School of Forest, Fisheries and Geomatics Sciences, University of Florida, Gainesville, Florida 32611, USA.
Department of Bacteriology, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
Plant Physiol. 2022 Jan 20;188(1):560-575. doi: 10.1093/plphys/kiab447.
Most legumes can establish a symbiotic association with soil rhizobia that trigger the development of root nodules. These nodules host the rhizobia and allow them to fix nitrogen efficiently. The perception of bacterial lipo-chitooligosaccharides (LCOs) in the epidermis initiates a signaling cascade that allows rhizobial intracellular infection in the root and de-differentiation and activation of cell division that gives rise to the nodule. Thus, nodule organogenesis and rhizobial infection need to be coupled in space and time for successful nodulation. The plant hormone cytokinin (CK) contributes to the coordination of this process, acting as an essential positive regulator of nodule organogenesis. However, the temporal regulation of tissue-specific CK signaling and biosynthesis in response to LCOs or Sinorhizobium meliloti inoculation in Medicago truncatula remains poorly understood. In this study, using a fluorescence-based CK sensor (pTCSn::nls:tGFP), we performed a high-resolution tissue-specific temporal characterization of the sequential activation of CK response during root infection and nodule development in M. truncatula after inoculation with S. meliloti. Loss-of-function mutants of the CK-biosynthetic gene ISOPENTENYLTRANSFERASE 3 (IPT3) showed impairment of nodulation, suggesting that IPT3 is required for nodule development in M. truncatula. Simultaneous live imaging of pIPT3::nls:tdTOMATO and the CK sensor showed that IPT3 induction in the pericycle at the base of nodule primordium contributes to CK biosynthesis, which in turn promotes expression of positive regulators of nodule organogenesis in M. truncatula.
大多数豆类植物可以与土壤根瘤菌建立共生关系，从而触发根瘤的发育。这些根瘤容纳根瘤菌并使它们能够有效地固定氮。表皮中对细菌脂寡糖（LCOs）的感知引发了一个信号级联反应，使根瘤菌能够在根部进行细胞内感染，并使细胞分化和激活，从而导致根瘤的形成。因此，根瘤器官发生和根瘤菌感染需要在空间和时间上协调，以实现成功的结瘤。植物激素细胞分裂素（CK）有助于协调这一过程，作为根瘤器官发生的必需正调节剂。然而，植物激素细胞分裂素（CK）在对 LCOs 或 Sinorhizobium meliloti 接种的响应中时空调节组织特异性 CK 信号转导和生物合成在 Medicago truncatula 中的机制仍知之甚少。在这项研究中，我们使用基于荧光的 CK 传感器（pTCSn::nls:tGFP），在接种 Sinorhizobium meliloti 后，对 M. truncatula 根部感染和根瘤发育过程中 CK 响应的顺序激活进行了高分辨率的组织特异性时间特征描述。CK 生物合成基因 ISOPENTENYLTRANSFERASE 3（IPT3）的功能丧失突变体显示出结瘤缺陷，表明 IPT3 是 M. truncatula 中根瘤发育所必需的。pIPT3::nls:tdTOMATO 和 CK 传感器的实时成像表明，在根瘤原基底部的周皮中 IPT3 的诱导有助于 CK 的生物合成，进而促进 M. truncatula 中根瘤器官发生的正调节剂的表达。
