Stone cells are often present in pear fruit, and they can seriously affect the fruit quality when present in large numbers. The plant growth regulator NAA, a synthetic auxin, is known to play an active role in fruit development regulation. However, the genetic mechanisms of NAA regulation of stone cell formation are still unclear. Here, we demonstrated that exogenous application of 200 μM NAA reduced stone cell content and also significantly decreased the expression level of PbrNSC encoding a transcriptional regulator. PbrNSC was shown to bind to an auxin response factor, PbrARF13. Overexpression of PbrARF13 decreased stone cell content in pear fruit and secondary cell wall (SCW) thickness in transgenic Arabidopsis plants. In contrast, knocking down PbrARF13 expression using virus-induced gene silencing had the opposite effect. PbrARF13 was subsequently shown to inhibit PbrNSC expression by directly binding to its promoter, and further to reduce stone cell content. Furthermore, PbrNSC was identified as a positive regulator of PbrMYB132 through analyses of co-expression network of stone cell formation-related genes. PbrMYB132 activated the expression of gene encoding cellulose synthase (PbrCESA4b/7a/8a) and lignin laccase (PbrLAC5) binding to their promotors. As expected, overexpression or knockdown of PbrMYB132 increased or decreased stone cell content in pear fruit and SCW thickness in Arabidopsis transgenic plants. In conclusion, our study shows that the ‘PbrARF13-PbrNSC-PbrMYB132’ regulatory cascade mediates the biosynthesis of lignin and cellulose in stone cells of pear fruit in response to auxin signals and also provides new insights into plant SCW formation.

中文翻译：

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生长素通过 PbrARF13-PbrNSC-PbrMYB132 转录调控级联抑制梨果实石细胞中的木质素和纤维素生物合成

梨果实中经常存在石细胞，大量存在会严重影响果实品质。植物生长调节剂 NAA 是一种合成生长素，已知在果实发育调节中发挥积极作用。然而，NAA 调节石细胞形成的遗传机制仍不清楚。在这里，我们证明外源应用 200 μM NAA会降低石细胞含量，并显着降低编码转录调节因子的PbrNSC的表达水平。PbrNSC 被证明可以与生长素反应因子 PbrARF13 结合。PbrARF13的过表达降低了梨果实中的石细胞含量和转基因拟南芥中的次生细胞壁 (SCW) 厚度植物。相反，使用病毒诱导的基因沉默来敲低PbrARF13 的表达却产生了相反的效果。随后显示 PbrARF13通过直接与其启动子结合来抑制PbrNSC表达，并进一步减少石细胞含量。此外，通过分析石细胞形成相关基因的共表达网络， PbrNSC 被确定为PbrMYB132的正调节因子。PbrMYB132 激活与其启动子结合的编码纤维素合酶 ( PbrCESA4b/7a/8a)和木质素漆酶 ( PbrLAC5 ) 的基因的表达。正如预期的那样， PbrMYB132的过度表达或敲低会增加或减少梨果实中的石细胞含量以及梨果实中的 SCW 厚度。拟南芥转基因植物。总之，我们的研究表明，“PbrARF13-PbrNSC-PbrMYB132”调节级联反应响应生长素信号介导梨果实石细胞中木质素和纤维素的生物合成，并且还为植物SCW形成提供了新的见解。