Universal maps downloader 9.922 image11/12/2023 The lower image is a section of a mature nodule showing the rhizobia in green. The upper pictures show a section of a nodule primordium showing a network of infection threads stained in blue (Left), and a symbiotic nodule cell densely packed with differentiated bacteroids (Right). The inset shows an enlarged image of nodules. The left part of the figure presents a nodulated M. Peptides contributing to nodule formation. In this review, we summarize these peptide signals and peptide effectors and the present knowledge on their identified or predicted functions in symbiosis (Figure 1).įIGURE 1. The field of Rhizobium-legume symbiosis is not lagging behind when it comes to discoveries of peptides with key roles in the nodulation process ( Djordjevic et al., 2015). Crucial importance of peptides has been demonstrated in embryogenesis ( Costa et al., 2014), fertilization ( Okuda et al., 2009 Higashiyama, 2010 Mecchia et al., 2017 Ge et al., 2017), cell expansion ( Haruta et al., 2014 Murphy and De Smet, 2014), cell differentiation ( Butenko et al., 2003 Hunt et al., 2009 Matsuzaki et al., 2010 Sugano et al., 2010 Lee et al., 2015 Santiago et al., 2016 Doblas et al., 2017 Nakayama et al., 2017), immunity ( Constabel et al., 1995 Pearce et al., 2001 Huffaker et al., 2006 Hou et al., 2014 Stegmann et al., 2017), nutrition ( Tabata et al., 2014 Ohkubo et al., 2017), as well as in other processes ( Whitford et al., 2012 Takahashi et al., 2018). Research on peptide-mediated signaling processes and other peptide functions in plants has gained momentum in the last decade. Ribosomally synthesized peptides with biological functions are arbitrarily (and loosely) defined as gene-encoded small proteins of 2 to about 100 amino acids. The NCRs and possibly also the GRP and SNARPs are targeted to the endosymbionts and play essential roles in the terminal differentiation of these bacteria. The PSK peptide is thought to be involved in the suppression of immunity in nodules and the nodule-specific cysteine-rich, GRP, and SNARP (LEED.PEED) peptide families are essential in the functioning of the nitrogen fixing root nodules. Still other peptides act at later stages of the symbiosis. CLE and CEP peptides are implicated in systemic and local control of nodule initiation during autoregulation of nodulation and in response to the nutritional demands of the plant. Several peptide classes, including RALF, DLV, ENOD40, and others, control Rhizobium infection and the initiation of cell divisions and the formation of nodule primordia. Peptides are further classified according to the stage of the symbiotic process where they act. We distinguish peptides that can be either cell-autonomous or secreted short-range or long-range signals, carrying messages in or between plant cells or that can act as effectors interacting with the symbiotic bacteria. Here, we discuss the function and the evolution of these peptides playing an important role in the formation or functioning of the symbiotic organs, the root nodules. Legumes are an unprecedented rich source of peptides, which are used to control the symbiosis of these plants with the nitrogen-fixing Rhizobium bacteria. Ribosomally synthesized peptides have wide ranges of functions in plants being, for example, signal molecules, transporters, alkaloids, or antimicrobial agents. 2Institute of Integrative Biology of the Cell, UMR 9198, CNRS – CEA – Université Paris-Sud, Gif-sur-Yvette, France.1Institute of Plant Biology, Biological Research Centre, Hungarian Academy of Sciences, Szeged, Hungary.Attila Kereszt 1 Peter Mergaert 2 Jesús Montiel 1† Gabriella Endre 1 Éva Kondorosi 1*
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