Abstract: Vascular sheath tissue-specific expression of phytochrome B or variants thereof in C3 plants increases photosynthesis rate and/or introduces a carbon refixation mechanism. The heritable genetic material of a C3 plant cell is altered such that one copy of phytochrome B, or active variant or functional fragment thereof is expressed specifically in vascular sheath cells. Whole plants are regenerated from these genetically altered plant cells. Alternatively, a Crispr modification of a native phytochrome locus in a plant cell is used to insert a vascular sheath-specific regulatory element, e.g. promoter or enhancer element, so that phytochrome B is expressed in vascular sheath cells of a regenerated whole plant. Genetically altered whole plants have increased yield-related traits, e.g. increased seed yield, resulting from the enhancement of photosynthesis and/or introduction of a carbon refixation mechanism.