In many tree species especially the rooting-recalcitrant woody perennials, the adventitious root (AR) in the juvenile phase can be easily induced by exogenous auxin, but AR formation becomes recalcitrant in the adult phase. Also, it is reported that the γ-aminobutyric acid (GABA) inhibits primary root growth in Arabidopsis and the AR formation in poplar (Populus ssp). So far, how GABA affects or is affected by the ontogenetic phase or auxin remains unclear. Here, we used an apple rootstock, Malus xiaojinensis, and tobacco (Nicotiana benthamiana) to investigate this question. We first analyzed the content of GABA, the activity of GABA synthetic enzyme GAD, and the expression of the coding gene MxGADs, respectively, in leafy cuttings of juvenile and adult phase. Next, the effect of exogenous GABA on AR formation was examined in in vitro shoots of M. xiaojinensis and tobacco. Interestingly, significant and consistent increases in GABA concentration, GAD activity, and expression of MxGAD genes in response to exogenous indole butyric acid (IBA) were detected in adult-phase cuttings, but not in juvenile-phase cuttings. Exogenous GABA application inhibited the AR formation by delaying rooting time and reducing root number and the total root length in in vitro shoots of both M. xiaojinensis and tobacco. The expression of MxPIN members increased in response to IBA application, but these changes were restrained by the addition of GABA. These results indicate that both the loss of juvenility and IBA are required to trigger GABA accumulation. GABA may affect the AR formation as a co-actor by inhibiting polar auxin transport. Together, these findings facilitate the understanding of the regulatory network among GABA, juvenility, and auxin signaling on the AR formation.

中文翻译：

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γ-氨基丁酸参与成虫期不定生根抵抗

在许多树种，尤其是顽固性多年生木本植物中，外源生长素很容易诱导幼年期的不定根（AR）形成，但成年期不定根（AR）的形成变得顽固。此外，据报道，γ-氨基丁酸 (GABA) 抑制拟南芥的初生根生长和杨树 (Populus ssp) 的 AR 形成。到目前为止，GABA 如何影响个体发育阶段或生长素或受其影响尚不清楚。在这里，我们使用苹果砧木 Malus xiaojinensis 和烟草 (Nicotiana bethamiana) 来研究这个问题。我们首先分别分析了幼年期和成年期叶插条中 GABA 的含量、GABA 合成酶 GAD 的活性和编码基因 MxGADs 的表达。接下来，在 M. 的体外芽中检查了外源 GABA 对 AR 形成的影响。小金和烟草。有趣的是，在成年期扦插中检测到 GABA 浓度、GAD 活性和 MxGAD 基因表达的显着和一致增加，以响应外源吲哚丁酸 (IBA)，但在幼虫期扦插中未检测到。外源GABA的施用通过延迟生根时间、减少根数和总根长来抑制小金烟和烟草离体枝条的AR形成。MxPIN 成员的表达随着 IBA 的应用而增加，但这些变化受到添加 GABA 的抑制。这些结果表明幼体和 IBA 的丧失都是触发 GABA 积累所必需的。GABA 可能通过抑制极性生长素转运来影响 AR 的形成。一起，