Plant proximity reduces seed yield in Arabidopsis plants by decreasing the number of ovule primordia.

Proximity of vegetation, which is influenced by planting density, significantly impacts plant development. In Arabidopsis thaliana, it is well established that simulated shade, which mimics the proximity of other plants, triggers hypocotyl and petiole elongation, accelerates flowering and suppresses axillary bud growth. Although there is evidence that simulated shade affects reproduction beyond accelerating flowering, its impact on the development of reproductive tissues after plant architecture establishment (i.e., once flowering has begun) remains poorly explored. Here, we report that simulated shade promotes silique and pedicel elongation while reducing seed production, primarily by decreasing ovule number formation. Shade perception triggers rapid changes in gene expression in reproductive tissues, with some genes showing tissue-specific responses and others being induced in both seedlings and reproductive tissues, highlighting a conserved core of shade-responsive genes associated with light perception, photosynthesis and hormone regulation. However, while shade-induced elongation responses occur rapidly, reduction in ovule number requires prolonged shade exposure, suggesting distinct regulatory pathways for these responses. These findings shed light on the complex interplay between common (e.g., elongation and core gene expression) and tissue-specific responses (e.g., ovule formation and specialized gene expression) to shade, contributing to the developmental plasticity of Arabidopsis. Furthermore, they enhance our understanding of how external signals, indicative of vegetation proximity, can modulate seed production, a genetically determined process.