# OsIPK2 Regulates Seed Vigor by Integrating IP6 Biosynthesis, Auxin Signaling, and H3K27me3 Deposition in Japonica Rice

## TL;DR

The study reveals how OsIPK2 regulates seed vigor in rice by affecting IP6 levels, auxin signaling, and epigenetic marks, offering new insights for improving seed quality.

## Contribution

The novel contribution is identifying OsIPK2's role in balancing seed vigor and seedling growth through IP6 metabolism, auxin signaling, and H3K27me3 regulation.

## Key findings

- Exogenous IP6 inhibits rice seed germination and root growth by disrupting auxin signaling.
- RNAi of OsIPK2 reduces seed IP6 content and germination but enhances seedling growth and resistance to IP6.
- OsIPK2 influences auxin-responsive gene regulation via H3K27me3 modification and affects zinc and iron accumulation in seeds.

## Abstract

Seed vigor is a crucial trait for crops to germinate and grow into healthy seedlings, which directly impacts crop yield. As a major phosphorus reservoir in seeds, inositol hexakisphosphate (IP6) is known to affect seed vigor, but the underlying mechanisms remain unclear. In this study, we found that exogenous application of IP6 inhibited rice seed germination and seedling growth by disrupting auxin signaling. Intriguingly, RNA interference (RNAi) of OsIPK2, a key enzyme in the IP6 biosynthesis pathway, led to reduced seed IP6 content and altered seed morphology, which consequently inhibited germination. Yet, the resulting seedlings exhibited enhanced growth performance. We also found that OsIPK2 is associated with altered plant response to auxin, which correlates with changes in the histone mark H3K27me3 at auxin-responsive gene loci, potentially contributing to the fine-tuning of their transcriptional activity. These findings provide novel molecular insights to inform future genetic approaches for improving seed quality in rice.

Seed vigor is a key agronomic trait that integrates germination capacity and seedling establishment, critically influencing rice productivity. Inositol hexakisphosphate (IP6) serves as a major phosphorus reservoir in seeds, yet its regulatory mechanism in seed vigor remains unclear. Here, we demonstrate that exogenous IP6 application inhibited seed germination and seedling growth of japonica rice (Oryza sativa L. ssp. japonica cv. Zhonghua11) in a dose-dependent manner; 10 mM IP6 reduced seed germination by 100%, while 100 μM IP6 suppressed primary root length by 33.6% compared to the control. This inhibitory effect is likely mediated by antagonizing auxin signaling, as supported by suppressed DR5::GUS expression and altered transcription of auxin-responsive genes. OsIPK2, a key enzyme in IP6 biosynthesis, showed high expression during early development in rice. RNA interference of OsIPK2 led to a 40.8–61.7% reduction in seed IP6 content, 45.3–65% higher zinc (Zn) and iron (Fe) accumulation, and a 35.4–53.5% lower germination rate compared to wild-type (WT). Conversely, OsIPK2-RNAi seedlings exhibited enhanced growth and resistance to IP6, which was associated with misregulation of auxin-responsive genes and a decrease in the repressive histone mark H3K27me3 at their loci. Furthermore, endogenous indole-3-acetic acid (IAA) levels significantly reduced in Ri-1 but unchanged in Ri-2, while abscisic acid (ABA) content and the IAA/ABA ratio remained unaltered compared to wild-type. Our findings reveal that OsIPK2 balances seed vigor and seedling development by modulating inositol phosphate metabolism, auxin responses, and epigenetic regulation, providing insights for improving seed quality in cereals. Whether the regulatory role of OsIPK2 in seed vigor is conserved across other rice subspecies requires further investigation.