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Summary: The plant hormone gibberellin (GA) is required at different stages of legume nodule development, with its spatiotemporal distribution tightly regulated. Transcriptomic and bioinformatic analyses established that several key GA biosynthesis and catabolism enzyme encoding genes are critical to soybean (Glycine max) nodule formation.We examined the expression of several GA oxidase genes and used a Förster resonance energy transfer‐based GA biosensor to determine the bioactive GA content of roots inoculated with DsRed‐labelled Bradyrhizobium diazoefficiens. We manipulated the level of GA by genetically disrupting the expression of GA oxidase genes. Moreover, exogenous treatment of soybean roots with GA3 induced the expression of key nodulation genes and altered infection thread and nodule phenotypes.GmGA20ox1a, GmGA3ox1a, and GmGA2ox1a are upregulated in soybean roots inoculated with compatible B. diazoefficiens. GmGA20ox1a expression is predominately localized to the transient meristem of soybean nodules and coincides with the spatiotemporal distribution of bioactive GA occurring throughout nodule organogenesis. GmGA2ox1a exhibits a nodule vasculature‐specific expression pattern, whereas GmGA3ox1a can be detected throughout the nodule and root. Disruptions in the level of GA resulted in aberrant rhizobia infection and reduced nodule numbers.Collectively, our results establish a central role for GAs in root hair infection by symbiotic rhizobia and in nodule organogenesis. [ABSTRACT FROM AUTHOR]

Legumes control their nodule numbers through the autoregulation of nodulation (AON). Rhizobia infection stimulates the production of root‐derived CLE peptide hormones that are translocated to the shoot where they regulate a new signal. We used soybean to demonstrate that this shoot‐derived signal is miR2111, which is transported via phloem to the root where it targets transcripts of Too Much Love (TML), a negative regulator of nodulation. Shoot perception of rhizobia‐induced CLE peptides suppresses miR2111 expression, resulting in TML accumulation in roots and subsequent inhibition of nodule organogenesis. Feeding synthetic mature miR2111 via the petiole increased nodule numbers per plant. Likewise, elevating miR2111 availability by over‐expression promoted nodulation, while target mimicry of TML induced the opposite effect on nodule development in wild‐type plants and alleviated the supernodulating and stunted root growth phenotypes of AON‐defective mutants. Additionally, in non‐nodulating wild‐type plants, ectopic expression of miR2111 significantly enhanced lateral root emergence with a decrease in lateral root length and average root diameter. In contrast, hairy roots constitutively expressing the target mimic construct exhibited reduced lateral root density. Overall, these findings demonstrate that miR2111 is both the critical shoot‐to‐root factor that positively regulates root nodule development and also acts to shape root system architecture. Our work used soybean to demonstrate that miR2111 is the critical shoot‐to‐root factor that positively regulates root nodule formation in the autoregulation of nodulation pathway and also acts to shape root system architecture via orchestrating the degree of root branching, as well as the length and thickness of lateral roots. This project was initially driven by the incentive to pinpoint the mobile factor that mediates the communication between shoots and roots in the downward signalling pathway of AON. We then aimed to address if miR2111 provides a genetic link for the concomitant aberrant root growth and supernodulation phenotypes seen in those AON‐defective mutants. [ABSTRACT FROM AUTHOR]

The prophage/phage region in the genome of 'Candidatus Liberibacter asiaticus', an alpha-proteobacterium associated with citrus Huanglongbing, included many valuable loci for genetic diversity studies. Previously, a mosaic genomic region (CLIBASIA_05640 to CLIBASIA_05650) was characterized, and this revealed inter- and intracontinental variations of 'Ca. L. asiaticus'. In this study, 267 'Ca. L. asiaticus' isolates collected from eight provinces in China were analysed with a primer set flanking the same mosaic region plus downstream sequence. While most amplicon sizes ranged from 1400 to 2000 bp, an amplicon of 550 bp (S550) was found in 14 samples collected from south-western China. Sequence analyses showed that S550 was the result of a 1033 bp deletion which included the previously known mosaic region. The genetic nature of the deletion event remains unknown. The regional restriction of S550 suggests that the 'Ca. L. asiaticus' population from south-western China is different from those in eastern China. The small and easy-to-detect S550 amplicon could serve as a molecular marker for 'Ca. L. asiaticus' epidemiology. [ABSTRACT FROM AUTHOR]

