Profiling of differentially expressed genes in roots of Robinia pseudoacacia during nodule development using suppressive subtractive hybridization

TRAPPC13 Is a Novel Target of Mesorhizobium amorphae Type III Secretion System Effector NopP

Similar to pathogenic bacteria, rhizobia can inject effector proteins into host cells directly to promote infection via the type III secretion system (T3SS). Nodulation outer protein P (NopP), a specific T3SS effector of rhizobia, plays different roles in the establishment of multiple rhizobia-legume symbiotic systems. Mesorhizobium amorphae CCNWGS0123 (GS0123), which infects Robinia pseudoacacia specifically, secretes several T3SS effectors, including NopP. Here, we demonstrate that NopP is secreted through T3SS-I of GS0123 during the early stages of infection, and its deficiency decreases nodule nitrogenase activity of R. pseudoacacia nodules. A trafficking protein particle complex subunit 13-like protein (TRAPPC13) has been identified as a NopP target protein in R. pseudoacacia roots by screening a yeast two-hybrid library. The physical interaction between NopP and TRAPPC13 is verified by bimolecular fluorescence complementation and coimmunoprecipitation assays. In addition, subcellular localization analysis reveals that both NopP and its target, TRAPPC13, are colocalized on the plasma membrane. Compared with GS0123-inoculated R. pseudoacacia roots, some genes associated with cell wall remodeling and plant innate immunity down-regulated in ΔnopP-inoculated roots at 36 h postinoculation. The results suggest that NopP in M. amorphae CCNWGS0123 acts in multiple processes in R. pseudoacacia during the early stages of infection, and TRAPPC13 could participate in the process as a NopP target.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Identification of Robinia pseudoacacia target proteins responsive to Mesorhizobium amphore CCNWGS0123 effector protein NopT

Nodulation outer proteins secreted via type 3 secretion systems are involved in the process of symbiosis between legume plants and rhizobia. To study the function of NopT in symbiosis, we mutated nopT in Mesorhizobium amphore CCNWGS0123 (GS0123), which can nodulate black locust (Robinia pseudoacacia). The nopT mutant induced higher levels of jasmonic acid, salicylic acid, and hydrogen peroxide accumulation in the roots of R. pseudoacacia compared with wild-type GS0123. The ΔnopT mutant induced higher disease-resistant gene expression 72 hours post-inoculation (hpi), whereas GS0123 induced higher disease-resistant gene expression earlier, at 36 hpi. Compared with the nopT mutant, GS0123 induced the up-regulation of most genes at 36 hpi and the down-regulation of most genes at 72 hpi. Proteolytically active NopT_GS0123 induced hypersensitive responses when expressed transiently in tobacco leaves (Nicotiana benthamiana). Two NopT_GS0123 targets in R. pseudoacacia were identified, ATP-citrate synthase alpha chain protein 2 and hypersensitive-induced response protein. Their interactions with NopT_GS0123 triggered resistance by the plant immune system. In conclusion, NopT_GS0123 inhibited the host plant immune system and had minimal effect on nodulation in R. pseudoacacia. Our results reveal the underlying molecular mechanism of NopT function in plant-symbiont interactions.

The Rpf84 gene, encoding a ribosomal large subunit protein, RPL22, regulates symbiotic nodulation in Robinia pseudoacacia

A homologue of the ribosomal protein L22e, Rpf84, regulates root nodule symbiosis by mediating the infection process of rhizobia and preventing bacteroids from degradation in Robinia pseudoacacia. Ribosomal proteins (RPs) are known to have extraribosomal functions, including developmental regulation and stress responses; however, the effects of RPs on symbiotic nodulation of legumes are still unclear. Ribosomal protein 22 of the large 60S subunit (RPL22), a non-typical RP that is only found in eukaryotes, has been shown to function as a tumour suppressor in animals. Here, a homologue of RPL22, Rpf84, was identified from the leguminous tree R. pseudoacacia. Subcellular localization assays showed that Rpf84 was expressed in the cytoplasm and nucleus. Knockdown of Rpf84 by RNA interference (RNAi) technology impaired the infection process and nodule development. Compared with the control, root and stem length, dry weight and nodule number per plant were drastically decreased in Rpf84-RNAi plants. The numbers of root hair curlings, infection threads and nodule primordia were also significantly reduced. Ultrastructure analyses showed that Rpf84-RNAi nodules contained fewer infected cells with fewer bacteria. In particular, remarkable deformation of bacteroids and fusion of multiple symbiosomes occurred in infected cells. By contrast, overexpression of Rpf84 promoted nodulation, and the overexpression nodules maintained a larger infection/differentiation region and had more infected cells filled with bacteroids than the control at 45 days post inoculation, suggesting a retarded ageing process in nodules. These results indicate for the first time that RP regulates the symbiotic nodulation of legumes and that RPL22 may function in initiating the invasion of rhizobia and preventing bacteroids from degradation in R. pseudoacacia.

