Table_2_Mutation of BAM2 rescues the sunn hypernodulation phenotype in Medicago truncatula, suggesting that a signaling pathway like CLV1/BAM in Arabidopsis affects nodule number.xlsx

The unique evolutionary adaptation of legumes for nitrogen-fixing symbiosis leading to nodulation is tightly regulated by the host plant. The autoregulation of nodulation (AON) pathway negatively regulates the number of nodules formed in response to the carbon/nitrogen metabolic status of the shoot and root by long-distance signaling to and from the shoot and root. Central to AON signaling in the shoots of Medicago truncatula is SUNN, a leucine-rich repeat receptor-like kinase with high sequence similarity with CLAVATA1 (CLV1), part of a class of receptors in Arabidopsis involved in regulating stem cell populations in the root and shoot. This class of receptors in Arabidopsis includes the BARELY ANY MERISTEM family, which, like CLV1, binds to CLE peptides and interacts with CLV1 to regulate meristem development. M. truncatula contains five members of the BAM family, but only MtBAM1 and MtBAM2 are highly expressed in the nodules 48 hours after inoculation. Plants carry mutations in individual MtBAMs, and several double BAM mutant combinations all displayed wild-type nodule number phenotypes. However, Mtbam2 suppressed the sunn-5 hypernodulation phenotype and partially rescued the short root length phenotype of sunn-5 when present in a sunn-5 background. Grafting determined that bam2 suppresses supernodulation from the roots, regardless of the SUNN status of the root. Overexpression of MtBAM2 in wild-type plants increases nodule numbers, while overexpression of MtBAM2 in some sunn mutants rescues the hypernodulation phenotype, but not the hypernodulation phenotypes of AON mutant rdn1-2 or crn. Relative expression measurements of the nodule transcription factor MtWOX5 downstream of the putative bam2 sunn-5 complex revealed disruption of meristem signaling; while both bam2 and bam2 sunn-5 influence MtWOX5 expression, the expression changes are in different directions. We propose a genetic model wherein the specific root interactions of BAM2/SUNN are critical for signaling in nodule meristem cell homeostasis in M. truncatula.

豆科植物独特的氮固定共生进化适应，即结瘤现象，受到宿主植物的高度调控。结瘤自调控（AON）途径通过从茎和根向远端的长距离信号传导，负向调节因茎和根的碳/氮代谢状态而产生的结瘤数量。在紫花苜蓿（Medicago truncatula）的茎部AON信号传导中，SUNN起着核心作用，它是一种富含亮氨酸重复序列的受体样激酶，其序列与CLAVATA1（CLV1）高度相似，CLV1是拟南芥中一类受体家族的成员，该受体家族参与调节根和茎中的干细胞群体。拟南芥中这类受体家族包括BARELY ANY MERISTEM（BAM）家族，该家族与CLV1类似，与CLE肽结合并相互作用，以调节芽苗发育。紫花苜蓿含有BAM家族的五个成员，但只有MtBAM1和MtBAM2在接种后48小时内在结瘤中高表达。植物中单个MtBAM发生突变，以及几种双BAM突变组合均表现出野生型结瘤数量表型。然而，在sunn-5背景中，Mtbam2抑制了sunn-5超结瘤表型，并部分恢复了sunn-5的短根长度表型。嫁接实验确定，bam2无论根的SUNN状态如何，都能抑制根部的超结瘤。在野生型植物中过表达MtBAM2会增加结瘤数量，而在某些sunn突变体中过表达MtBAM2可以挽救超结瘤表型，但不能挽救AON突变体rdn1-2或crn的超结瘤表型。对疑似bam2 sunn-5复合物下游的结瘤转录因子MtWOX5的相对表达量测量揭示了芽苗信号传导的破坏；尽管bam2和bam2 sunn-5都影响MtWOX5的表达，但其表达变化方向不同。我们提出了一种遗传模型，其中BAM2/SUNN的特定根部相互作用对于紫花苜蓿结瘤芽苗细胞稳态中的信号传导至关重要。