Nodule organogenesis in legumes is regulated temporally and spatially through gene networks. Genome-wide transcriptome, proteomic, and metabolomic analyses have been used previously to define the functional role of various plant genes in the nodulation process. However, while significant progress has been made, most of these studies have suffered from tissue dilution since only a few cells/root regions respond to rhizobial infection, with much of the root non-responsive. To partially overcome this issue, we adopted translating ribosome affinity purification (TRAP) to specifically monitor the response of the root cortex to rhizobial inoculation using a cortex-specific promoter. While previous studies have largely focused on the plant response within the root epidermis (e.g., root hairs) or within developing nodules, much less is known about the early responses within the root cortex, such as in relation to the development of the nodule primordium or growth of the infection thread. We focused on identifying genes specifically regulated during early nodule organogenesis using roots inoculated with Bradyrhizobium japonicum . A number of novel nodulation gene candidates were discovered, as well as soybean orthologs of nodulation genes previously reported in other legumes. The differential cortex expression of several genes was confirmed using a promoter-GUS analysis, and RNAi was used to investigate gene function. Notably, a number of differentially regulated genes involved in phytohormone signaling, including auxin, cytokinin, and gibberellic acid (GA), were also discovered, providing deep insight into phytohormone signaling during early nodule development.
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GmMAX2–D14 and –KAI interaction‐mediated SL and KAR signaling play essential roles in soybean root nodulation
Despite of important functions of strigolactones (SLs) and karrikins (KARs) in plant development, plant–parasite and plant–fungi interactions, their roles in soybean–rhizobia interaction remain elusive. SL/KAR signaling genes
- PAR ID:
- 10456572
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- The Plant Journal
- Volume:
- 101
- Issue:
- 2
- ISSN:
- 0960-7412
- Page Range / eLocation ID:
- p. 334-351
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Nod factors secreted by nitrogen-fixing rhizobia are lipo-chitooligosaccharidic signals required for establishment of the nodule symbiosis with legumes. In
Medicago truncatula , the Nod factor hydrolase 1 (MtNFH1) was found to cleave Nod factors ofSinorhizobium meliloti . Here, we report that the class V chitinase MtCHIT5b ofM. truncatula expressed inEscherichia coli can release lipodisaccharides from Nod factors. Analysis ofM. truncatula mutant plants indicated that MtCHIT5b, together with MtNFH1, degradesS. meliloti Nod factors in the rhizosphere.MtCHIT5b expression was induced by treatment of roots with purified Nod factors or inoculation with rhizobia. MtCHIT5b with a fluorescent tag was detected in the infection pocket of root hairs. Nodulation of aMtCHIT5b knockout mutant was not significantly altered whereas overexpression ofMtCHIT5b resulted in fewer nodules. Reduced nodulation was observed whenMtCHIT5b andMtNFH1 were simultaneously silenced in RNA interference experiments. Overall, this study shows that nodule formation ofM. truncatula is regulated by a second Nod factor cleaving hydrolase in addition to MtNFH1. -
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The unique evolutionary adaptation of legumes for nitrogen-fixing symbiosis leading to nodulation is tightly regulated by the host plant. The autoregulation of nodulation (AON) pathway negatively regulates the number of nodules formed in response to the carbon/nitrogen metabolic status of the shoot and root by long-distance signaling to and from the shoot and root. Central to AON signaling in the shoots of
