| Summary: | The phytochrome-interacting factor (PIF) proteins are part of a subfamily of basic helix–loop–helix (bHLH) transcription factors that integrate with phytochromes (PHYs) and are known to play important roles in adaptive changes in plant architecture. However, the characterization and function of <i>PIFs</i> in potatoes are currently poorly understood. In this study, we identified seven <i>PIF</i> members in potatoes and named them <i>StPIF01-1</i>, <i>StPIF01-2</i>, <i>StPIF03</i>, <i>StPIF06-1</i>, <i>StPIF06-2</i>, <i>StPIF07</i>, and <i>StPIF09</i> based on their location in potato chromosomes. The chromosomal location, gene structures, physicochemical characteristics, phylogenetic tree, and tissue-specific expression of <i>StPIFs</i> were also analyzed. RT-qPCR analysis revealed that the <i>StPIF3</i> gene was highly induced by shade and may play a crucial regulatory role in potato responses to shade stress. Also, multiple cis-regulatory elements involved in light response were detected in the promoter of the <i>StPIF</i> genes. Subcellular localization analysis indicated that the StPIF3-encoding protein is mainly localized in the nucleus. Transgenic overexpression of <i>StPIF3</i> in potatoes increased stem length, chlorophyll accumulation, and enhanced shade-avoidance symptoms, whereas the <i>StPIF3</i>-interfering lines had a lower plant height and more chlorophyll accumulation. These findings enhance our comprehension of <i>StPIF</i> gene roles, potentially advancing potato yield and quality research. This study provides detailed information about <i>StPIFs</i> and identifies the function of <i>StPIF3</i>, which is involved in shade-avoidance syndrome.
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