背景回顾：The enzyme DWARF27 (D27) catalyzes the reversible isomerization of all-trans- into 9-cis-β-carotene, initiating strigolactone (SL) biosynthesis. 

提出问题：Genomes of higher plants encode two D27-homologs, D27-like1 and -like2, with unknown functions. 

主要研究：Here, we investigated the enzymatic activity and biological function of the ArabidopsisD27-like1. 

结果1-体外酶活验证：In vitro enzymatic assays and expression in Synechocystis sp. PCC6803 revealed a yet not reported 13-cis/15-cis/9-cis- and a 9-cis/all-trans-β-carotene isomerization. 

结果2-D27-like1促进SL生物合成：Although disruption of AtD27-like1 did not cause SL deficiency phenotypes, overexpression of AtD27-like1 in the d27 mutant restored the more-branching phenotype, indicating a contribution of AtD27-like1 to SL biosynthesis. Accordingly, generated d27 d27like1 double mutants showed more pronounced branching phenotype, compared to d27.The contribution of AtD27-like1 to SL biosynthesis is likely due to its formation of 9-cis-β-carotene that was present at higher levels in AtD27-like1 overexpressing lines.  

结果3-D27-like1负调控ABA生物合成：In contrast, AtD27-like1 expression correlated negatively with the content of 9-cis-violaxanthin, a precursor of abscisic acid (ABA), in shoots. Consistently, ABA levels were higher in shoots and also in dry seeds of the d27like1 and d27 d27like1 mutants. 

结果4-D27-like1表达模式：Transgenic lines expressing β-glucuronidase (GUS) driven by the AtD27LIKE1 promoter and transcript analysis of hormone-treated Arabidopsis seedlings unraveled that AtD27LIKE1 is expressed in different tissues and regulated ABA and auxin. 

结论：Taken together, our work revealed a cis/cis-β-carotene isomerase that affects the content of both cis-carotenoid derived plant hormones ABA and SLs.

Hypothetical model for the roles of AtD27LIKE1 in SL and ABA biosynthesis.

DWARF27（D27）催化all-trans-β-carotene到9-cis-β-carotene的可逆异构化，从而起始独脚金内酯（SL）的生物合成。高等植物编码两个D27同源基因，即D27-like1和D27-like2，但是具体的功能未知。本文中，作者研究了拟南芥D27-like1的酶活和生物学功能。通过在集胞藻PCC6803中的体外酶活实验和表达，发现了一个未曾报道的13-cis/15-cis/9-cis-和9-cis/all-trans-β-carotene异构化。尽管扰乱AtD27-like1基因并不会导致SL缺陷的表型，但是在d27突变体中过表达AtD27-like1基因能够恢复分枝增多的表型，表明AtD27-like1能够作用于SL的生物合成。与预期相符，d27 d27like1双突变体相比于d27单突，表现出更加明显的分枝表型。AtD27-like1对SL生物合成的作用很有可能是通过形成9-cis-β-carotene来发挥功能，而后者在AtD27-like1过表达株系中的含量水平确实更高。相反，AtD27-like1基因的表达与地上组织中ABA前体物质9-cis-violaxanthin的含量呈负相关。与此一致，d27like1单突和d27 d27like1双突的地上组织和干燥种子中，ABA的含量水平更高。通过AtD27LIKE1启动子驱动GUS进行遗传转化以及激素处理的拟南芥实生苗的转录分析，作者发现AtD27LIKE1基因在不同组织中均表达，并且能够调控ABA和生长素。综上，本文的研究揭示了一个cis/cis-β-carotene异构酶能够影响源自于cis-carotenoid的两大植物激素ABA和SLs。

** Salim Al-Babili **