Phototropism: at the crossroads of light-signaling pathways

• Phototropins are the primary photoreceptors controlling growth towards light.
• Phototropism is enhanced by phytochromes and cryptochromes.
• Phytochromes and cryptochromes promote phototropism by controlling gene expression.
• Phototropin–phytochrome interaction further enhances phototropism in some plant species.

Phototropism enables plants to orient growth towards the direction of light and thereby maximizes photosynthesis in low-light environments. In angiosperms, blue-light photoreceptors called phototropins are primarily involved in sensing the direction of light. Phytochromes and cryptochromes (sensing red/far-red and blue light, respectively) also modulate asymmetric hypocotyl growth, leading to phototropism. Interactions between different light-signaling pathways regulating phototropism occur in cryptogams and angiosperms. In this review, we focus on the molecular mechanisms underlying the co-action between photosensory systems in the regulation of hypocotyl phototropism in Arabidopsis thaliana. Recent studies have shown that phytochromes and cryptochromes enhance phototropism by controlling the expression of important regulators of phototropin signaling. In addition, phytochromes may also regulate growth towards light via direct interaction with the phototropins.