The MdAux/IAA2 Transcription Repressor Regulates Cell and Fruit Size in Apple Fruit

Abstract

Auxin plays an important role in regulating plant development, and <i>Auxin/indole acetic acid</i> (<i>Aux/IAA</i>) is a type of auxin-responsive gene and plays an important role in auxin signaling; to date, although 29 Aux/IAA proteins have been reported in <i>Abrabidopsis thaliana</i>, only parts of the <i>Aux/IAA</i> family gene functions have been identified. We previously reported that a bud sport of 'Longfeng' (LF) apple (<i>Malus domestica</i>), named 'Grand longfeng' (GLF), which showed a larger fruit size than LF, has lower expression of <i>MdAux/IAA2</i>. In this study, we identified the function of the <i>MdAux/IAA2</i> gene in apple fruit size difference using <i>Agrobacterium</i>-mediated genetic transformation. Overexpression of <i>MdAux/IAA2</i> decreased the apple flesh callus increment and caused a smaller globular cell size. In addition, overexpression of <i>MdAux/IAA2</i> in GLF fruit resulted in the reduction of apple fruit size, weight, and cell size, while silencing <i>MdAux/IAA2</i> in LF apple fruit resulted in an increase in apple fruit weight and cell size. We suggest that the high auxin content depressed the expression of <i>MdAux/IAA2</i>, and that the downregulated expression of <i>MdAux/IAA2</i> led to the formation of GLF. Our study suggests a mechanism for fruit size regulation in plants and we will explore the transcription factors functioning in this process in the future.