Significance: Plants often grow at high plant densities where they risk being shaded by surrounding plants. Neighbors are detected through changes in the composition of reflected light, and plants respond to such changes by growing their photosynthetic organs away from their competitors. This research shows thatArabidopsisplants first detect these light cues in the tips of their leaves and that this information then is transmitted through the mobile plant hormone auxin to the very base of the organ, where it induces an upward leaf movement response. 3D computational models show that this spatial separation in signal detection and response is adaptive for plant performance in dense stands.

Light absorption by plants changes the composition of light inside vegetation. Blue (B) and red (R) light are used for photosynthesis whereas far-red (FR) and green light are reflected. A combination of UV-B, blue and R:FR-responsive photoreceptors collectively measures the light and temperature environment and adjusts plant development accordingly. This developmental plasticity to photoreceptor signals is largely regulated through the phytohormone auxin. The phytochrome, cryptochrome and UV Resistance Locus 8 (UVR8) photoreceptors are inactivated in shade and/or elevated temperature, which releases their repression of Phytochrome Interacting Factor (PIF) transcription factors. Active PIFs stimulate auxin synthesis and reinforce auxin signalling responses through direct interaction with Auxin Response Factors (ARFs). It was recently discovered that shade-induced hypocotyl elongation and petiole hyponasty depend on long-distance auxin transport towards target cells from the cotyledon and leaf tip, respectively. Other responses, such as phototropic bending, are regulated by auxin transport and signalling across only a few cell layers. In addition, photoreceptors can directly interact with components in the auxin signalling pathway, such as Auxin/Indole Acetic Acids (AUX/IAAs) and ARFs. Here we will discuss the complex interactions between photoreceptor and auxin signalling, addressing both mechanisms and consequences of these highly interconnected pathways.

Plastids are crucial for fuelling and regulating plant growth and development. Photosynthesising chloroplasts provide energy for growth, while other plastids play additional key roles in various aspects of plant physiology. For function and development, plastids greatly depend on nucleus‐encoded proteins, and they can modulate the synthesis of these proteins by sending retrograde signals to the nucleus. These signals communicate the developmental and operational status of the plastid, both of which are sensitive to the environment. Abiotic stressors such as drought, salinity, and suboptimal light and temperature conditions can induce changes in chloroplast metabolism, ultrastructure and cellular positioning. In response to specific environmental triggers, retrograde signals reprogramme nuclear gene expression to fine‐tune plastid form and function, but also influence whole‐plant morphology. Over the past years, the chloroplast responses to stress have become clearer. Various sources of retrograde signals, derived from plastid metabolism, plastid gene expression and altered photosynthetic redox balance, are now known to directly interfere with canonical signalling pathways. However, most of what is known about retrograde signalling originates from studies using artificial stressors, such as chemical treatments or genetic mutations, and its importance in natural environments is still poorly understood. This review highlights the understanding of plastid responses to the environment, as well as the impact generated downstream of retrograde signals, to better understand the role of plastids in abiotic stress resilience of flowering plants.

Summary: Although plants are immobile, many of their organs are flexible to move in response to environmental cues. In dense vegetation plants detect neighbours through far-red light perception with their leaf tip. They respond remotely, with asymmetrical growth between the abaxial and adaxial sides of the leafstalk, the petiole. This results in upward movement that brings the leaf blades into better lit zones of the canopy. The plant hormone auxin is required for this response, but it is not understood how non-differential leaf tip-derived auxin can remotely regulate movement. Here we show that remote light signalling promotes auxin accumulation in the abaxial petiole by reinforcing an intrinsic auxin transport directionality. In the petiole, auxin elicits a response of both auxin as well as a second growth promoter; gibberellin. We show that this dual regulation is necessary for hyponastic leaf movement in response to light. Our results reveal how plants can spatially relay information about neighbour proximity from their sensory leaf tips to the petiole base, thus driving adaptive growth.

Seedling establishment is highly sensitive to environmental cues, with light serving as a principal regulator. In darkness, seedlings enter skotomorphogenesis, marked by hypocotyl elongation and apical hook formation, which helps seedlings emerge from the soil to reach the light. Here, we show that salinity impairs soil emergence of Arabidopsis thaliana seedlings by inducing a partially photomorphogenic like phenotype in darkness, characterized by reduced apical hook curvature. Within the hook, salt stress diminished differential epidermal cell elongation required to drive hook bending, and reduced both the auxin signalling maximum and PIN3 abundance on the concave side of the hook. Transcriptome analysis revealed that salt and osmotic stresses directly alter organ-specific transcriptional profiles, including the NaCl-mediated repression of B-BOX DOMAIN PROTEIN 28 (BBX28) in hooks and cotyledons. Under control conditions, BBX28 protein accumulated asymmetrically across the hook, but this pattern was lost under salinity. Notably, the ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) exhibited an opposite gradient to BBX28, which was similarly disrupted by salt stress or by the inhibition of polar auxin transport, presenting auxin as an upstream regulator of COP1 spatial distribution. Together, these findings establish COP1 asymmetry as a novel feature of the apical hook, indicating that spatial regulation, not just absolute levels, shapes COP1 function, and reveal how salinity disrupts hormonal, transcriptional and protein networks to compromise seedling establishment under stress.