Table_1_Mutation of BAM2 rescues the sunn hypernodulation phenotype in Medicago truncatula, suggesting that a signaling pathway like CLV1/BAM in Arabidopsis affects nodule number.docx

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.

豆科植物在氮固定共生过程中所特有的进化适应——结瘤，其过程受到宿主植物的严格调控。结瘤的自动调节（AON）途径通过长距离信号从茎和根向彼此传递，对茎和根的碳/氮代谢状态做出反应，从而负向调控形成的结瘤数量。在紫花苜蓿的茎部，AON信号传导的核心是SUNN，这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，该受体家族参与调控根和茎中的干细胞群。拟南芥中的这一类受体家族包括BARELY ANY MERISTEM（BAM）家族，该家族与CLV1类似，与CLE肽结合并相互作用，以调节分生组织发育。紫花苜蓿含有BAM家族的五个成员，但在接种后48小时，只有MtBAM1和MtBAM2在结瘤中高表达。植物中单个MtBAM的突变体，以及几种双BAM突变体组合均表现出野生型结瘤数量表型。然而，当存在于sunn-5背景下时，Mtbam2抑制了sunn-5的超结瘤表型，并部分挽救了sunn-5的短根长度表型。嫁接实验表明，bam2无论根中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。在假设的bam2 sunn-5复合物下游的结瘤转录因子MtWOX5的相对表达测量中，揭示了分生组织信号的破坏；虽然bam2和bam2 sunn-5均影响MtWOX5的表达，但表达变化的方向不同。我们提出一个遗传模型，其中BAM2/SUNN的特定根相互作用对于在紫花苜蓿结瘤分生组织细胞稳态中的信号传导至关重要。