Complete genome sequence of the Robinia pseudoacacia L. symbiont Mesorhizobium amorphae CCNWGS0123

Mesorhizobium amorphae CCNWGS0123 was isolated in 2006, from effective nodules of Robinia pseudoacacia L. grown in lead-zinc mine tailing site, in Gansu Province, China. M. amorphae CCNWGS0123 is an aerobic, Gram-negative, non-spore-forming rod strain. This paper characterized M. amorphae CCNWGS0123 and presents its complete genome sequence information and genome annotation. The 7,374,589 bp long genome which encodes 7136 protein-coding genes and 63 RNA coding genes, contains one chromosome and four plasmids. Moreover, a chromosome with no gaps was assembled.

Effects of Rhizophagus irregularis on Photosynthesis and Antioxidative Enzymatic System in Robinia pseudoacacia L. under Drought Stress

Arbuscular mycorrhizal (AM) fungi colonize roots improving plant water status and tolerance to drought. However, it is not clear whether the presence of AM would affect the photosynthesis and antioxidant gene-enzymes response, which help to alleviate drought stress of the host plant. Here, pot experiments were performed to investigate the effects of Rhizophagus irregularis, an AM fungus, on the tissue water content, photosynthesis, reactive oxygen species (ROS) production, antioxidant enzyme activity and gene expression in black locust (Robinia pseudoacacia L.) seedlings which were subjected to well-watered or moderate drought stress. Mycorrhizal symbiosis increased relative water content (RWC) of plant roots and leaves, promoted the accumulation of biomass and chlorophyll (Chl) content, and improved photochemistry efficiency, regardless of watering regimes. Mycorrhizal plants had higher SOD, POD, CAT, APX, and GR activities, and the transcript levels of Cu/Zn-SOD. APX and GR, but lower O2-, H2O2 and MDA concentrations in leaves and roots of black locust under drought and well-watered conditions. Results from the present study indicate that AM fungus (R. irregularis) symbiosis can enhance photosynthesis and ROS scavenging capabilities and increase RWC of leaves and roots to alleviate drought stress in black locust. Further research is needed to elucidate the relations among AM fungi and the metabolic pathways of antioxidant enzymes, and the function of antioxidant genes regulated by mycorrhizal symbiosis with the purpose of revealing the mechanisms of mycorrhizal-induced plant tolerance to drought stress.

Aquaporin gene expression and physiological responses of Robinia pseudoacacia L. to the mycorrhizal fungus Rhizophagus irregularis and drought stress

The influence of arbuscular mycorrhiza (AM) and drought stress on aquaporin (AQP) gene expression, water status, and photosynthesis was investigated in black locust (Robinia pseudoacacia L.). Seedlings were grown in potted soil inoculated without or with the AM fungus Rhizophagus irregularis, under well-watered and drought stress conditions. Six full-length AQP complementary DNAs (cDNAs) were isolated from Robinia pseudoacacia, named RpTIP1;1, RpTIP1;3, RpTIP2;1, RpPIP1;1, RpPIP1;3, and RpPIP2;1. A phylogenetic analysis of deduced amino acid sequences demonstrated that putative proteins coded by these RpAQP genes belong to the water channel protein family. Expression analysis revealed higher RpPIP expression in roots while RpTIP expression was higher in leaves, except for RpTIP1;3. AM symbiosis regulated host plant AQPs, and the expression of RpAQP genes in mycorrhizal plants depended on soil water condition and plant tissue. Positive effects were observed for plant physiological parameters in AM plants, which had higher dry mass and lower water saturation deficit and electrolyte leakage than non-AM plants. Rhizophagus irregularis inoculation also slightly increased leaf net photosynthetic rate and stomatal conductance under well-watered and drought stress conditions. These findings suggest that AM symbiosis can enhance the drought tolerance in Robinia pseudoacacia plants by regulating the expression of RpAQP genes, and by improving plant biomass, tissue water status, and leaf photosynthesis in host seedlings.

