Pomegranate Aux/IAA9A gene offers new insights into seed development and tissue lignification

Abstract

The auxin/indole-3-acetic acid (Aux/IAA) transcription repressor acts as a key switch in the auxin signaling pathway and plays important roles in various aspects of plant development and growth, including fruit and seed development. However, little is known about Aux/IAA genes in pomegranate. We carried out a genome-wide association study to identify candidate Aux/IAA genes involved in pomegranate seed coat development and performed a gene function study of the candidate gene PgIAA9A. Twenty-four PgIAA genes were identified in the pomegranate genome. Among them, PgIAA15, PgIAA16, PgIAA9A, PgIAA3B, PgIAA27B, and PgIAA11 exhibited high levels of expression in the seed coat, including the fleshy, edible outer seed coat and the sclerotic inner seed coat. Furthermore, the transcription of these six genes in the inner seed coat was up-regulated by exogenous IAA. Overexpression of PgIAA9Am encoding a stabilized PgIAA9A protein in Arabidopsis resulted in restricted plant growth and development, exemplified by reduced silique length and seed size and decreased lignin contents in stems, leaves, and seeds. Moreover, the expression of genes related to vascular development was reduced. These results shed light on the potential roles of PgIAA9A in pomegranate seed coat development, particularly in terms of seed size and hardness.