背景回顾：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. 

提出问题：However, the genetic mechanisms of NAA regulation of stone cell formation are still unclear. 

结果1-NAA处理促进石细胞减少：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. 

结果2-ARF13-石细胞：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. 

结果3-ARF13-NSC-石细胞：PbrARF13 was subsequently shown to inhibit PbrNSC expression by directly binding to its promoter, and further to reduce stone cell content. 

结果4-NSC-MYB132-纤维素/木质素：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. 

结论：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.

梨果实中经常会含有石细胞，当石细胞的数量较多时会严重影响果实的品质。植物生长调节剂NAA是一个人工合成的生长素，其在果实发育调控中发挥积极作用。但是，NAA调控石细胞形成的遗传机制仍不清楚。本文中，作者发现外施200μM NAA会降低石细胞含量，同时也会降低转录调节因子PbrNSC的表达。PbrNSC能够直接结合生长素响应因子PbrARF13。过表达PbrARF13会降低梨果实中石细胞的含量，也会降低拟南芥中的次生细胞壁厚度。而利用VIGS介导的PbrARF13表达敲低会导致相反的表型。随后，作者发现PbrARF13通过直接结合到PbrNSC启动子上，抑制PbrNSC基因的表达，从而进一步降低石细胞的含量。此外，作者通过分析石细胞形成相关基因的共表达网络，发现PbrNSC是PbrMYB132基因的正调控因子。PbrMYB132能够通过直接结合启动子序列，激活纤维素合成基因PbrCESA4b/7a/8a和木质素漆酶编码基因PbrLAC5的表达。正有如预期一样，过表达或者敲低PbrMYB132基因能够增加或降低梨果实中的石细胞含量以及拟南芥中的次生细胞壁厚度。综上，本文的研究结果揭示了“PbrARF13-PbrNSC-PbrMYB132”调控级联能够响应生长素信号，调控梨果实石细胞中的纤维素和木质素生物合成，同时也为植物次生细胞壁形成提供了新的视野。

** 吴 俊 **