The Arabidopsis genome contains genes encoding High Mobility Group (HMG) chromatin remodelers which function as transcription activators crucial for transcription of a plethora of genes. AtHMGB15, an ARID-HMG protein that harbors an AT rich interaction domain along with the classic HMG-Box DNA binding domain exhibits heightened expression primarily in flowers and pollen grains. The intricate process of male gametophyte development in flowering plants is regulated by jasmonic acid (JA) signaling. JA signalling initiates with the activation of MYC2 transcription factor, leading to the expression of numerous JA-responsive genes during stamen development, and pollen maturation.

This study investigates the broader molecular analysis and biological relevance of transcriptional regulation of AtHMGB15 in orchestrating pollen development via the hormone pathway. Phenotypic characterization of athmgb15-4 mutant plants revealed delayed bolting, shorter siliques, and reduced seed set compared to the wildtype. Additionally, the mutation of AtHMGB15 resulted in defective pollen morphology, delayed pollen germination, aberrant pollen tube growth, and a higher percentage of non-viable pollen grains. In silico and molecular analysis validated the down-regulation of JA biosynthesis and signaling genes in the athmgb15-4 mutant. Quantitative analysis demonstrated that jasmonic acid and its derivatives were approximately tenfold lower in athmgb15-4 flowers. Exogenous application of methyl jasmonate could restore pollen morphology and germination alongside elevating the expression of previously down-regulated JA signaling genes, suggesting that the low jasmonate content in athmgb15-4 impaired JA signaling during pollen development. Furthermore, our study suggests that AtHMGB15 physically interacts with the MYC2 protein to form a transcription activation complex further promoting the transcription of two key JA signaling genes, namely MYB21 and MYB24, during stamen, and pollen development.

We thereby propose the quintessential role of AtHMGB15 as a positive regulator of JA pathway, controlling the spatial, and temporal expression of key regulators involved in stamen and pollen development.

(Sachdev et. al., Plant Physiology 2024)