Transgenic insertion of the cyanobacterial membrane protein ictB increases grain yield in Zea mays through increased photosynthesis and carbohydrate production

Abstract

The C4 crop maize (Zea mays) is the most widely grown cereal crop worldwide and is an essential feedstock for food and bioenergy. Improving maize yield is important to achieve food security and agricultural sustainability in the 21st century. One potential means to improve crop productivity is to enhance photosynthesis. ictB, a membrane protein that is highly conserved across cyanobacteria, has been shown to improve photosynthesis, and often biomass, when introduced into diverse C3 plant species. Here, ictB from Synechococcus sp. strain PCC 7942 was inserted into maize using Agrobacterium-mediated transformation. In three controlled-environment experiments, ictB insertion increased leaf starch and sucrose content by up to 25% relative to controls. Experimental field trials in four growing seasons, spanning the Midwestern United States (Summers 2018 & 2019) and Argentina (Winter 2018 & 2019), showed an average of 3.49% grain yield improvement, by as much as 5.4% in a given season and up to 9.4% at certain trial locations. A subset of field trial locations was used to test for modification of ear traits and ФPSII, a proxy for photosynthesis. Results suggested that yield gain in transgenics could be associated with increased ФPSII, and the production of longer, thinner ears with more kernels. ictB localized primarily to the microsome fraction of leaf bundle-sheath cells, but not to chloroplasts. Extramembrane domains of ictB interacted in vitro with proteins involved in photosynthesis and carbohydrate metabolism. To our knowledge, this is the first published evidence of ictB insertion into a species using C4 photosynthesis and the largest-scale demonstration of grain yield enhancement from ictB insertion in planta. Results show that ictB is a valuable yield gene in the economically important crop maize, and are an important proof of concept that transgenic manipulation of photosynthesis can be used to create economically viable crop improvement traits.

A Year at the Forefront of Engineering Photosynthesis

“…In agreement with this, transgenic RbcS::ictB maize lines had a 3.49% increase in yield compared to controls in field trials, with the increased performance attributed to enhanced carbohydrate production ( Koester et al, 2021 ). Yield increase was associated with an increase in photosystem II operating efficiency, suggesting that enhanced photosynthesis was responsible for the improvements ( Koester et al, 2021 ). These results support the hypothesis that IctB is not a CCM bicarbonate transporter: a complete understanding of how CCMs operate therefore remains lacking.…”

“…If IctB is not a bicarbonate transporter, previous suggestions that ictB expression in C 3 species enhances photosynthesis by facilitating a CCM ( Lieman-Hurwitz et al, 2003 ; Simkin et al, 2015 ; Gong et al, 2015 ; Hay et al, 2017 ) must be overlooking an alternative function. ictB expression in a C 4 species should therefore recapitulate the enhanced photosynthesis phenotype of ictB expression in C 3 plants ( Koester et al, 2021 ). In agreement with this, transgenic RbcS::ictB maize lines had a 3.49% increase in yield compared to controls in field trials, with the increased performance attributed to enhanced carbohydrate production ( Koester et al, 2021 ).…”

“…ictB expression in a C 4 species should therefore recapitulate the enhanced photosynthesis phenotype of ictB expression in C 3 plants ( Koester et al, 2021 ). In agreement with this, transgenic RbcS::ictB maize lines had a 3.49% increase in yield compared to controls in field trials, with the increased performance attributed to enhanced carbohydrate production ( Koester et al, 2021 ). Yield increase was associated with an increase in photosystem II operating efficiency, suggesting that enhanced photosynthesis was responsible for the improvements ( Koester et al, 2021 ).…”

Field-grown ictB tobacco transformants show no difference in photosynthetic efficiency for biomass relative to the wild type

“…The results regarding the effect of ictB on crop biomass production have so far been inconclusive. For example, in some cases, ictB -overexpressing plants have produced significantly greater biomass ( Lieman-Hurwitz et al , 2003 ; Hay et al , 2017 ; Koester et al , 2021 ), while in other studies overexpression of ictB did not significantly alter biomass production ( Gong et al , 2015 ). This suggests that the mechanism that underlies ictB may be greatly affected by environmental factors, and that an increase in crop productivity may only happen under certain conditions, although these conditions have yet to be identified.…”

“…Paddy-grown rice expressing ictB had significantly (10.5%) higher mesophyll conductance ( g m ) and 13.5% higher A compared with the WT ( Gong et al , 2015 ). Field-grown maize also benefited with increases in A and carbohydrate production, with increases in yield of up to 9.4% ( Koester et al , 2021 ). Replicated field trials of ictB -expressing soybean showed significant increases of 25% in g m , 14% in A , and 15% in seed yield relative to the WT ( Hay et al , 2017 ).…”