Highlights d Rhizobia modify b-1,3-glucan (callose) levels in developing nitrogen-fixing nodules d The b-1,3-glucanase MtBG2 promotes symplastic connectivity in nodule primordia d Symplastic communication is necessary for nodule development and colonization d Symplastic connectivity regulates the spatial expression of key symbiotic genes
Root growth is critical for the effective exploitation of the rhizosphere and productive plant growth. Our recent work1 showed that root architecture was dependent upon the degree of symplastic connectivity between neighboring cells during the specification of lateral root primordia and was affected by genes regulating callose deposition at plasmodesmata (PD). Here we provide additional evidence that both symplastic connectivity and callose are also important during the later phase of lateral root development: emergence. Callose immunolocalization assays indicated that transient symplastic isolation of the primordium occur immediately prior to emergence through the overlaying tissues to produce the mature lateral root.1 Here we could corroborate these results by analyzing the mobility of a symplastic tracer and the expression of PD genes in lateral roots and in response to auxins. Moreover, we show that altering callose deposition affects the number of emerged lateral roots suggesting that PD regulation is important for emergence.
Colonization of the land by plants required major modifications in cellular structural composition and metabolism. Intercellular communication through plasmodesmata (PD) plays a critical role in the coordination of growth and cell activities. Changes in the form, regulation or function of these channels are likely linked to plant adaptation to the terrestrial environments. Constriction of PD aperture by deposition of callose is the best-studied mechanism in PD regulation. Glycosyl hydrolases family 17 (GHL17) are callose degrading enzymes. In Arabidopsis this is a large protein family, few of which have been PD-localized. The objective here is to identify correlations between evolution of this protein family and their role at PD and to use this information as a tool to predict the localization of candidates isolated in a proteomic screen. With this aim, we studied phylogenetic relationship between Arabidopsis GHL17 sequences and those isolated from fungi, green algae, mosses and monocot representatives. Three distinct phylogenetic clades were identified. Clade alpha contained only embryophytes sequences suggesting that this subgroup appeared during land colonization in organisms with functional PD. Accordingly, all PD-associated GHL17 proteins identified so far in Arabidopsis thaliana and Populus are grouped in this ‘embryophytes only’ phylogenetic clade. Next, we tested the use of this knowledge to discriminate between candidates isolated in the PD proteome. Transient and stable expression of GFP protein fusions confirmed PD localization for candidates contained in clade alpha but not for candidates contained in clade beta. Our results suggest that GHL17 membrane proteins contained in the alpha clade evolved and expanded during land colonization to play new roles, among others, in PD regulation.
Background A major route for cell-to-cell signalling in plants is mediated by cell wall-embedded pores termed plasmodesmata forming the symplasm. Plasmodesmata regulate the plant development and responses to the environment; however, our understanding of what factors or regulatory cues affect their structure and permeability is still limited. In this paper, a meta-analysis was carried out for the identification of conditions affecting plasmodesmata transport and for the in silico prediction of plasmodesmata proteins in species for which the plasmodesmata proteome has not been experimentally determined. Results Using the information obtained from experimental proteomes, an analysis pipeline (named plasmodesmata in silico proteome 1 or PIP1) was developed to rapidly generate candidate plasmodesmata proteomes for 22 plant species. Using the in silico proteomes to interrogate published transcriptomes, gene interaction networks were identified pointing to conditions likely affecting plasmodesmata transport capacity. High salinity, drought and osmotic stress regulate the expression of clusters enriched in genes encoding plasmodesmata proteins, including those involved in the metabolism of the cell wall polysaccharide callose. Experimental determinations showed restriction in the intercellular transport of the symplasmic reporter GFP and enhanced callose deposition in Arabidopsis roots exposed to 75-mM NaCl and 3% PEG (polyethylene glycol). Using PIP1 and transcriptome meta-analyses, candidate plasmodesmata proteins for the legume Medicago truncatula were generated, leading to the identification of Medtr1g073320, a novel receptor-like protein that localises at plasmodesmata. Expression of Medtr1g073320 affects callose deposition and the root response to infection with the soil-borne bacteria rhizobia in the presence of nitrate. Conclusions Our study shows that combining proteomic meta-analysis and transcriptomic data can be a valuable tool for the identification of new proteins and regulatory mechanisms affecting plasmodesmata function. We have created the freely accessible pipeline PIP1 as a resource for the screening of experimental proteomes and for the in silico prediction of PD proteins in diverse plant species.
SummaryA major route for cell-to-cell signaling is via cell wall-embedded pores termed plasmodesmata (PD). PD regulate many aspects of plant development and responses to the environment however, our understanding of what factors affect their structure and permeability is limited. In this paper, a meta-analysis is presented as a tool for the identification of conditions affecting PD transport and generation of PD proteomes in species in which experimental data is not available. A custom-built pipeline was developed to rapidly generate candidate PD proteomes for species of interest. The pipeline uses protein structural features and conserved domains to predict and prioritize validation of PD candidate proteins. Co-expression tables generated using published microarrays identify putative interactions between PD genes and conditions affecting PD function. We provide evidence that high salinity and osmotic stress regulate the expression of PD candidate proteins and symplasmic phloem unloading of GFP. The PD proteome for Medicago truncatula was generated in silico leading to the identification and experimental determination of a receptor like protein that target PD in root cortical cells. Together the results highlight the power of our newly designed tool in the identification of new factors and proteins influencing PD in diverse plant species.
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