Plant roots are constantly exposed to a diverse microbiota of pathogens and mutualistic partners. The host's immune system is an essential component for its survival, enabling it to monitor nearby microbes for potential threats and respond with a defence response when required. Current research suggests that the plant immune system has also been employed in the legume-rhizobia symbiosis as a means of monitoring different rhizobia strains and that successful rhizobia have evolved to overcome this system to infect the roots and initiate nodulation. With clear implications for host-specificity, the immune system has the potential to be an important target for engineering versatile crops for effective nodulation in the field. However, current knowledge of the interacting components governing this pathway is limited, and further research is required to build on what is currently known to improve our understanding. This review provides a general overview of the plant immune system's role in nodulation. With a focus on the cycles of microbe-associated molecular pattern-triggered immunity (MTI) and effector-triggered immunity (ETI), we highlight key molecular players and recent findings while addressing the current knowledge gaps in this area. [ABSTRACT FROM AUTHOR]

Summary: Legumes form a symbiosis with atmospheric nitrogen (N2)‐fixing soil rhizobia, resulting in new root organs called nodules that enable N2‐fixation. Nodulation is a costly process that is tightly regulated by the host through autoregulation of nodulation (AON) and nitrate‐dependent regulation of nodulation. Both pathways require legume‐specific CLAVATA/ESR‐related (CLE) peptides.Nitrogen‐induced nodulation‐suppressing CLE peptides have not previously been investigated in Medicago truncatula, for which only rhizobia‐induced MtCLE12 and MtCLE13 have been characterised. Here, we report on novel peptides MtCLE34 and MtCLE35 in nodulation control.The nodulation‐suppressing CLE peptides of five legume species were classified into three clades based on sequence homology and phylogeny. This approached identified MtCLE34 and MtCLE35 and four new CLE peptide orthologues of Pisum sativum. Whereas MtCLE12 and MtCLE13 are induced by rhizobia, MtCLE34 and MtCLE35 respond to both rhizobia and nitrate. MtCLE34 was identified as a pseudogene lacking a functional CLE‐domain. MtCLE35 was found to inhibit nodulation in a SUNN‐ and RDN1‐dependent manner via overexpression analysis.Together, our findings indicate that MtCLE12 and MtCLE13 have a specific role in AON, while MtCLE35 regulates nodule numbers in response to both rhizobia and nitrate. MtCLE34 likely had a similar role to MtCLE35, but its function was lost due to a premature nonsense mutation. [ABSTRACT FROM AUTHOR]

Global demand to increase food production and simultaneously reduce synthetic nitrogen fertilizer inputs in agriculture are underpinning the need to intensify the use of legume crops. The symbiotic relationship that legume plants establish with nitrogen‐fixing rhizobia bacteria is central to their advantage. This plant–microbe interaction results in newly developed root organs, called nodules, where the rhizobia convert atmospheric nitrogen gas into forms of nitrogen the plant can use. However, the process of developing and maintaining nodules is resource intensive; hence, the plant tightly controls the number of nodules forming. A variety of molecular mechanisms are used to regulate nodule numbers under both favourable and stressful growing conditions, enabling the plant to conserve resources and optimize development in response to a range of circumstances. Using genetic and genomic approaches, many components acting in the regulation of nodulation have now been identified. Discovering and functionally characterizing these components can provide genetic targets and polymorphic markers that aid in the selection of superior legume cultivars and rhizobia strains that benefit agricultural sustainability and food security. This review addresses recent findings in nodulation control, presents detailed models of the molecular mechanisms driving these processes, and identifies gaps in these processes that are not yet fully explained. Legumes form a beneficial symbiosis with soil bacteria, known as rhizobia, which is signified by the production of nodules on the host plant's roots. The host plant prevents over colonization by rhizobia through various molecular mechanisms that tightly control nodule numbers in response to biotic and abiotic factors. Components of these pathways are key to understanding and optimizing nodulation processes and are the focus of this review. [ABSTRACT FROM AUTHOR]