148
In addition to unclear C3 or C4 nature of seagrass photosynthesis, a basic understanding of Rubisco kinetics in seagrasses is poor. In many terrestrial plant species,
Rubisco is activated by light via an enzyme rubisco activase (Zhang and Portis
1999). A decline in photosynthetic activity under moderate heat stress in these
plants has been attributed to either a decreasing capacity of photosynthetic electron
transport or a reduction in carbon fixation capacity as a result of an inactivation of
Rubisco activase which is heat-sensitive (Salvucci and Crafts-Brandner 2004;
Salvucci et al. 2001; Salvucci and Crafts-Brandner 2004; Portis et al. 2008). It has
also been suggested that the heat tolerance of photosynthesis is controlled by the
thermal properties of Rubisco activase (Salvucci and Crafts-Brandner 2004). Little
is known about such role of Rubisco activase in seagrasses. Until now, it has been
addressed only in the studies of the seagrass, P. oceanica, in relation to photoacclimation (Dattolo et al. 2013, 2014) and heat stress (Marín-Guirao et al. 2016). Global
warming and the rise in atmospheric CO 2 will likely increase the temperature of
seawater in the coming decades, particularly in the shallow areas. Identifying the
major limiting process at elevated temperature is essential when predicting the photosynthetic response of seagrasses. In terrestrial plants system, it has been demonstrated that when taking into consideration the regulation of Rubisco activase,
kinetic properties of Rubisco can effectively predict the temperature response of
photosynthesis (Crafts-Brandner and Salvucci 2000).
The most common carbon-concentrating mechanism (CCM) proposed to operate
in seagrasses is associated with extracellular carbonic anhydrase (discussed in the
previous section). This conclusion, however, is mainly drawn from inhibitors experiments. Identification and localization of extracellular carbonic anhydrase, as well
as analysis of molecular responses induced by low CO 2 availability, could provide
further evidence for CCM in this group of plants. For example, advanced understanding of CCM induction and regulation in algae was obtained from comparative
transcriptome, proteome, and metabolome analyses when algae were transferred
from high to low carbon availability. As a result, genes, proteins, and key metabolites that may be involved with the CCM including carbonic anhydrase have been
identified (Yamano and Fukuzawa 2009; Renberg et al. 2010; Baba et al. 2011;
Ramanan et al. 2012; Winck et al. 2013).
7.3.3 Linkage Between Photosynthesis and Other Metabolic
Pathways
In the past decade, much more effort has been made toward understanding the photobiology of seagrasses at physiological to molecular levels (as discussed earlier).
However, knowledge on the interactions between photosynthesis and other metabolic pathways across the whole system is still lacking. It is important to have a
better understanding of complex interplay within the plant primary metabolism networks because such interactions might affect the responses to stresses at a wholeplant level.
P. Buapet
In addition to unclear C3 or C4 nature of seagrass photosynthesis, a basic understanding of Rubisco kinetics in seagrasses is poor. In many terrestrial plant species,
Rubisco is activated by light via an enzyme rubisco activase (Zhang and Portis
1999). A decline in photosynthetic activity under moderate heat stress in these
plants has been attributed to either a decreasing capacity of photosynthetic electron
transport or a reduction in carbon fixation capacity as a result of an inactivation of
Rubisco activase which is heat-sensitive (Salvucci and Crafts-Brandner 2004;
Salvucci et al. 2001; Salvucci and Crafts-Brandner 2004; Portis et al. 2008). It has
also been suggested that the heat tolerance of photosynthesis is controlled by the
thermal properties of Rubisco activase (Salvucci and Crafts-Brandner 2004). Little
is known about such role of Rubisco activase in seagrasses. Until now, it has been
addressed only in the studies of the seagrass, P. oceanica, in relation to photoacclimation (Dattolo et al. 2013, 2014) and heat stress (Marín-Guirao et al. 2016). Global
warming and the rise in atmospheric CO 2 will likely increase the temperature of
seawater in the coming decades, particularly in the shallow areas. Identifying the
major limiting process at elevated temperature is essential when predicting the photosynthetic response of seagrasses. In terrestrial plants system, it has been demonstrated that when taking into consideration the regulation of Rubisco activase,
kinetic properties of Rubisco can effectively predict the temperature response of
photosynthesis (Crafts-Brandner and Salvucci 2000).
The most common carbon-concentrating mechanism (CCM) proposed to operate
in seagrasses is associated with extracellular carbonic anhydrase (discussed in the
previous section). This conclusion, however, is mainly drawn from inhibitors experiments. Identification and localization of extracellular carbonic anhydrase, as well
as analysis of molecular responses induced by low CO 2 availability, could provide
further evidence for CCM in this group of plants. For example, advanced understanding of CCM induction and regulation in algae was obtained from comparative
transcriptome, proteome, and metabolome analyses when algae were transferred
from high to low carbon availability. As a result, genes, proteins, and key metabolites that may be involved with the CCM including carbonic anhydrase have been
identified (Yamano and Fukuzawa 2009; Renberg et al. 2010; Baba et al. 2011;
Ramanan et al. 2012; Winck et al. 2013).
7.3.3 Linkage Between Photosynthesis and Other Metabolic
Pathways
In the past decade, much more effort has been made toward understanding the photobiology of seagrasses at physiological to molecular levels (as discussed earlier).
However, knowledge on the interactions between photosynthesis and other metabolic pathways across the whole system is still lacking. It is important to have a
better understanding of complex interplay within the plant primary metabolism networks because such interactions might affect the responses to stresses at a wholeplant level.
P. Buapet