Increased Cuticle Permeability Caused by a New Allele of ACETYL-COA CARBOXYLASE1 Enhances CO2 Uptake

Carbon dioxide (CO₂) is an essential substrate for photosynthesis in plants. CO₂ is absorbed mainly through the stomata in land plants because all other aerial surfaces are covered by a waxy layer called the cuticle. The cuticle is an important barrier that protects against extreme water loss; however, this anaerobic layer limits CO₂ uptake. Simply, in the process of adapting to a terrestrial environment, plants have acquired drought tolerance in exchange for reduced CO₂ uptake efficiency. To evaluate the extent to which increased cuticle permeability enhances CO₂ uptake efficiency, we investigated the CO₂ assimilation rate, carbon content, and dry weight of the Arabidopsis (Arabidopsis thaliana) mutant excessive transpiration1 (extra1), whose cuticle is remarkably permeable to water vapor. We isolated the mutant as a new allele of ACETYL-COA CARBOXYLASE1, encoding a critical enzyme for fatty acid synthesis, thereby affecting cuticle wax synthesis. Under saturated water vapor conditions, the extra1 mutant demonstrated a higher CO₂ assimilation rate, carbon content, and greater dry weight than did the wild-type plant. On the other hand, the stomatal mutant slow-type anion channel-associated1, whose stomata are continuously open, also exhibited a higher CO₂ assimilation rate than the wild-type plant; however, the increase was only half of the amount exhibited by extra1. These results indicate that the efficiency of CO₂ uptake via a permeable cuticle is greater than the efficiency via stomata and confirm that land plants suffer a greater loss of CO₂ uptake efficiency by developing a cuticle barrier.

To absorb carbon dioxide (CO₂) for photosynthesis, land plants expose their wet surfaces to a dry atmosphere and suffer evaporative water loss as a consequence (Hall et al., 1993). As too much water loss would result in dehydration, plants cover most of their aerial surfaces with a relatively impermeable layer, called the cuticle, and take in CO₂ mainly through stomatal pores, which make up only about 2% per a leaf area (Willmer and Fricker, 1996). In other words, the cuticle provides drought tolerance to plants in exchange for reduced efficiency in CO₂ uptake.The cuticle is a continuous membrane consisting of a polymer matrix (cutin), polysaccharides, and organic solvent‐soluble lipids (cuticular waxes; Holloway, 1982; Jeffree, 1996; Riederer and Schreiber, 2001). The cuticle is an important structure to protect plants against excess drought, high temperature, strong UV radiation, pathogens, and harmful insects (Kerstiens, 1996a, 1996b; Burghardt and Riederer, 2006; Riederer and Müller, 2006; Domínguez et al., 2011; Yeats and Rose, 2013). The cuticle limits the transpiration through plant surfaces other than through the stomatal pores to <10% of the total (Mohr and Schopfer, 1995). On the other hand, this impermeable layer also strongly restricts CO₂ influx. Boyer et al. (1997) and Boyer (2015a, 2015b) reported a lower conductance for CO₂ than for water vapor in cuticles of intact leaves of grape (Vitis vinifera) and sunflower (Helianthus annuus) due to the differences in molecular size and diffusion paths between the two gases. However, although many studies have explored the water permeability of cuticles in various conditions and species (Kerstiens, 1996a; Riederer and Müller, 2006; Kosma et al., 2009; Schreiber and Schönherr, 2009), much less attention has been directed to CO₂, despite its substantial role in photosynthesis.In this study, we verified the hypothesis that plants could absorb CO₂ more efficiently under non-drought stress conditions if their cuticles are more permeable. In addition, we also investigated the extent to which a permeable cuticle can enhance CO₂ uptake efficiency. To verify the hypothesis, we investigated whether the CO₂ uptake efficiency is increased in a mutant with a high cuticle permeability. For this research, we isolated an Arabidopsis (Arabidopsis thaliana) mutant named excessive transpiration1 (extra1), which exhibited marked evaporative water loss due to an increased cuticle permeability caused by a new allele of ACETYL-COA CARBOXYLASE1 (ACC1). ACC1 encodes a critical enzyme for the synthesis of malonyl-CoA, an essential substrate for fatty acid synthesis (Baud et al., 2003). To evaluate CO₂ uptake efficiency, we investigated CO₂ assimilation rate, carbon content, and dry weight of the extra1 mutant and compared them to that of wild-type plants as well as that of another mutant, slow-type anion channel-associated1 (slac1) with continuously open stomata (Negi et al., 2008; Vahisalu et al., 2008). Our results reveal that the increased cuticle permeability strongly and constantly enhances CO₂ uptake efficiency under non-drought stress conditions.