The auxin IAA is a vital plant hormone in controlling growth and development, but our knowledge about its complicated biosynthetic pathways and molecular regulation are still limited and fragmentary. cytokinin induced root waving 2 (ckrw2) was isolated as one of the auxin-deficient mutants in a large-scale forward genetic screen aiming to find more genes functioning in auxin homeostasis and/or its regulation. Here we show that CKRW2 is identical to Histone Monoubiquitination 1 (HUB1), a gene encoding an E3 ligase required for histone H2B monoubiquitination (H2Bub1) in Arabidopsis. In addition to pleiotropic defects in growth and development, loss of CKRW2/HUB1 function also led to typical auxin-deficient phenotypes in roots, which was associated with significantly lower expression levels of several functional auxin synthetic genes, namely TRP2/TSB1, WEI7/ASB1, YUC7 and AMI1. Corresponding defects in H2Bub1 were detected in the coding regions of these genes by chromatin immunoprecipitation (ChIP) analysis, indicating the involvement of H2Bub1 in regulating auxin biosynthesis. Importantly, application of exogenous cytokinin (CK) could stimulate CKRW2/HUB1 expression, providing an epigenetic avenue for CK to regulate the auxin homeostasis. Our results reveal a previously unknown mechanism for regulating auxin biosynthesis via HUB1/2-mediated H2Bub1 at the chromatin level.

生长素 IAA 是控制生长和发育的重要植物激素，但我们对其复杂的生物合成途径和分子调控的了解仍然有限且不完整。在大规模正向遗传筛选中，细胞分裂素诱导的根波2 ( ckrw2 ) 被分离为生长素缺陷突变体之一，旨在寻找更多在生长素稳态和/或其调节中起作用的基因。在这里我们表明CKRW2与组蛋白单泛素化 1 (HUB1) 相同，编码拟南芥中组蛋白 H2B 单泛素化 (H2Bub1) 所需的 E3 连接酶的基因。除了生长发育中的多效性缺陷外，CKRW2/HUB1 功能的丧失还导致根中典型的生长素缺乏表型，这与几个功能性生长素合成基因，即TRP2/TSB1、WEI7/ASB1 的表达水平显着降低有关、YUC7和AMI1。通过染色质免疫沉淀 (ChIP) 分析在这些基因的编码区检测到 H2Bub1 中的相应缺陷，表明 H2Bub1 参与调节生长素生物合成。重要的是，外源性细胞分裂素（CK）的应用可以刺激CKRW2/HUB1表达，为 CK 调节生长素稳态提供了表观遗传途径。我们的结果揭示了一种以前未知的通过 HUB1/2 介导的 H2Bub1 在染色质水平上调节生长素生物合成的机制。