Regulation of tomato fruit growth and development during different plant developmental stages by far‐red light: a combined anatomical, physiological, and transcriptomic analysis

SUMMARY Far‐red (FR) light, a critical environmental signal perceived via phytochrome photoreceptors, modulates numerous aspects of plant development; yet, its role in reproductive processes such as fruit growth and ripening remains less clear. In this study, we investigated the developmental stage‐specific effects of FR light on fruit development, anatomy, and metabolism in dwarf tomato ( Solanum lycopersicum ) plants. FR light was applied during three distinct stages: vegetative phase, early fruit development, and late fruit development. Three corresponding plant batches were analyzed: (i) full‐growth cycle plants for final fruit weight and plant architecture, (ii) plants harvested at the mature green stage for anatomical measurements, and (iii) plants during early fruit formation for integrated transcriptomic and metabolomic profiling of source (leaf) and sink (fruit) tissues. FR exposure from flowering to the full‐grown green fruit stage increased total fruit weight by enhancing fruit size, linked to an increase in both mesocarp cell layers and cell size. These anatomical changes were associated with transcriptional upregulation of genes related to auxin, gibberellins, and brassinosteroid biosynthesis and signaling in both leaves and fruits. Additionally, FR light accelerated the onset of ripening, coinciding with transcriptional upregulation of abscisic acid and ethylene biosynthesis pathways. Furthermore, our results reveal that FR light is initially perceived in leaves and subsequently modulates fruit development and ripening through hormone‐mediated signaling. This study provides new insights into the light‐regulated plasticity of reproductive development and highlights the importance of FR timing in optimizing fruit fresh weight and sugar content.