The IAA- and ABA-responsive transcription factor CgMYB58 upregulates lignin biosynthesis and triggers juice sac granulation in pummelo

Abstract

In citrus, lignin overaccumulation in the juice sac results in granulation and an unpleasant fruit texture and taste. By integrating metabolic phenotyping and transcriptomic analyses, we found 702 differentially expressed genes (DEGs), including 24 transcription factors (TFs), to be significantly correlated with lignin content. <i>CgMYB58</i> was further identified as a critical R2R3 MYB TF involved in lignin overaccumulation owing to its high transcript levels in Huanong Red-fleshed pummelo (HR, <i>Citrus grandis</i>) fruits. Transient expression of <i>CgMYB58</i> led to an increase in the lignin content in the pummelo fruit mesocarp, whereas its stable overexpression significantly promoted lignin accumulation and upregulated 19 lignin biosynthetic genes. Among these genes, <i>CgPAL1</i>, <i>CgPAL2</i>, <i>Cg4CL1</i>, and <i>CgC3H</i> were directly modulated by CgMYB58 through interaction with their promoter regions. Moreover, we showed that juice sac granulation in pummelo fruits could be affected by indole-3-acetic acid (IAA) and abscisic acid (ABA) treatments. In HR pummelo, ABA significantly accelerated this granulation, whereas IAA effectively inhibited this process. Taken together, these results provide novel insight into the lignin accumulation mechanism in citrus fruits. We also revealed the theoretical basis via exogenous IAA application, which repressed the expression of <i>CgMYB58</i> and its target genes, thus alleviating juice sac granulation in orchards.