Department of Plant Molecular Biology, University of Lausanne, Lausanne, Switzerland.
PLoS Genet. 2013 Jun;9(6):e1003564. doi: 10.1371/journal.pgen.1003564. Epub 2013 Jun 20.
Observations gained from model organisms are essential, yet it remains unclear to which degree they are applicable to distant relatives. For example, in the dicotyledon Arabidopsis thaliana (Arabidopsis), auxin biosynthesis via indole-3-pyruvic acid (IPA) is essential for root development and requires redundant TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1 (TAA1) and TAA1-RELATED (TAR) genes. A promoter T-DNA insertion in the monocotyledon Brachypodium distachyon (Brachypodium) TAR2-LIKE gene (BdTAR2L) severely down-regulates expression, suggesting reduced tryptophan aminotransferase activity in this mutant, which thus represents a hypomorphic Bdtar2l allele (Bdtar2l(hypo) ). Counterintuitive however, Bdtar2l(hypo) mutants display dramatically elongated seminal roots because of enhanced cell elongation. This phenotype is also observed in another, stronger Bdtar2l allele and can be mimicked by treating wild type with L-kynerunine, a specific TAA1/TAR inhibitor. Surprisingly, L-kynerunine-treated as well as Bdtar2l roots display elevated rather than reduced auxin levels. This does not appear to result from compensation by alternative auxin biosynthesis pathways. Rather, expression of YUCCA genes, which are rate-limiting for conversion of IPA to auxin, is increased in Bdtar2l mutants. Consistent with suppression of Bdtar2l(hypo) root phenotypes upon application of the ethylene precursor 1-aminocyclopropane-1-carboxylic-acid (ACC), BdYUCCA genes are down-regulated upon ACC treatment. Moreover, they are up-regulated in a downstream ethylene-signaling component homolog mutant, Bd ethylene insensitive 2-like 1, which also displays a Bdtar2l root phenotype. In summary, Bdtar2l phenotypes contrast with gradually reduced root growth and auxin levels described for Arabidopsis taa1/tar mutants. This could be explained if in Brachypodium, ethylene inhibits the rate-limiting step of auxin biosynthesis in an IPA-dependent manner to confer auxin levels that are sub-optimal for root cell elongation, as suggested by our observations. Thus, our results reveal a delicate homeostasis of local auxin and ethylene activity to control cell elongation in Brachypodium roots and suggest alternative wiring of auxin-ethylene crosstalk as compared to Arabidopsis.
从模式生物中获得的观察结果是必不可少的，但它们在多大程度上适用于远缘生物仍不清楚。例如，在双子叶植物拟南芥（Arabidopsis）中，通过吲哚-3-丙酮酸（IPA）合成生长素对于根的发育是必不可少的，并且需要冗余的色氨酸氨基转移酶 OF ARABIDOPSIS 1（TAA1）和 TAA1-RELATED（TAR）基因。单子叶植物粗山羊草（Brachypodium）TAR2-LIKE 基因（BdTAR2L）中的启动子 T-DNA 插入严重下调了表达，表明该突变体中的色氨酸氨基转移酶活性降低，因此代表一个功能减弱的 Bdtar2l 等位基因（Bdtar2l(hypo)）。然而，令人费解的是，Bdtar2l(hypo)突变体表现出明显伸长的初生根，因为细胞伸长增强。这种表型也在另一个更强的 Bdtar2l 等位基因中观察到，并且可以通过用 L-kynerunine（一种特定的 TAA1/TAR 抑制剂）处理野生型来模拟。令人惊讶的是，用 L-kynerunine 处理以及 Bdtar2l 根显示出升高而不是降低的生长素水平。这似乎不是由替代生长素生物合成途径的补偿引起的。相反，IPA 转化为生长素的限速因子 YUCCA 基因的表达在 Bdtar2l 突变体中增加。与乙烯前体 1-氨基环丙烷-1-羧酸（ACC）施加时抑制 Bdtar2l(hypo)根表型一致，BdYUCCA 基因在 ACC 处理时下调。此外，它们在下游乙烯信号成分同源突变体 Bd 乙烯不敏感 2 样 1 中上调，该突变体也表现出 Bdtar2l 根表型。总之，Bdtar2l 表型与 Arabidopsis taa1/tar 突变体描述的逐渐减少的根生长和生长素水平形成对比。如果在粗山羊草中，乙烯以 IPA 依赖的方式抑制生长素生物合成的限速步骤，以赋予对根细胞伸长来说次优的生长素水平，正如我们的观察结果所示，那么这可以解释 Bdtar2l 表型。因此，我们的结果揭示了 Brachypodium 根中局部生长素和乙烯活性的微妙平衡，以控制细胞伸长，并表明与拟南芥相比，生长素-乙烯相互作用的替代布线。
