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

Jia Feng,Yafan Han,Cheng Dai,Zhongchi Liu,Huifeng Luo,Chunying Kang

doi:10.1093/jxb/ery384

Abstract

The critical role of auxin in strawberry fruit set and receptacle enlargement was demonstrated previously. While fertilization is known to trigger auxin biosynthesis, the specific tissue source of fertilization-induced auxin is not well understood. Here, the auxin reporter DR5ver2::GUS was introduced into wild strawberry (Fragaria vesca) to reveal auxin distribution 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 catalysing the two-step auxin biosynthesis pathway was investigated using their respective promoters fused to the β-glucuronidase (GUS) reporter. Two FveTARs and four FveYUCs were shown to be expressed primarily in the endosperm and embryo inside the achenes as well as in root tips and lateral root primordia. Expression of these reporters 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, we generated CRISPR-mediated knock-out mutants of FveYUC10, the most abundant YUC in seeds; 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 vasculature, root tip, and lateral root primordia and highlighted the endosperm as the main auxin biosynthesis site for fruit set.

Highlights

Cultivated strawberry (Fragaria ananassa) is one of the major fruit crops grown worldwide
Auxin biosynthesis genes were not expressed in the receptacle during early-stage fruit development (Kang et al, 2013) and free auxin in the fruit receptacle was about 5 ng g−1 fresh weight, significantly lower than that in the achenes (Dreher and Poovaiah, 1982; Symons et al, 2012; Estrada-Johnson et al, 2017).Together, these observations suggest that fertilization-induced auxin biosynthesis primarily occurs in the achenes of strawberry
As auxin distribution in roots was well documented in Arabidopsis, GUS staining was first carried out in F. vesca roots to assess whether DR5ver2::GUS acts correctly in these F. vesca transgenic lines in the T0 generation

Summary

Introduction

Cultivated strawberry (Fragaria ananassa) is one of the major fruit crops grown worldwide. As the source of the auxin for promotion of receptacle fruit set This early observation was supported later by RNA-seq data and measurement of free auxin. Auxin biosynthesis genes were not expressed in the receptacle during early-stage fruit development (Kang et al, 2013) and free auxin in the fruit receptacle was about 5 ng g−1 fresh weight, significantly lower than that in the achenes (Dreher and Poovaiah, 1982; Symons et al, 2012; Estrada-Johnson et al, 2017).Together, these observations suggest that fertilization-induced auxin biosynthesis primarily occurs in the achenes of strawberry

Methods

Results

Conclusion