Conserved and newly acquired roles of PIF1 homologs in tomato (Solanum lycopersicum)

PHYTOCHROME INTERACTING FACTORS (PIFs) are transcription factors that interact with the photoreceptors phytochromes and integrate multiple signaling pathways related to light, temperature, defense and hormone responses. PIFs have been extensively studied in Arabidopsis thaliana, but less is known about their roles in other species. Here, we investigate the role of the two homologs of PIF1 found in tomato (Solanum lycopersicum), namely PIF1a and PIF1b. Analysis of gene expression showed very different patterns, indicating a potential evolutionary divergence in their roles. At the protein level, light regulated the stability of PIF1a, but not PIF1b, further supporting a functional divergence. Phenotypic analyses of CRISPR-Cas9-generated tomato mutants defective in PIF1a or PIF1b or both revealed conserved and newly acquired roles compared to Arabidopsis PIF1. Both PIF1a or PIF1b were found to regulate seed germination, photosynthetic pigment biosynthesis and fruit production. However, only PIF1a-defective mutants showed defects on root hair elongation, flowering time and fruit growth and softening. We did not identify any process altered only in plants lacking PIF1b. Together, these data show that neofunctionalization has taken place in tomato, illustrating the potential of these transcription factors to acquiring new roles in different species.