A translationally controlled tumor protein gene Rpf41 is required for the nodulation of Robinia pseudoacacia

Translationally controlled tumor protein (TCTP) is fundamental for the regulation of development and general growth in eukaryotes. Its multiple functions have been deduced from its involvement in several cell pathways, but its potential involvement in symbiotic nodulation of legumes cannot be suggested a priori. In the present work, we identified and characterized from the woody leguminous tree Robinia pseudoacacia a homolog of TCTP, Rpf41, which was up-regulated in the infected roots at 15 days post-inoculation but decreased in the matured nodules. Subcellular location assay showed that Rpf41 protein was located in the plasma membrane, cytoplasm, nucleus, and also maybe in cytoskeleton. Knockdown of Rpf41 via RNA interference (RNAi) resulted in the impaired development of both nodule and root hair. Compared with wild plants, the root and stem length, fresh weight and nodule number per plant was decreased dramatically in Rpf41 RNAi plants. The number of ITs or nodule primordia was also significantly reduced in the Rpf41 RNAi roots. The analyses of nodule ultrastructure showed that the infected cell development in Rpf41 RNAi nodules remained in zone II, which had fewer infected cells. Furthermore, the symbiosomes displayed noticeable shrinkage of bacteroid and peribacteroid space enlargement in the infected cells of Rpf41 RNAi nodules. In the deeper cell layers, a more remarkable aberration of the infected cell ultrastructure was observed, and electron-transparent lesions in the bacteroid cytoplasm were detected. These results identify TCTP as an important regulator of symbiotic nodulation in legume for the first time, and it may be involved in symbiotic cell differentiation and preventing premature aging of the young nodules in R. pseudoacacia.

Arabidopsis Synaptotagmin 2 Participates in Pollen Germination and Tube Growth and Is Delivered to Plasma Membrane via Conventional Secretion

Arabidopsis synaptotagmin 2 (SYT2) has been reported to participate in an unconventional secretory pathway in somatic cells. Our results showed that SYT2 was expressed mainly in the pollen of Arabidopsis thaliana. The pollen of syt2 T-DNA and RNA interference mutant lines exhibited reduced total germination and impeded pollen tube growth. Analysis of the expression of SYT2-GFP fusion protein in the pollen tube indicates that SYT2 was localized to distinct, patchy compartments but could co-localize with the Golgi markers, BODIPY TR C5 ceramide and GmMan1-mCherry. However, SYT2-DsRed-E5 was localized to the plasma membrane in Arabidopsis suspension cells, in addition to the Golgi apparatus. The localization of SYT2 at the plasma membrane was further supported by immunofluorescence staining in pollen tubes. Moreover, brefeldin A treatment inhibited the transport of SYT2 to the plasma membrane and caused SYT2 to aggregate and form enlarged compartments. Truncation of the SYT2-C2AB domains also resulted in retention of SYT2 in the Golgi apparatus. An in vitro phospholipid-binding assay showed that SYT2-C2AB domains bind to the phospholipid membrane in a calcium-dependent manner. Take together, our results indicated that SYT2 was required for pollen germination and pollen tube growth, and was involved in conventional exocytosis.

RNA-Seq analysis identifies key genes associated with haustorial development in the root hemiparasite Santalum album

Santalum album (sandalwood) is one of the economically important plant species in the Santalaceae for its production of highly valued perfume oils. Sandalwood is also a hemiparasitic tree that obtains some of its water and simple nutrients by tapping into other plants through haustoria which are highly specialized organs in parasitic angiosperms. However, an understanding of the molecular mechanisms involved in haustorium development is limited. In this study, RNA sequencing (RNA-seq) analyses were performed to identify changes in gene expression and metabolic pathways associated with the development of the S. album haustorium. A total of 56,011 non-redundant contigs with a mean contig size of 618 bp were obtained by de novo assembly of the transcriptome of haustoria and non-haustorial seedling roots. A substantial number of the identified differentially expressed genes were involved in cell wall metabolism and protein metabolism, as well as mitochondrial electron transport functions. Phytohormone-mediated regulation might play an important role during haustorial development. Especially, auxin signaling is likely to be essential for haustorial initiation, and genes related to cytokinin and gibberellin biosynthesis and metabolism are involved in haustorial development. Our results suggest that genes encoding nodulin-like proteins may be important for haustorial morphogenesis in S. album. The obtained sequence data will become a rich resource for future research in this interesting species. This information improves our understanding of haustorium development in root hemiparasitic species and will allow further exploration of the detailed molecular mechanisms underlying plant parasitism.