**Background:** Light is a critical environmental factor influencing all stages of plant growth and development, including root architecture. Auxin is an essential plant hormone that mediates rooting under different light treatments. While the crosstalk between light and auxin signaling in shoot–root communication has been studied, the molecular mechanisms of light–auxin crosstalk during rooting, phototropism, and geotropism had not been systematically reviewed. This review aims to summarize how light-regulated auxin signaling controls root development and to catalog the target genes and proteins involved.
**Methods:** This is a narrative review that synthesizes findings from primary research articles on light perception, photoreceptor function, auxin signaling components, and their interactions in root development across multiple plant species including Arabidopsis thaliana, Nicotiana attenuata, Lotus japonicus, Glycine max, Oryza sativa, Zea mays, Physcomitrella patens, Eucalyptus spp., Solanum lycopersicum, and others. The review covers studies on primary roots, lateral roots, adventitious roots, root hairs, rhizoids, seminal roots, crown roots, root phototropism, gravitropism, root greening, and root branching.
**Key Results:** The review identifies multiple photoreceptors involved in root development: phytochromes (PhyA, PhyB, PhyC, PhyD, PhyE), cryptochromes (CRY1, CRY2, CRY3), phototropins (PHOT1, PHOT2), and UVR8. PhyA and PhyB regulate root elongation and root hair formation; phyA-211 and phyB-9 mutants showed slow root elongation and irregular root hair formation. PhyA, PhyB, and PhyE induced lateral rooting while PhyD suppressed it. CRY1 and CRY2 antagonistically regulate primary root elongation under blue light — cry1 mutants had enhanced lateral root numbers while CRY1ox lines showed decreased lateral roots. PHOT1 mediates root phototropism over a wide range of blue light intensities. UVR8 overexpression decreased primary root length and lateral root density compared to wild-type.
Key signaling components include PIFs (PIF1-6), COP1, HY5, and MYB73/MYB77. PIF3 participates in nitric oxide-mediated root growth inhibition under continuous white light (~100 μmol m⁻² s⁻¹). PIF4 promotes primary root growth under aluminum stress via auxin signaling. PIF1, PIF3, PIF4, and PIF5 positively regulate hypocotyl elongation and adventitious root formation. HY5 mediates light-promoted root growth and nitrate uptake by activating NRT2.1 expression. Under far-red light, HY5 accumulates in lateral root primordia and represses ARF19, PIN3, and LAX3 expression, reducing lateral root density.
Specific auxin-related target genes identified include PIN1, PIN2, PIN3, PIN4, PIN7, AUX1, LAX3, ARF7, ARF19, IAA7, IAA14, IAA29, YUC1, YUC4, SAUR23, SAUR28, SAUR68, HAT2, and miR775. Under blue light, PHOT1 and PHOT2 enhance adventitious root number and density via PIN3. In maize under continuous white light (~36 μmol m⁻² s⁻¹), IAA content increased in root tips, especially in the transition zone, and the Zmyuc gene ZM2G141383 was upregulated in the 0–1 mm tip region. UV-B irradiation via UVR8 inhibited lateral root growth by suppressing MYB73/MYB77 DNA-binding activity.
**Clinical Implications:** This review has no direct clinical or medical applications. Its significance lies in agricultural and horticultural contexts — understanding light–auxin crosstalk could improve propagation techniques (e.g., adventitious rooting in cuttings), optimize light conditions for crop root systems, and enhance nutrient uptake efficiency. The authors note that mechanisms differ across plant species (e.g., barley vs. wheat) and that further research is needed to identify additional photoreceptors and complete the molecular understanding of light–auxin signaling during rooting.