The spacing of lateral roots (LRs) along the main root in plants is driven by an oscillatory signal, often referred to as the “root clock” that represents a pre-patterning mechanism that can be influenced by environmental signals. Light is an important environmental factor that has been previously reported to be capable of modulating the root clock, although the effect of light signaling on the LR pre-patterning has not yet been fully investigated. In this study, we reveal that light can activate the transcription of a photomorphogenic gene HY1 to maintain high frequency and amplitude of the oscillation signal, leading to the repetitive formation of pre-branch sites. By grafting and tissue-specific complementation experiments, we demonstrated that HY1 generated in the shoot or locally in xylem pole pericycle cells was sufficient to regulate LR branching. We further found that HY1 can induce the expression of HY5 and its homolog HYH, and act as a signalosome to modulate the intracellular localization and expression of auxin transporters, in turn promoting auxin accumulation in the oscillation zone to stimulate LR branching. These fundamental mechanistic insights improve our understanding of the molecular basis of light-controlled LR formation and provide a genetic interconnection between shoot- and root-derived signals in regulating periodic LR branching.

植物中沿主根的侧根 (LR) 的间距由振荡信号驱动，通常称为“根时钟”，它代表一种受环境信号影响的预模式机制。光是一个重要的环境因素，之前据报道它能够调制根时钟，尽管尚未充分研究光信号对 LR 预图案化的影响。在这项研究中，我们揭示了光可以激活光形态发生基因HY1的转录保持振荡信号的高频和振幅，导致前分支位点的重复形成。通过嫁接和组织特异性互补实验，我们证明在枝条中或在木质部极周细胞中局部产生的 HY1 足以调节 LR 分支。我们进一步发现HY1可以诱导HY5及其同源物HYH的表达，并作为信号体调节生长素转运蛋白的细胞内定位和表达，进而促进生长素在振荡区的积累，从而刺激 LR 分支。这些基本的机制见解提高了我们对光控 LR 形成的分子基础的理解，并在调节周期性 LR 分支中提供了芽和根衍生信号之间的遗传相互联系。