Modifying Plant Photosynthesis and Growth via Simultaneous Chloroplast Transformation of Rubisco Large and Small Subunits

Abstract

Engineering improved Rubisco for the enhancement of photosynthesis is challenged by the alternate locations of the chloroplast <i>rbcL</i> gene and nuclear <i>RbcS</i> genes. Here we develop an RNAi-<i>RbcS</i> tobacco (<i>Nicotiana tabacum</i>) master-line, tobRrΔS, for producing homogenous plant Rubisco by <i>rbc</i>L-<i>rbc</i>S operon chloroplast transformation. Four genotypes encoding alternative <i>rbcS</i> genes and adjoining 5'-intergenic sequences revealed that Rubisco production was highest (50% of the wild type) in the lines incorporating a <i>rbcS</i> gene whose codon use and 5' untranslated-region matched <i>rbcL</i> Additional tobacco genotypes produced here incorporated differing potato (<i>Solanum tuberosum</i>) <i>rbcL</i>-<i>rbcS</i> operons that either encoded one of three mesophyll small subunits (pS1, pS2, and pS3) or the potato trichome pS^T-subunit. The pS3-subunit caused impairment of potato Rubisco production by ∼15% relative to the lines producing pS1, pS2, or pS^T However, the βA-βB loop Asn-55-His and Lys-57-Ser substitutions in the pS3-subunit improved carboxylation rates by 13% and carboxylation efficiency (CE) by 17%, relative to potato Rubisco incorporating pS1 or pS2-subunits. Tobacco photosynthesis and growth were most impaired in lines producing potato Rubisco incorporating the pS^T-subunit, which reduced CE and CO₂/O₂ specificity 40% and 15%, respectively. Returning the <i>rbcS</i> gene to the plant plastome provides an effective bioengineering chassis for introduction and evaluation of novel homogeneous Rubisco complexes in a whole plant context.