Distinct and conserved transcriptomic changes during nematode-induced giant cell development in tomato compared with Arabidopsis: a functional role for gene repression.

Root-knot nematodes (RKNs) induce giant cells (GCs) from root vascular cells inside the galls. Accompanying molecular changes as a function of infection time and across different species, and their functional impact, are still poorly understood. Thus, the transcriptomes of tomato galls and laser capture microdissected (LCM) GCs over the course of parasitism were compared with those of Arabidopsis, and functional analysis of a repressed gene was performed. Microarray hybridization with RNA from galls and LCM GCs, infection-reproduction tests and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) transcriptional profiles in susceptible and resistant (Mi-1) lines were performed in tomato. Tomato GC-induced genes include some possibly contributing to the epigenetic control of GC identity. GC-repressed genes are conserved between tomato and Arabidopsis, notably those involved in lignin deposition. However, genes related to the regulation of gene expression diverge, suggesting that diverse transcriptional regulators mediate common responses leading to GC formation in different plant species. TPX1, a cell wall peroxidase specifically involved in lignification, was strongly repressed in GCs/galls, but induced in a nearly isogenic Mi-1 resistant line on nematode infection. TPX1 overexpression in susceptible plants hindered nematode reproduction and GC expansion. Time-course and cross-species comparisons of gall and GC transcriptomes provide novel insights pointing to the relevance of gene repression during RKN establishment.

Strigolactone signaling in the endodermis is sufficient to restore root responses and involves SHORT HYPOCOTYL 2 (SHY2) activity.

Strigolactones (SLs) are plant hormones and regulators of root development, including lateral root (LR) formation, root hair (RH) elongation and meristem cell number, in a MORE AXILLARY GROWTH 2 (MAX2)-dependent way. However, whether SL signaling is acting cell-autonomously or in a non-cell-autonomous way in roots is unclear. We analyzed root phenotype, hormonal responses and gene expression in multiple lines of Arabidopsis thaliana max2-1 mutants expressing MAX2 under various tissue-specific promoters and shy2 mutants. The results demonstrate for the first time that expression of MAX2 under the SCARECROW (SCR) promoter, expressed mainly in the root endodermis, is sufficient to confer SL sensitivity in the root for RH, LR and meristem cell number. Moreover, loss of function mutation of SHORT HYPOCOTYL 2 (SHY2), a key component in auxin and cytokinin regulation of meristem size, has been found to be insensitive to SLs in relation to LR formation and meristem cell number. Endodermal SL signaling, mediated by MAX2, is sufficient to confer SL sensitivity in root, and SHY2 may participate in SL signaling to regulate meristem size and LR formation. These SL signaling pathways thus may act through modulation of auxin flux in the root tip, and may indicate a root-specific, yet non-cell-autonomous regulatory mode of action.

Structure-function relations of strigolactone analogs: activity as plant hormones and plant interactions.

Strigolactones (SLs) have several functions as signaling molecules in their interactions with symbiotic arbuscular mycorrhizal (AM) fungi and the parasitic weeds Orobanche and Striga. SLs are also a new class of plant hormone regulating plant development. In all three organisms, a specific and sensitive receptor-mediated perception system is suggested. By comparing the activity of synthetic SL analogs on Arabidopsis root-hair elongation, Orobanche aegyptiaca seed germination, and hyphal branching of the AM fungus Glomus intraradices, we found that each of the tested organisms differs in its response to the various examined synthetic SL analogs. Structure-function relations of the SL analogs suggest substitutions on the A-ring as the cause of this variation. Moreover, the description of competitive antagonistic analogs suggests that the A-ring of SL can affect not only affinity to the receptor, but also the molecule's ability to activate it. The results support the conclusion that Arabidopsis, Orobanche, and AM fungi possess variations in receptor sensitivity to SL analogs, probably due to variation in SL receptors among the different species.

Strigolactones are positive regulators of light-harvesting genes in tomato.

