (Lagarde et al. 2000). Despite the fact that it was reported that phytoene desaturase constitutes the rate limiting step in Synechococcus sp PCC 7942 (Chamovitz
et al. 1993), the heterologous overexpression of crtP in the cyanobacterium Synechocystis sp. PCC 6803 did not yield to an increase of the carotenoid cellular quota
(Lagarde et al. 2000). However, the overexpression of both crtP and crtB resulted
in an increase of the carotenoid content by 50% (Lagarde et al. 2000), suggesting
that the two enzymes form a complex in which the product of the first reaction is
funnelled to the second enzyme. The limiting steps of astaxanthin biosynthesis are
phytoene, lycopene and canthaxanthin synthesis (Su et al. 2014; Gao et al. 2015).
Carotenoid accumulation under stress conditions may require de novo enzyme
biosynthesis as in the case of astaxanthin accumulation in Haematococcus pluvialis
(Gwak et al. 2014; Hu et al. 2019; Schoefs et al. 2001).