In legumes, the symbiotic nodules are formed as a result of dedifferentiation and reactivation of cortical root cells. A shootacting receptor complex, similar to the Arabidopsis (Arabidopsis thaliana) CLAVATA1 (CLV1)/CLV2 receptor, regulating development of the shoot apical meristem, is involved in autoregulation of nodulation (AON), a mechanism that systemically controls nodule number. The targets of CLV1/CLV2 in the shoot apical meristem, the WUSCHEL (WUS)-RELATED HOMEOBOX (WOX) family transcription factors, have been proposed to be important regulators of apical meristem maintenance and to be expressed in apical meristem "organizers." Here, we focus on the role of the WOX5 transcription factor upon nodulation in Medicago truncatula and pea (Pisum sativum) that form indeterminate nodules. Analysis of temporal WOX5 expression during nodulation with quantitative reverse transcription-polymerase chain reaction and promoter-reporter fusion revealed that the WOX5 gene was expressed during nodule organogenesis, suggesting that WOX genes are common regulators of cell proliferation in different systems. Furthermore, in nodules of supernodulating mutants, defective in AON, WOX5 expression was higher than that in wild-type nodules. Hence, a conserved WUS/WOX-CLV regulatory system might control cell proliferation and differentiation not only in the root and shoot apical meristems but also in nodule meristems. In addition, the link between nodule-derived CLE peptides activating AON in different legumes and components of the AON system was investigated. We demonstrate that the identified AON component, NODULATION3 of pea, might act downstream from or beside the CLE peptides during AON.Legume plants can thrive on nitrogen-poor soils because they interact symbiotically with soil-resident bacteria, the rhizobia. After a complex signal exchange between both partners, new root organs are formed: the nodules, in which the bacteria fix atmospheric nitrogen for the plant in return for a protective niche and carbon sources. Nodulation requires well-controlled bacterial invasion and initiation of cortical cell division after perception of the bacterially produced Nodulation (Nod) factors. Studies in several legumes have elucidated many elements of the signaling cascade (Catoira et al.,

Highlight: KNOX3 gene is involved in symbiotic nodule development in Medicago truncatula and Pisum sativum, providing evidence that it may regulate cytokinin biosynthesis/activation upon nodulation.