The leafy head characteristic is a special phenotype of Chinese cabbage resulting from artificial selection during domestication and breeding. BREVIS RADIX (BRX) has been suggested to control root elongation, shoot growth, and tiller angle in Arabidopsis and rice. In Brassica rapa, three BrBRX homoeologs have been identified, but only BrBRX.1 and BrBRX.2 were found to be under selection in leaf-heading accessions, indicating their functional diversification in leafy head formation. Here, we show that these three BrBRX genes belong to a plant-specific BRX gene family but that they have significantly diverged from other BRX-like members on the basis of different phylogenetic classifications, motif compositions and expression patterns. Moreover, although the expression of these three BrBRX genes differed, compared with BrBRX.3, BrBRX.1, and BrBRX.2 displayed similar expression patterns. Arabidopsis mutant complementation studies showed that only BrBRX.1 could rescue the brx root phenotype, whereas BrBRX.2 and BrBRX.3 could not. However, overexpression of each of the three BrBRX genes in Arabidopsis resulted in similar pleiotropic leaf phenotypes, including epinastic leaf morphology, with an increase in leaf number and leaf petiole length and a reduction in leaf angle. These leaf traits are associated with leafy head formation. Further testing of a SNP (T/C) in BrBRX.2 confirmed that this allele in the heading accessions was strongly associated with the leaf-heading trait of B. rapa. Our results revealed that all three BrBRX genes may be involved in the leaf-heading trait, but they may have functionally diverged on the basis of their differential expression.

中文翻译：

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油菜中三种 BRX 同源物的差异及其对叶片形态的影响

叶头特征是大白菜在驯化和育种过程中经过人工选择而产生的一种特殊表型。布雷维斯基数(BRX) 已被建议用于控制拟南芥和水稻的根伸长、枝条生长和分蘖角度。在油菜， 三溴化橡胶已鉴定出同源物，但仅BrBRX.1和BrBRX.2发现在叶头种质中处于选择状态，表明它们在叶头形成中的功能多样化。在这里，我们展示了这三个溴化橡胶基因属于植物特异性BRX基因家族，但它们与其他基因明显不同类 BRX成员基于不同的系统发育分类、基序组成和表达模式。而且，虽然这三者的表达溴化橡胶基因不同，与BrBRX.3,BrBRX.1,和BrBRX.2表现出相似的表达模式。拟南芥突变体互补研究表明，只有BrBRX.1可以拯救brx根表型，而BrBRX.2和BrBRX.3不能。然而，这三个中的每一个都过度表达溴化橡胶拟南芥中的基因导致相似的多效性叶片表型，包括表位叶片形态，叶片数量和叶柄长度增加，叶角减小。这些叶子特征与多叶头的形成有关。SNP (T/C) 的进一步测试BrBRX.2证实抽穗材料中的该等位基因与叶抽穗性状密切相关B. rapa. 我们的结果表明，这三个溴化橡胶基因可能与抽叶性状有关，但由于它们的差异表达，它们可能在功能上有所不同。