Medicago truncatula is SUNN, a leucine-rich repeat receptor-like kinase with high sequence similarity with CLAVATA1 (CLV1), part of a class of receptors inArabidopsis involved in regulating stem cell populations in the root and shoot. This class of receptors inArabidopsis includes the BARELY ANY MERISTEM family, which, like CLV1, binds to CLE peptides and interacts with CLV1 to regulate meristem development.M. truncatula contains five members of theBAM family, but onlyMtBAM1 andMtBAM2 are highly expressed in the nodules 48 hours after inoculation. Plants carry mutations in individualMtBAM s, and several doubleBAM mutant combinations all displayed wild-type nodule number phenotypes. However,Mtbam2 suppressed thesunn-5 hypernodulation phenotype and partially rescued the short root length phenotype ofsunn- 5 when present in asunn-5 background. Grafting determined thatbam2 suppresses supernodulation from the roots, regardless of theSUNN status of the root. Overexpression ofMtBAM2 in wild-type plants increases nodule numbers, while overexpression ofMtBAM2 in somesunn mutants rescues the hypernodulation phenotype, but not the hypernodulation phenotypes of AON mutantrdn1-2 orcrn . Relative expression measurements of the nodule transcription factor MtWOX5 downstream of the putativebam2 sunn-5 complex revealed disruption of meristem signaling; while bothbam2 andbam2 sunn-5 influenceMtWOX5 expression, the expression changes are in different directions. We propose a genetic model wherein the specific root interactions of BAM2/SUNN are critical for signaling in nodule meristem cell homeostasis inM. truncatula . -
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SUMMARY The conservation of GOLVEN (GLV)/ROOT MERISTEM GROWTH FACTOR (RGF) peptide encoding genes across plant genomes capable of forming roots or root‐like structures underscores their potential significance in the terrestrial adaptation of plants. This study investigates the function and role of GOLVEN peptide‐coding genes in
Medicago truncatula . Five out of fifteen GLV/RGF genes were notably upregulated during nodule organogenesis and were differentially responsive to nitrogen deficiency and auxin treatment. Specifically, the expression ofMtGLV9 andMtGLV10 at nodule initiation sites was contingent upon the NODULE INCEPTION transcription factor. Overexpression of these five nodule‐induced GLV genes in hairy roots ofM. truncatula and application of their synthetic peptide analogues led to a decrease in nodule count by 25–50%. Uniquely, the GOLVEN10 peptide altered the positioning of the first formed lateral root and nodule on the primary root axis, an observation we term ‘noduletaxis’; this decreased the length of the lateral organ formation zone on roots. Histological section of roots treated with synthetic GOLVEN10 peptide revealed an increased cell number within the root cortical cell layers without a corresponding increase in cell length, leading to an elongation of the root likely introducing a spatiotemporal delay in organ formation. At the transcription level, the GOLVEN10 peptide suppressed expression of microtubule‐related genes and exerted its effects by changing expression of a large subset of Auxin responsive genes. These findings advance our understanding of the molecular mechanisms by which GOLVEN peptides modulate root morphology, nodule ontogeny, and interactions with key transcriptional pathways. -
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Abstract The symbiotic relationship between soybean [
Glycine max L. (Merr.)] roots and bacteria (Bradyrhizobium japonicum ) lead to the development of nodules, important legume root structures where atmospheric nitrogen (N2) is fixed into bio‐available ammonia (NH3) for plant growth and development. With the recent development of the Soybean Nodule Acquisition Pipeline (SNAP), nodules can more easily be quantified and evaluated for genetic diversity and growth patterns across unique soybean root system architectures. We explored six diverse soybean genotypes across three field year combinations in three early vegetative stages of development and report the unique relationships between soybean nodules in the taproot and non‐taproot growth zones of diverse root system architectures of these genotypes. We found unique growth patterns in the nodules of taproots showing genotypic differences in how nodules grew in count, size, and total nodule area per genotype compared to non‐taproot nodules. We propose that nodulation should be defined as a function of both nodule count and individual nodule area resulting in a total nodule area per root or growth regions of the root. We also report on the relationships between the nodules and total nitrogen in the seed at maturity, finding a strong correlation between the taproot nodules and final seed nitrogen at maturity. The applications of these findings could lead to an enhanced understanding of the plant‐Bradyrhizobium relationship and exploring these relationships could lead to leveraging greater nitrogen use efficiency and nodulation carbon to nitrogen production efficiency across the soybean germplasm.
