Figure 8.

Model of the autoregulation system in soybean and proposed function of GmNF-YA1a/b in AOM. (a) Both, AM interaction and nodulation are based on an early signal exchange between the partners. Root exudates induce production of microbial signals (Myc/Nod factors) that are perceived by symbiosis-specific plasma membrane-bound receptor kinases (RKs). The signal is then translocated to the nucleus via activation of the common early signaling cascade finally leading to the induction of specific TFs that mediate successful mycorrhization or nodulation. Additionally, the so-called activated state of the root is induced resulting most likely in production of CLE peptides as putative root-derived signals activating NARK in the shoot. NARK reduces, probably via the shoot-derived inhibitor SDI, shoot-to-root auxin transport and JA biosynthesis in the shoot. Downstream of NARK unknown components finally suppress the common early signaling cascade leading to reduced subsequent infections with AM fungi and rhizobia. (b) Proposed model of GmNF-YA1a/b function in establishment and autoregulation in AM symbiosis. GmNF-YA1a/b promote formation of AM either via stimulating production of root exudates attracting AM fungi or by acting as positive regulator of one of the components of the early signal transduction cascade of AM establishment. After AM fungal infection, in soybean plants with functional NARK the gene expression of GmNF-YA1a/b is systemically downregulated in roots. This might be caused directly by SDI or by other signals downstream of SDI.