Fig. 7: Possible working model of HAN2-mediated chilling adaptation in rice.

HAN2 encodes the OsABCB5 transporter, which mediates auxin efflux from the cell. In rice, HAN2 is highly expressed in the parenchyma cells of the vascular bundle sheath in mature leaves, where it facilitates the transport of auxin synthesised in these leaves to young tissues. In temperate japonica rice, the insertion of a Copia LTR downstream of HAN2 significantly reduces active histone modifications, including H3K4me3 and H3K36me3, across the gene body, leading to transcriptional suppression of HAN2. In contrast, indica cultivars lack this retrotransposon insertion, resulting in elevated HAN2 expression and increased auxin accumulation in young tissues. Higher auxin levels promote the release of OsARF24, which binds to the ARF-motif in the OsMAP1/OsMPK3 promoter and represses its expression. This regulatory cascade could ultimately compromise chilling tolerance at both the seedling and booting stages in indica rice. Conversely, temperate japonica rice, which carries the Copia LTR insertion, exhibits reduced HAN2 expression and improved chilling tolerance at both developmental stages.