Plants form mutualistic nutrient acquiring symbioses with microbes, including arbuscular mycorrhizal fungi. The formation of these symbioses is costly and plants employ a negative feedback loop termed autoregulation of mycorrhizae (AOM) to limit arbuscular mycorrhizae (AM) formation. We provide evidence for the role of one leucine-rich-repeat receptor like kinase (FAB), a hydroxyproline O-arabinosyltransferase enzyme (FIN) and additional evidence for one receptor like protein (SlCLV2) in the negative regulation of AM formation in tomato. Reciprocal grafting experiments suggest that the FAB gene acts locally in the root, while the SlCLV2 gene may act in both the root and the shoot. External nutrients including phosphate and nitrate can also strongly suppress AM formation. We found that FAB and FIN are required for nitrate suppression of AM but are not required for the powerful suppression of AM colonisation by phosphate. This parallels some of the roles of legume homologs in the autoregulation of the more recently evolved symbioses with nitrogen-fixing bacteria leading to nodulation. This deep homology in the symbiotic role of these genes suggests that in addition to the early signalling events that lead to the establishment of AM and nodulation, the autoregulation pathway might also be considered part of the common symbiotic toolkit that enabled plants to form beneficial symbioses.

Background The presence of a polar auxin transport stream has long been correlated with the differentiation and patterning of vascular cells across vascular plants. As our understanding of auxin transport and vascular development has grown, so too has evidence for the correlation between these processes. However, a clear understanding of the cellular and molecular mechanisms driving this correlation has not been elucidated.• Scope This article examines the hypothesis that canalization via polar auxin transport regulates vascular reconnection and patterning in the stem after wounding or grafting. We examine the evidence for the causal nature of the relationship and the suggested role that other hormones may play. Data are presented indicating that in grafted plants the degree of auxin transport may not always correlate with vascular reconnection. Furthermore, data on grafting success using plants with a range of hormone-related mutations indicate that these hormones may not be critical for vascular reconnection.• Conclusions In the past, excellent work examining elements of auxin synthesis, transport and response in relation to vascular development has been carried out. However, new experimental approaches are required to test more directly the hypothesis that auxin transport regulates stem vascular reconnection after wounding or grafting. This could include studies on the timing of the re-establishment of auxin transport and vascular reconnection after grafting and the influence of auxin transport mutants and inhibitors on these processes using live imaging.

The underlying mechanisms that determine whether two species can form a successful graft union (graft compatibility) remain obscure. Two prominent hypotheses are (1) the more closely related species are, the higher the graft success and (2) the vascular anatomy at the graft junction influences graft success. In this paper these two hypotheses are examined in a systematic way using graft combinations selected from a range of (a) phylogenetically close and more distant legume species, (b) species displaying different germination patterns and (c) scions and rootstocks possessing contrasting stem tissues and vascular patterns. Relatedness of species was not a good predictor of graft compatibility, as vascular reconnection can occur between distantly related species and can fail to occur in some more closely related species. Similarly, neither the stem tissues present at the graft junction nor the vascular anatomy correlated with the success of vascular reconnection. Relatedness and stem anatomy therefore do not appear to be the determining factors in successful vascular reconnection after grafting in legumes. These results are discussed in conjunction with other hypotheses such as the role of auxin.

Symbioses with beneficial microbes are widespread in plants, but these relationships must balance the energy invested by the plants with the nutrients acquired. Symbiosis with arbuscular mycorrhizal (AM) fungi occurs throughout land plants but our understanding of the genes and signals that regulate colonisation levels is limited. Here, we demonstrate that in tomato two CLV3/EMBRYO-SURROUNDING REGION (CLE) peptides, SlCLE10 and SlCLE11, act to suppress AM colonisation of roots. Mutant studies and overexpression via hairy transformation indicate SlCLE11 acts locally in the root to limit AM colonisation. Indeed, SlCLE11 expression is strongly induced in AM colonised roots but SlCLE11 is not required for phosphate suppression of AM colonisation. SlCLE11 may act through as yet uncharacterised signalling pathways, as SlCLE11 does not suppress AM colonisation by acting through two previously characterised receptors with roles in regulating AM colonisation, SlFAB (CLAVATA1 orthologue) or SlCLV2. SlCLE10 appears to play a more minor or redundant role, as cle10 mutants did not influence AM, although the fact that ectopic overexpression of SlCLE10 did suppress colonisation suggests SlCLE10 may play a role in regulating AM colonisation. Our findings show that CLE peptides regulate AM colonisation in the non-legume species tomato.

AbstractPlants form mutualistic nutrient acquiring symbioses with microbes, including arbuscular mycorrhizal fungi. The formation of these symbioses is costly and plants employ a negative feedback loop termed autoregulation of mycorrhizae (AOM) to limit arbuscular mycorrhizae (AM) formation. We provide evidence for the role of one leucine-rich-repeat receptor like kinase (FAB), a hydroxyproline O-arabinosyltransferase enzyme (FIN), one CLE peptide, SlCLE11 and additional evidence for the one receptor like protein (SlCLV2) in the negative regulation of AM formation in tomato. Reciprocal grafting experiments suggest that the FAB gene acts locally in the root, while the SlCLV2 gene may act in both the root and the shoot. External nutrients including phosphate and nitrate can also strongly suppress AM formation. We found that FAB and FIN are required for nitrate suppression of AM but are not required for the powerful suppression of AM colonisation by phosphate. This parallels some of the roles of legume homologs in the autoregulation of the more recently evolved symbioses with nitrogen-fixing bacteria leading to nodulation. This deep homology in the symbiotic role of these genes suggests that in addition to the early signalling events that lead to the establishment of AM and nodulation, the autoregulation pathway might also be considered part of the common symbiotic toolkit that enabled plants to form beneficial symbioses.HighlightWe describe the role of CLV signalling elements in the negative regulation of arbuscular mycorrhizal symbioses of tomato, including influencing nitrate but not phosphate suppression of mycorrhizal colonisation.