Strigolactones are newly identified plant hormones, shown to participate in the regulation of lateral shoot branching and root development. However, little is known about their effects on biological processes, genes, and proteins. Transcription profiling of roots treated with GR24, a synthetic strigolactone with proven biological activity, and/or indole acetic acid (IAA) was combined with physiological and transcriptional analysis of a tomato mutant (Sl-ORT1) deficient in strigolactone production. GR24 treatment led to markedly induced expression of genes putatively involved in light harvesting. This was apparent in both the presence and absence of exogenously applied IAA, but not with IAA treatment alone. Following validation of the microarray results, transcriptional induction by light of the GR24-induced genes was demonstrated in leaves exposed to high or low light intensities. Sl-ORT1 contained less chlorophyll and showed reduced expression of light harvesting-associated genes than the wild type (WT). Moreover, perfusion of GR24 into WT and Sl-ORT1 leaves led to induction of most of the examined light harvesting-associated genes. Results suggest that GR24 treatment interferes with the root's response to IAA treatment and that strigolactones are potentially positive regulators of light harvesting in plants.

A tomato strigolactone-impaired mutant displays aberrant shoot morphology and plant interactions.

Strigolactones are considered a new group of plant hormones. Their role as modulators of plant growth and signalling molecules for plant interactions first became evident in Arabidopsis, pea, and rice mutants that were flawed in strigolactone production, release, or perception. The first evidence in tomato (Solanum lycopersicon) of strigolactone deficiency is presented here. Sl-ORT1, previously identified as resistant to the parasitic plant Orobanche, had lower levels of arbuscular mycorrhizal fungus (Glomus intraradices) colonization, possibly as a result of its reduced ability to induce mycorrhizal hyphal branching. Biochemical analysis of mutant root extracts suggested that it produces only minute amounts of two of the tomato strigolactones: solanacol and didehydro-orobanchol. Accordingly, the transcription level of a key enzyme (CCD7) putatively involved in strigolactone synthesis in tomato was reduced in Sl-ORT1 compared with the wild type (WT). Sl-ORT1 shoots exhibited increased lateral shoot branching, whereas exogenous application of the synthetic strigolactone GR24 to the mutant restored the WT phenotype by reducing the number of lateral branches. Reduced lateral shoot branching was also evident in grafted plants which included a WT interstock, which was grafted between the mutant rootstock and the scion. In roots of these grafted plants, the CCD7 transcription level was not significantly induced, nor was mycorrhizal sensitivity restored. Hence, WT-interstock grafting, which restores mutant shoot morphology to WT, does not restore mutant root properties to WT. Characterization of the first tomato strigolactone-deficient mutant supports the putative general role of strigolactones as messengers of suppression of lateral shoot branching in a diversity of plant species.

Microarray analysis and functional tests suggest the involvement of expansins in the early stages of symbiosis of the arbuscular mycorrhizal fungus Glomus intraradices on tomato (Solanum lycopersicum).

Arbuscular mycorrhizal (AM) symbiosis occurs between fungi of the phylum Glomeromycota and most terrestrial plants. However, little is known about the molecular symbiotic signalling between AM fungi (AMFs) and non-leguminous plant species. We sought to further elucidate the molecular events occurring in tomato, a non-leguminous host plant, during the early, pre-symbiotic stage of AM symbiosis, i.e. immediately before and after contact between the AMF (Glomus intraradices) and the host. We adopted a semi-synchronized AMF root infection protocol, followed by genomic-scale, microarray-based, gene expression profiling at several defined time points during pre-symbiotic AM stages. The microarray results suggested differences in the number of differentially expressed genes and in the differential regulation of several functional groups of genes at the different time points examined. The microarray results were validated and one of the genes induced during contact between AMF and tomato, the expansin-like EXLB1, was functionally analysed. Expansins, encoded by a large multigene family, facilitate plant cell expansion. However, no biological or biochemical function has yet been established for plant-originated expansin-like proteins. EXLB1 transcripts were localized early during the association to cells that may perceive the fungal signal, and later during the association in close proximity to sites of AMF hypha-root colonization. Moreover, in transgenic roots, we demonstrated that a reduction in the steady-state level of EXLB1 transcript was correlated with a reduced rate of infection, reduced arbuscule expansion and reduced AMF spore formation.