C4 mechanisms in aquatic angiosperms: comparisons with terrestrial C4 systems

Area of Science:

• Plant Physiology
• Evolutionary Biology
• Biochemistry

Background:

• Aquatic C4 photosynthesis is rare, with only about a dozen identified species among over 7600 C4 plants.
• While terrestrial C4 plants utilize Kranz anatomy, aquatic species show variations, with some lacking it in submersed leaves.
• The evolution of C4 photosynthesis may have originated in aquatic environments due to dissolved CO2 limitations.

Purpose of the Study:

• To investigate the mechanisms and evolutionary significance of C4 photosynthesis in aquatic plants.
• To explore the unique biochemical and anatomical features of submersed C4 species, particularly Hydrilla verticillata.
• To understand the regulation and early development of C4 photosynthesis using Hydrilla as a model system.

Main Methods:

• Comparative analysis of C3, C3-C4, and C4 photosynthetic pathways in amphibious and aquatic plants.
• Biochemical studies on enzyme kinetics and localization, including phosphoenolpyruvate carboxylase (PEPC) and NADP-malic enzyme (NADP-ME).
• Physiological measurements of gas exchange, internal CO2 concentrations, growth rates, and quantum yield in Hydrilla verticillata.

Main Results:

• Submersed C4 species exhibit cytosolic β-carboxylation and chloroplastic decarboxylation, allowing C4 and Calvin cycles within the same cell, often without Kranz anatomy.
• Hydrilla verticillata functions as a facultative C4 plant, switching between C3 and C4 pathways based on CO2 availability.
• Hydrilla possesses multiple PEPC isoforms, with Hvpepc4 potentially being the photosynthetic form, differing from terrestrial C4 isoforms. NADP-ME also resembles a C3 isoform.

Conclusions:

• Aquatic C4 photosynthesis, exemplified by Hydrilla, presents a 'minimalist' system for studying C4 evolution and regulation due to its lack of complex anatomy.
• Hydrilla verticillata, belonging to the ancient Hydrocharitaceae family, provides crucial insights into the early development of C4 photosynthesis.
• The unique biochemical strategy in submersed C4 plants highlights the adaptability of photosynthesis in diverse environments.