Rhizobia reside as symbiosomes in the infected cells of legume nodules to fix atmospheric nitrogen. The symbiotic relation is strictly controlled, lasts for some time, but eventually leads to nodule senescence. We present a comprehensive transcriptomics study to understand the onset of nodule senescence in the legume Medicago truncatula. Distinct developmental stages with characteristic gene expression were delineated during which the two symbiotic partners were degraded consecutively, marking the switch in nodule tissue status from carbon sink to general nutrient source. Cluster analysis discriminated an early expression group that harbored regulatory genes that might be primary tools to interfere with pod filling-related or stress-induced nodule senescence, ultimately causing prolonged nitrogen fixation. Interestingly, the transcriptomes of nodule and leaf senescence had a high degree of overlap, arguing for the recruitment of similar pathways.

Mature indeterminate Medicago truncatula nodules are zonated with an apical meristem, an infection zone, a fixation zone with nitrogen-fixing bacteroids, and a “developmental” senescence zone that follows nodule growth with a conical front originating in the center of the fixation zone. In nitrogen-fixing cells, senescence is initiated coincidently with the expression of a family of conserved cysteine proteases that might be involved in the degradation of symbiotic structures. Environmental stress, such as prolonged dark treatment, interferes with nodule functioning and triggers a fast and global nodule senescence. Developmental and dark stress-induced senescence have several different structural and expression features, suggesting at least partly divergent underlying molecular mechanisms.

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CASE DESCRIPTION 4 alpacas and 2 llamas (11 months to 11 years old) from 2 properties were examined for lethargy (6/6), salivation and regurgitation (4/6), and recumbency (3/6). Signs developed approximately 48 to 72 hours after accidental access to black oil sunflower seeds. CLINICAL FINDINGS 3 alpacas died suddenly prior to treatment and were necropsied. One llama survived, and 1 alpaca and 1 llama died after days of medical treatment. All 3 treated animals had systemic inflammatory signs including tachycardia, fever, and hematologic changes. Biochemical anomalies included azotemia, hyperglycemia, hyponatremia, hypochloremia, and hypoalbuminemia. Necropsy identified numerous sunflower seeds in the gastrointestinal tract of all 5 animals that died, with pulmonary congestion (5/5 animals), mild centrilobular vacuolar hepatic degeneration (4/5), and erosions of the esophagus (3/5) and first (3/5) and third (1/5) compartments of the forestomach. Renal tubular necrosis was found in the 2 animals that died on day 4 of treatment. TREATMENT AND OUTCOME One llama responded successfully to intensive medical management including supplemented IV fluid therapy, oral and partial parenteral nutrition, and administration of antimicrobials, furosemide, and insulin and was clinically normal with plasma biochemical analysis values within reference range 12 weeks later. Vitamin D, oxalates, heavy metals, and mycotoxins were excluded as the cause of clinical signs on the basis of screening of uneaten seeds and tissue samples and gastric content from the treated llama that died. CLINICAL RELEVANCE Inadvertent large volume black oil sunflower seed ingestion resulted in a high mortality rate in camelids. A specific toxic principle was not identified. Feeding this product to camelids is not recommended to avoid the risk of accidental overingestion and subsequent disease. (J Am Vet Med Assoc 2021;259:406–414)

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