PbDELLA-PbMYB56-PbCYP78A6 module regulates GA4 + 7-induced pseudo-embryo development and parthenocarpy in pear (Pyrus bretschneideri)

Abstract

Parthenocarpy can ensure fruit setting without fertilization and generate seedless fruits. PbCYP78A6 has been shown to play a role in gibberellin (GA)-induced parthenocarpy in pears. However, the transcriptional response mechanism of PbCYP78A6 to GA remains unclear. In this study, using a yeast one-hybrid assay combined with co-expression analysis, PbMYB56 was initially identified as a transcription regulator of PbCYP78A6, which was further confirmed by electrophoretic mobility shift assay (EMSA) and dual-luciferase reporter assays. The biofunction of PbMYB56 was further verified using transient transgene tests, stable transgenic pear callus and tomato. PbMYB56 overexpression resulted in reduced cell death and higher f luorescence intensity after f luoresce diacetate (FDA) staining, as well as delayed fruit-drop by increasing PbCYP78A6 expression in unpollinated pear fruitlets and callus. In contrast, silencing PbMYB56 caused cell death and early fruit-drop with decreased PbCYP78A6 expression. Moreover, after emasculation, heterologous overexpression of PbMYB56 induced parthenocarpy and enlarged seed size in pollinated tomato fruits. Silencing SlMYB56, a homolog of PbMYB56 in tomatoes, resulted in smaller fruit and seed size, and these traits were restored by co-overexpression with PbCYP78A6. Furthermore, we investigated the protein interaction between PbMYB56 and PbDELLA, which is crucial component of the GA signaling pathway. This interaction inhibited PbMYB56-induced transcriptional activation of PbCYP78A6. Co-overexpression of PbMYB56 and PbDELLA contributed to reduced seed development and loss of parthenocarpy potential in tomatoes. Collectively, our study identifies PbDELLA-PbMYB56-PbCYP78A6 as a regulatory module of GA4 + 7-induced pseudo-embryo and parthenocarpy development, offering insights into the mechanism underlying parthenocarpy formation in pears.