The HD-Zip transcription factor S1HB15A regulates abscission by modulating jasmonoyl-isoleucine biosynthesis

Authors: LIU, XIANFENG - Shenyang Agricultural University; CHENG, LINA - Shenyang Agricultural University; LI, RUIZHEN - Shenyang Agricultural University; CAI, YUE - Shenyang Agricultural University; WANG, XIAOYANG - Shenyang Agricultural University; FU, XIN - Shenyang Agricultural University; DONG, XIUFEN - Shenyang Agricultural University; QI, MINGFANG - Shenyang Agricultural University; Jiang, Cai-Zhong; XU, TAO - Shenyang Agricultural University; LI, TAINLAI - Shenyang Agricultural University

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2022
Publication Date: 5/6/2022
Citation: Liu, X., Cheng, L., Li, R., Cai, Y., Wang, X., Fu, X., Dong, X., Qi, M., Jiang, C., Xu, T., Li, T. 2022. The HD-Zip transcription factor S1HB15A regulates abscission by modulating jasmonoyl-isoleucine biosynthesis. Plant Physiology. 189(4):2396-2412. https://doi.org/10.1093/plphys/kiac212.
DOI: https://doi.org/10.1093/plphys/kiac212

Interpretive Summary: The process of abscission in plants involves the detachment of multicellular organs from the main body at specialized regions called abscission zones (AZs), and has been studied in various organs, including flowers, fruits, leaves and roots using physiological, molecular and genetic approaches. Abscission is triggered by various developmental cues and environmental stimuli and is precisely regulated by phytohormones, with auxin acting as a major inhibitor. Polar auxin transport across the AZ forms an auxin gradient from the distal to the proximal side that is vital for inhibiting abscission, and impairing this auxin flux results in abscission initiation. This is accompanied by changes in transcript abundance of thousands of genes; however, the mechanism by which auxin inhibits abscission remains largely unknown. Jasmonic acid (JA) is a phytohormone involved in various aspects of development, including root growth, stamen development, flowering, leaf senescence and abscission. Application of JA and its methyl ester (JA-Me) promoted the abscission of bean petiole explants in the dark and light without induced ethylene production but by change of sugar metabolism. JA signaling has also been reported to impair gravistimulation-induced lateral auxin redistribution by altering the subcellular distribution of the PIN2 auxin transporter. Auxin, on the other hand, up-regulates the expression of the GH3 genes to inactivate JA, which in turn promotes adventitious root formation. Recent studies suggest that auxin suppresses regulates JA biosynthesis. However, to date, the molecular details of how auxin interacts with JA in the AZ are not well resolved. Furthermore, the molecular mechanisms underlying the antagonistic effects of auxin and JA are unknown. Here, we identified a tomato (Solanum lycopersicum) HD-ZIP III transcription factor, SlHB15A, which was highly expressed in the flower pedicel abscission zone (AZ) and was induced by auxin. Knocking out SlHB15A using clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 (CRISPR-associated protein 9) technology resulted in significantly accelerated abscission. In contrast, overexpression of miR166-resistant mSlHB15A delayed abscission. RNA-seq and quantitative reverse transcription (qRT)-PCR analysis showed that knocking out SlHB15A was associated with altered expression of genes related to JA biosynthesis and signaling. Functional analysis furthermore indicated that SlHB15A regulates abscission by modifying JA-isoleucine (JA-Ile) levels, and increasing the expression of SlJAR1, a gene involved in JA-Ile biosynthesis, which could induce abscission dependent and independent ethylene. SlHB15A was observed to bind directly to the SlJAR1 promoter and silencing SlJAR1 delayed abscission. We also found that flower removal impaired auxin flux and enhanced the JA-Ile content, and that application of JA-Ile severely impaired the inhibitory effects of auxin on abscission. These results indicated that SlHB15A mediates the antagonistic effect of auxin and JA-Ile during tomato pedicel abscission and auxin inhibit abscission through an SlHB15A-SlJAR1 modular. Our results suggest a new regulatory model of the molecular basis by which auxin antagonizes JA action during flower pedicel abscission.

Technical Abstract: Plant organ abscission, a process that is important for development and reproductive success, is both inhibited by the phytohormone auxin and promoted by another phytohormone, jasmonic acid (JA). However, the molecular mechanisms underlying the antagonistic effects of auxin and JA are unknown. We identified a tomato (Solanum lycopersicum) HD-ZIP III transcription factor, SlHB15A, which was highly expressed in the flower pedicel abscission zone (AZ) and was induced by auxin. Knocking out SlHB15A using clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 (CRISPR-associated protein 9) technology resulted in significantly accelerated abscission. In contrast, overexpression of miR166-resistant mSlHB15A delayed abscission. RNA-seq and quantitative reverse transcription (qRT)-PCR analysis showed that knocking out SlHB15A was associated with altered expression of genes related to JA biosynthesis and signaling. Functional analysis furthermore indicated that SlHB15A regulates abscission by modifying JA-isoleucine (JA-Ile) levels, and increasing the expression of SlJAR1, a gene involved in JA-Ile biosynthesis, which could induce abscission dependent and independent ethylene. SlHB15A was observed to bind directly to the SlJAR1 promoter and silencing SlJAR1 delayed abscission. We also found that flower removal impaired auxin flux and enhanced the JA-Ile content, and that application of JA-Ile severely impaired the inhibitory effects of auxin on abscission. These results indicated that SlHB15A mediates the antagonistic effect of auxin and JA-Ile during tomato pedicel abscission and auxin inhibit abscission through an SlHB15A-SlJAR1 modular.