Multilayered Organization of Jasmonate Signalling in the Regulation of Root Growth

Physical damage can strongly affect plant growth, reducing the biomass of developing organs situated at a distance from wounds. These effects, previously studied in leaves, require the activation of jasmonate (JA) signalling. Using a novel assay involving repetitive cotyledon wounding in Arabidopsis seedlings, we uncovered a function of JA in suppressing cell division and elongation in roots. Regulatory JA signalling components were then manipulated to delineate their relative impacts on root growth. The new transcription factor mutant myc2-322B was isolated. In vitro transcription assays and whole-plant approaches revealed that myc2-322B is a dosage-dependent gain-of-function mutant that can amplify JA growth responses. Moreover, myc2-322B displayed extreme hypersensitivity to JA that totally suppressed root elongation. The mutation weakly reduced root growth in undamaged plants but, when the upstream negative regulator NINJA was genetically removed, myc2-322B powerfully repressed root growth through its effects on cell division and cell elongation. Furthermore, in a JA-deficient mutant background, ninja1 myc2-322B still repressed root elongation, indicating that it is possible to generate JA-responses in the absence of JA. We show that NINJA forms a broadly expressed regulatory layer that is required to inhibit JA signalling in the apex of roots grown under basal conditions. By contrast, MYC2, MYC3 and MYC4 displayed cell layer-specific localisations and MYC3 and MYC4 were expressed in mutually exclusive regions. In nature, growing roots are likely subjected to constant mechanical stress during soil penetration that could lead to JA production and subsequent detrimental effects on growth. Our data reveal how distinct negative regulatory layers, including both NINJA-dependent and -independent mechanisms, restrain JA responses to allow normal root growth. Mechanistic insights from this work underline the importance of mapping JA signalling components to specific cell types in order to understand and potentially engineer the growth reduction that follows physical damage.

物理损伤可显著影响植物生长，降低伤口远端发育器官的生物量。这类此前已在叶片中研究过的效应，依赖于茉莉酸（jasmonate, JA）信号通路的激活。我们利用一套全新的拟南芥幼苗重复子叶损伤实验体系，揭示了JA在抑制根系细胞分裂与伸长中的功能。随后，研究人员通过调控JA信号通路的各类调控组分，明确了它们对根系生长的相对影响。我们分离得到了新的转录因子突变体myc2-322B。体外转录实验与整株植株分析表明，myc2-322B是一种剂量依赖性的功能获得性突变体，能够放大JA介导的生长响应。此外，myc2-322B对JA表现出极端超敏性，其根系伸长完全被抑制。该突变体在未受损伤的植株中仅轻微抑制根系生长，但当上游负调控因子NINJA被遗传敲除后，myc2-322B可通过影响细胞分裂与细胞伸长，强力抑制根系生长。进一步研究发现，在茉莉酸缺陷型突变体背景下，ninja1 myc2-322B仍可抑制根系伸长，这表明无需JA即可激活JA响应。我们证实，NINJA形成了广泛表达的调控层级，可在基础生长条件下抑制根顶端的JA信号通路。与之相反，MYC2、MYC3与MYC4呈现细胞层特异性定位，且MYC3与MYC4的表达区域相互排斥。在自然环境中，根系在穿透土壤的过程中可能持续遭受机械胁迫，这会诱导JA产生并进而对生长产生不利影响。本研究的数据揭示了两类不同的负调控机制——包括依赖NINJA与不依赖NINJA的调控层级——如何约束JA响应，以保障根系正常生长。本研究的机制性见解强调，将JA信号通路组分映射至特定细胞类型，对于理解乃至工程化改造物理损伤后出现的生长抑制现象具有重要意义。