Antisense Repression of the Medicago truncatula Nodule-Enhanced Sucrose Synthase Leads to a Handicapped Nitrogen Fixation Mirrored by Specific Alterations in the Symbiotic Transcriptome and Metabolome

Abstract

We analyzed the role of the sucrose (Suc) synthase MtSucS1 during nodulation of the model legume Medicago truncatula, integrating data for the developmental, transcriptional, and metabolic processes affected downstream of an impaired Suc cleavage in root nodules. To reduce carbohydrate supply to nodule tissues, transgenic plants expressing a p35S-driven MtSucS1-antisense fusion were constructed. These plants displayed an up to 90% reduction of MtSucS1 proteins in roots and nodules. Phenotypic studies of two independent MtSucS1-reduced lines demonstrated that only under conditions depending on nodulation, these plants appeared to be impaired in above-ground growth. Specifically plant height, shoot weight, leaf development, flowering, as well as seed maturation were reduced, and the efficiency of photosynthesis was affected. Concomitantly, a significantly enhanced root to shoot ratio with a marked increase in root tip numbers was observed. Root nodule formation was found retarded and the impaired nodulation was accompanied by a less efficient nitrogen (N) acquisition. The decreased total N content of MtSucS1-antisense lines and an enhanced carbon to N ratio in roots, nodules, and shoots correlated with the extent of MtSucS1 knockdown. On the level of transcription, effects of an MtSucS1 reduction were evident for genes representing important nodes of the nodule carbon and N metabolism, while metabolite profiling revealed significantly lower levels of amino acids and their derivatives particularly in strongly MtSucS1-reduced nodules. Our results support the model that nodule-enhanced Suc synthase 1 of the model legume M. truncatula is required for the establishment and maintenance of an efficient N-fixing symbiosis.

High specificity in plant leaf metabolic responses to arbuscular mycorrhiza

“…Indeed, plant species have distinct metabolic phenotypes due to qualitative and quantitative differences in metabolites 7,[11][12][13] emerging from different evolutionary selection pressures and ecological niches occupied by the respective species. The high responsiveness of the Fabaceae M. truncatula may be attributed to its extraordinary high affinity to root symbionts, including AMF but also nodule-forming rhizobia 25 (not detected under our experimental conditions), somehow priming the plants to strongly respond to these endosymbionts. In contrast to the high proportion of species-specific changes, the decrease in organic acids of the citric acid cycle in AM plants was a common response in all dicots of our study.…”

“…Veronica chamaedrys (Vc, Plantaginaceae) was included to allow betweengenus comparisons within the same plant family. Medicago truncatula (Mt, Fabaceae), for which the association with root symbionts is well characterized 8,25 , represented another dicot family, whereas Poa annua (Pa, Poaceae) was included as representative of the monocots. The chosen species are herbaceous with comparable statures and share largely similar habitats.…”

The Root Hair “Infectome” of Medicago truncatula Uncovers Changes in Cell Cycle Genes and Reveals a Requirement for Auxin Signaling in Rhizobial Infection

“…Several other genes that had not been previously linked to infection were identified. The sucrose synthase gene SUCS1 (Mtr.22018.1.S1_s_at: Medtr4g124660), which is expressed in nodules and required for nitrogen fixation (Baier et al, 2007) and is required for colonization by arbuscular mycorrhizal fungi (Hohnjec et al, 1999;Baier et al, 2010), had increased expression in skl after S. meliloti inoculation. SWEET13, which encodes a sugar transporter (Chen et al, 2012), and a gene involved in actin nucleation ABIL1 (Mtr.20281.1.S1_at: Medtr7g116710), which had very low expression in root hairs in the absence of Nod factorproducing rhizobia, were also induced by S. meliloti.…”

An RNA Sequencing Transcriptome Analysis Reveals Novel Insights into Molecular Aspects of the Nitrate Impact on the Nodule Activity of Medicago truncatula

“…Gordon et al (1999) have shown that under several stress conditions, there was a coincidental decline in both Suc synthase and nitrogenase activity. An antisense down-regulation of the nodule-enhanced Suc synthase in M. truncatula resulted in impaired plant growth, especially when the plants were totally dependent on nitrogen fixation (Baier et al, 2007). However, a detailed study on the relationship of soybean Suc synthase activity and nitrogen fixation after nitrate application revealed that the decreased Suc synthase activity was the result of, rather than the reason for, the decreased nitrogen fixation (Gordon et al, 2002).…”