Molecular Adaptation of rbcL in the Heterophyllous Aquatic Plant Potamogeton

Heterophyllous aquatic plants show marked phenotypic plasticity. They adapt to environmental changes by producing different leaf types: submerged, floating and terrestrial leaves. By contrast, homophyllous plants produce only submerged leaves and grow entirely underwater. Heterophylly and submerged homophylly evolved under selective pressure modifying the species-specific optima for photosynthesis, but little is known about the evolutionary outcome of habit. Recent evolutionary analyses suggested that rbcL, a chloroplast gene that encodes a catalytic subunit of RuBisCO, evolves under positive selection in most land plant lineages. To examine the adaptive evolutionary process linked to heterophylly or homophylly, we analyzed positive selection in the rbcL sequences of ecologically diverse aquatic plants, Japanese Potamogeton.Phylogenetic and maximum likelihood analyses of codon substitution models indicated that Potamogeton rbcL has evolved under positive Darwinian selection. The positive selection has operated specifically in heterophyllous lineages but not in homophyllous ones in the branch-site models. This suggests that the selective pressure on this chloroplast gene was higher for heterophyllous lineages than for homophyllous lineages. The replacement of 12 amino acids occurred at structurally important sites in the quaternary structure of RbcL, two of which (residue 225 and 281) were identified as potentially under positive selection.Our analysis did not show an exact relationship between the amino acid replacements and heterophylly or homophylly but revealed that lineage-specific positive selection acted on the Potamogeton rbcL. The contrasting ecological conditions between heterophyllous and homophyllous plants have imposed different selective pressures on the photosynthetic system. The increased amino acid replacement in RbcL may reflect the continuous fine-tuning of RuBisCO under varying ecological conditions

Biochemical and biophysical CO2 concentrating mechanisms in two species of freshwater macrophyte within the genus Ottelia (Hydrocharitaceae)

Two freshwater macrophytes, Ottelia alismoides and Ottelia acuminata, were grown at low (mean 5 µmol L-1) and high (mean 400 µmol L-1) CO2 concentrations under natural conditions. The ratio of PEPC to RubisCO was 1.8 in O. acuminata in both treatments. In O. alismoides, this ratio was 2.8 and 5.9 when grown at high and low CO2, respectively, as a result of a 2-fold increase of PEPC activity. The activity of PPDK was similar to and changed in-line with PEPC (1.9-fold change). The activity of the decarboxylating NADP-malic enzyme (ME) was very low in both species while NAD-ME activity was high and increased with PEPC activity in O. alismoides. These results suggest that O. alismoides might perform a type of C4 metabolism with NAD-ME decarboxylation, despite lacking Kranz anatomy. The C4-activity was still present at high CO2 suggesting that it could be constitutive. O. alismoides at low CO2 showed diel acidity variation of up to 34 μequiv g-1 FW indicating it may also operate a form of Crassulacean Acid Metabolism (CAM). pH-drift experiments showed that both species were able to use bicarbonate. In O. acuminata, the kinetics of carbon uptake were altered by CO2 growth conditions, unlike in O. alismoides. Thus the two species appear to regulate their carbon concentrating mechanisms differently in response to changing CO2. The Hydrocharitaceae have many species with evidence for C4, CAM, or a metabolism involving organic acids, and are worthy of further study

Trade-offs and synergies in the structural and functional characteristics of leaves photosynthesizing in aquatic environments

Aquatic plants, comprising different divisions of embryophytes, derive from terrestrial ancestors. They have evolved to live in water, both fresh and salty, an environment that presents unique challenges and opportunities for photosynthesis and growth. These include, compared to air, a low water stress, a greater density, and attenuation of light, and a more variable supply of inorganic carbon, both in concentration and chemical species, but overall a lower carbon availability, and the opportunity to take up nutrients from the water. The leaves of many aquatic plants are linear, dissected, whorled, or cylindrical with a large volume of air spaces. They tend to have a high specific leaf area, thin cuticles, and usually lack functional stomata. Exploiting the availability of chemicals in their environment, freshwater macrophytes may incorporate silica in their cell wall, while seagrasses contain sulphated polysaccharides, similar to those of marine macroalgae; both groups have low lignin content. This altered cell wall composition produces plants that are more flexible and therefore more resistant to hydraulic forces (mechanical stress arising from water movement). Aquatic plants may have enhanced light harvesting complexes conferring shade adaptation, but also have mechanisms to cope with high light. Aquatic plants have evolved numerous strategies to overcome potential carbon-limitation in water. These include growing in micro-environments where CO2 is high, producing leaves and roots that exploit CO2 from the air or sediment and operating concentrating mechanisms that increase CO2 (CCM) around the primary carboxylating enzyme, ribulose-1,5-bisphosphate carboxylase-oxygenase. These comprise C4 metabolism, crassulacean acid metabolism, and the ability to exploit the often high concentrations of HCO3−, and ~50% of freshwater macrophytes and ~85% of seagrasses have one or more CCM. Many of these adaptations involve trade-offs between conflictin constraints and opportunities while others represent ‘synergies’ that help to maximize the productivity of this important group of plants