背景回顾：The basic helix-loop-helix (bHLH) transcription factor PHYTOCHROME-INTERACTING FACTOR 7 (PIF7) is a central regulator that promotes stem growth by activating growth-related gene expression during shade-avoidance responses. 

研究意义：Studying the co-factors of PIF7 can facilitate understanding of the mechanism of PIFs and light signal transduction.

主要发现：Here, we describe the identification of two bHLH transcription factors, bHLH48 and bHLH60 (bHLH48/bHLH60), as essential partners for PIF7-dependent modulation of hypocotyl elongationand function downstream of phytochrome B. 

结果1-分子功能：These two bHLH factors display DNA binding activity and interact with PIF7. 

结果2-遗传分析：Genetic analysis indicated that bHLH48/bHLH60 and PIF7 are interdependent in promoting hypocotyl elongation. 

结果3-ChIP-seq：Chromatin immunoprecipitation sequencing (ChIP-seq) analysis identified the substantially overlapping downstream targets of bHLH60 and PIF7. 

结果3-生化分析：Biochemical analysis revealed that bHLH48/bHLH60 enhance the DNA binding ability of PIF7. 

结论：These results provide evidence that bHLH48/bHLH60 act as positive partners of PIF7 for mutual benefit in the regulation of hypocotyl elongation.

植物中，bHLH转录因子光敏色素互作因子PIF7是促进茎生长的主要调控因子，其在植物避阴响应中能够激活与生长相关基因的表达。而研究PIF7共因子能够加深我们对于PIFs和光信号转导的认识。本文中，作者鉴定了两个bHLH转录因子，分别是bHLH48和bHLH60，这两个转录因子在PIF7依赖型的下胚轴伸长调控和光敏色素B下游功能的发挥都是必不可少的组分。bHLH48/bHLH60均能够结合PIF7，并与之互作。遗传分析显示，bHLH48/bHLH60和PIF7在促进下胚轴伸长方面的功能是相互依赖的。染色质免疫共沉淀测序分析鉴定到了bHLH60和PIF7对于下游靶向区域存在重叠峰。生化实验显示bHLH48/bHLH60能够增强PIF7的DNA结合能力。本文的研究结果揭示了bHLH48/bHLH60作为PIF7的正向“伙伴”蛋白，共同作用于下胚轴伸长的调控。