Summary Root‐knot nematodes (RKNs; Meloidogyne spp.) induce new post‐embryogenic organs within the roots (galls) where they stablish and differentiate nematode feeding cells, giant cells (GCs). The developmental programmes and functional genes involved remain poorly defined. Arabidopsis root apical meristem (RAM), lateral root (LR) and callus marker lines, SHORT‐ROOT/SHR, SCARECROW/SCR, SCHIZORIZA/SCZ, WUSCHEL‐RELATED‐HOMEOBOX‐5/WOX5, AUXIN‐RESPONSIVE‐FACTOR‐5/ARF5, ARABIDOPSIS‐HISTIDINE PHOSPHOTRANSFER‐PROTEIN‐6/AHP6, GATA‐TRANSCRIPTION FACTOR‐23/GATA23 and S‐PHASE‐KINASE‐ASSOCIATED‐PROTEIN2B/SKP2B, were analysed for nematode‐dependent expression. Their corresponding loss‐of‐function lines, including those for LR upstream regulators, SOLITARY ROOT/SLR/IAA14, BONDELOS/BDL/IAA12 and INDOLE‐3‐ACETIC‐ACID‐INDUCIBLE‐28/IAA28, were tested for RKN resistance/tolerance. LR genes, for example ARF5 (key factor for root stem‐cell niche regeneration), GATA23 (which specifies pluripotent founder cells) and AHP6 (cytokinin‐signalling‐inhibitor regulating pericycle cell‐divisions orientation), show a crucial function during gall formation. RKNs do not compromise the number of founder cells or LR primordia but locally induce gall formation possibly by tuning the auxin/cytokinin balance in which AHP6 might be necessary. Key RAM marker genes were induced and functional in galls. Therefore, the activation of plant developmental programmes promoting transient‐pluripotency/stemness leads to the generation of quiescent‐centre and meristematic‐like cell identities within the vascular cylinder of galls. Nematodes enlist developmental pathways of new organogenesis and/or root regeneration in the vascular cells of galls. This should determine meristematic cell identities with sufficient transient pluripotency for gall organogenesis.

Developmental plasticity is one of the most striking features of plant morphogenesis, as plants are able to vary their shapes in response to environmental cues. Biotic or abiotic stimuli often promote organogenesis events in plants not observed under normal growth conditions. Root-knot nematodes (RKNs) are known to parasitize multiple species of rooting plants and to induce characteristic tissue expansion called galls or root-knots on the roots of their hosts by perturbing the plant cellular machinery. Galls contain giant cells (GCs) and neighboring cells, and the GCs are a source of nutrients for the parasitizing nematode. Highly active cell proliferation was observed in galls. However, the underlying mechanisms that regulate the symptoms triggered by the plant-nematode interaction have not yet been elucidated. In this study, we deciphered the molecular mechanism of gall formation with an in vitro infection assay system using RKN Meloidogyne incognita, and the model plant Arabidopsis thaliana. By taking advantages of this system, we performed next-generation sequencing-based transcriptome profiling, and found that the expression of procambium identity-associated genes were enriched during gall formation. Clustering analyses with artificial xylogenic systems, together with the results of expression analyses of the candidate genes, showed a significant correlation between the induction of gall cells and procambium-associated cells. Furthermore, the promoters of several procambial marker genes such as ATHB8, TDR and WOX4 were activated not only in M. incognita-induced galls, but similarly in M. javanica induced-galls and Heterodera schachtii-induced syncytia. Our findings suggest that phytoparasitic nematodes modulate the host’s developmental regulation of the vascular stem cells during gall formation.

Root-knot nematodes (RKNs; Meloidogyne spp.) induce feeding cells (giant cells; GCs) inside a pseudo-organ (gall) from still unknown root cells. Understanding GCs ontogeny is essential to the basic knowledge of RKN–plant interaction and to discover novel and effective control strategies. Hence, we report for the first time in a model plant, Arabidopsis, molecular, and cellular features concerning ectopic de novo organogenesis of RKNs GCs in leaves. RKNs induce GCs in leaves with irregular shape, a reticulated cytosol, and fragmented vacuoles as GCs from roots. Leaf cells around the nematode enter G2-M shown by ProCycB1;1:CycB1;1(NT)-GUS expression, consistent to multinucleated GCs. In addition, GCs nuclei present irregular and varied sizes. All these characteristics mentioned, being equivalent to GCs in root-galls. RKNs complete their life cycle forming a gall/callus-like structure in the leaf vascular tissues resembling auxin-induced callus with an auxin-response maxima, indicated by high expression of DR5::GUS that is dependent on leaf auxin-transport. Notably, induction of leaves calli/GCs requires molecular components from roots crucial for lateral roots (LRs), auxin-induced callus and root-gall formation, i.e., LBD16. Hence, LBD16 is a xylem pole pericycle specific and local marker in LR primordia unexpectedly induced locally in the vascular tissue of leaves after RKN infection. LBD16 is also fundamental for feeding site formation as RKNs could not stablish in 35S::LBD16-SRDX leaves, and likely it is also a conserved molecular hub between biotic and developmental signals in Arabidopsis either in roots or leaves. Moreover, RKNs induce the ectopic development of roots from leaf and root-galls, also formed in mutants compromised in LR formation, arf7/arf19, slr, and alf4. Therefore, nematodes must target molecular signatures to induce post-embryogenic de novo organogenesis through the LBD16 callus formation pathway partially different from those prevalent during normal LR development.

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