Expression of a CO₂-permeable aquaporin enhances mesophyll conductance in the C₄speciesSetaria viridis

Abstract

A fundamental limitation of photosynthetic carbon fixation is the availability of CO2. In C4 plants, primary carboxylation occurs in mesophyll cytosol, and little is known about the role of CO2 diffusion in facilitating C4 photosynthesis. We have examined the expression, localization, and functional role of selected plasma membrane intrinsic aquaporins (PIPs) from Setaria italica (foxtail millet) and discovered that SiPIP2;7 is CO2-permeable. When ectopically expressed in mesophyll cells of S. viridis (green foxtail), SiPIP2;7 was localized to the plasma membrane and caused no marked changes in leaf biochemistry. Gas-exchange and C18O16O discrimination measurements revealed that targeted expression of SiPIP2;7 enhanced the conductance to CO2 diffusion from the intercellular airspace to the mesophyll cytosol. Our results demonstrate that mesophyll conductance limits C4 photosynthesis at low pCO2 and that SiPIP2;7 is a functional CO2 permeable aquaporin that can improve CO2 diffusion at the airspace/mesophyll interface and enhance C4 photosynthesis.

Here comes the sun: How optimization of photosynthetic light reactions can boost crop yields

“…In recent years, different strategies to improve crop yields have been proposed and reviewed, such as: (i) introducing photorespiratory bypasses (Betti et al, 2016; Hagemann and Bauwe, 2016; South et al, 2018; Eisenhut et al, 2019; López‐Calcagno et al, 2019; Maurino, 2019; Shen et al, 2019; Khurshid et al, 2020; Wang et al, 2020; Abbasi et al, 2021); (ii) introducing algal/cyanobacterial carbon concentrating mechanisms (McGrath and Long, 2014; Rae et al, 2017; Long et al, 2018; Atkinson et al, 2020; Hennacy and Jonikas, 2020; Chen et al, 2021; Rottet et al, 2021); (iii) introducing the C4 photosynthesis pathway into C3 plants (Ermakova et al, 2020, 2021a); (iv) improving mesophyll conductance (Hanba et al, 2004; Xu et al, 2019; Lundgren and Fleming, 2020; Ermakova et al, 2021c); (v) modifying metabolic processes (Rossi et al, 2015; South et al, 2019); and (vi) modifying circadian rhythms and introducing chronocultures (Steed et al, 2021). During the second half of the 20th century, the Green Revolution led to improved grain yields through conventional breeding techniques and improved pest/disease control.…”

Mesophyll conductance response to short‐term changes in pCO2 is related to leaf anatomy and biochemistry in diverse C4 grasses

“…The role of CO 2 -permeable aquaporins in enhancing g m has been well characterised in C 3 species (Uehlein et al, 2008). Only recently, it has been demonstrated that overexpressing a CO 2 -permeable aquaporin in plasma membranes of Setaria viridis (C 4 grass) can enhance C 4 -g m (Ermakova et al, 2021). We did not investigate the role of aquaporins in affecting the CO 2 response of C 4 -g m .…”

Engineered Accumulation of Bicarbonate in Plant Chloroplasts: Known Knowns and Known Unknowns

“…Nonetheless, a number of oxyanions, such as phosphate, nitrate, and sulfate evidently do diffuse through the OEM (Bölter et al, 1999). Notably, simple diffusion of CO 2 through leaf tissue is insufficient to support the supply rates needed for observed rates of CO 2 assimilation by plants (Morison et al, 2005), and it is likely that CO 2 entry into the chloroplast is also facilitated by CO 2permeable aquaporins (Flexas et al, 2006;Evans et al, 2009;Tolleter et al, 2017;Ermakova et al, 2021) and CA-driven distribution of C i between predominant species (HCO 3 − and CO 2 ; Price et al, 1994). Therefore, the facilitated entry of C i into C 3 chloroplasts is conceptually not counter to contemporary chloroplast function, and on face value would appear beneficial.…”