Reporter gene expression reveals precise auxin synthesis sites during fruit and root development in the wild strawberry.

The critical role of auxin in strawberry fruit set and receptacle enlargement was previously demonstrated. While fertilization was known to trigger auxin biosynthesis, the specific tissue source of fertilization-induced auxin is not well understood. Here, auxin reporter DR5ver2::GUS was introduced into wild strawberry Fragaria vesca to reveal auxin distributions in the seed and fruit receptacle pre- and post-fertilization as well as in the root. In addition, the expression of TAR and YUCCA genes coding for enzymes catalyzing the two-step auxin biosynthesis pathway was investigated using their respective promoters fused to the GUS reporter. Two FveTARs and four FveYUCs were shown to be expressed primarily in the endosperms and embryo inside the achenes as well as in root tips and lateral root primordia. These reporter expressions in dissected tissues provided more detailed and precise spatial (cell and tissue) and temporal (pre- and post-fertilization) information on where auxin is synthesized and accumulates than previous studies in strawberry. Moreover, CRISPR-mediated knock-out mutants of FveYUC10, the most abundant YUC in seeds, were obtained; the mutants had a lower free auxin level in young fruit, but displayed no obvious morphological phenotypes. However, overexpression of FveYUC10 resulted in elongated hypocotyls in Arabidopsis caused by elevated auxin level. Overall, the study revealed auxin accumulation in the chalazal seed coat, embryo, receptacle vasculatures, root tip, and lateral root primordia and highlighted endosperm as the main auxin biosynthesis site for fruit set.