Pomegranate ATP-binding cassette transporter PgABCG9 plays a negative regulatory role in lignin accumulation

Seed hardness is an important quality characteristic of pomegranate fruit. The development of seed hardness relies on the deposition of lignin in the inner seed coat, but the underlying molecular mechanisms remain unclear. In this study, we identified a member of ABCG transporters, PgABCG9, which may function in seed hardening by negatively regulating lignin biosynthesis. PgABCG9 was expressed at high levels in the inner seed coats of pomegranate fruit, and its transcript level was negatively correlated with seed hardness. PgABCG9-transgenic Arabidopsis plants exhibited weaker growth and thinner stems than the wild-type. The number of xylem cells, xylem cell wall thickness, and lignin deposition in the PgABCG9 transgenic plants were significantly reduced. In addition, overexpression of PgABCG9 in Arabidopsis enhanced plant tolerance to exogenous monolignols. Targeted metabolite profiling revealed that the contents of metabolites involved in lignin biosynthesis, including monolignols and monolignol precursors, were also reduced in PgABCG9- transgenic plants. We found that PgABCG9 is localized to the Golgi. These findings indicate that PgABCG9 plays a negative regulatory role in lignin biosynthesis and potentially contributes to soft-seed development in pomegranate through a mechanism that includes the reduction of lignin content in the seed coat by sequestration of monolignols in intracellular compartments.