Histopathology of Sclerotinia sclerotiorum infection and oxalic acid function in susceptible and resistant soybean

Abstract

The success of the necrotrophic fungus Sclerotinia sclerotiorum is largely dependent on its major virulence factor, oxalic acid (OA). Virulence is lost in transgenic plants that express OA degrading enzymes, e.g. oxalate oxidase (OxO). The histopathology of S. sclerotiorum infection and OA accumulation was examined in a transgenic soybean line over-expressing OxO (OxO-OE) and its isogenic parent (WT). In situ flower inoculation showed that the OxO-OE plants were highly resistant to the pathogen while the WT parents were susceptible. This difference in resistance was not apparent in the floral tissues, as aggressive hyphal activity was similar on both hosts, showing that high OxO activity and low OA accumulation in OxO-OE was not a deterrent. However, the process of fungal infection on excised leaf tissue differed on the two hosts. Primary lesions developed and showed similar severe ultrastructural damage on both hosts but rapid lesion expansion (colonization) proceeded only on the WT, concomitant with OA accumulation. Oxalic acid rose in OxO-OE 1 day post-inoculation and did not change over the following 3 days, showing that colonization can be blocked by maintaining low levels of OA. However, OxO degradation of OA did not deter initial host penetration and primary lesion formation. This shows that OA, the major virulence factor of S. sclerotiorum, is critical for host colonization but may not be required during primary lesion formation, suggesting that other factors are contributing to the establishment of the primary lesion.

Changes in the Sclerotinia sclerotiorum transcriptome during infection of Brassica napus

“…Oxalic acid (OA) is crucial for S. sclerotiorum infection and is required for suppression of host defenses [15], regulation of hydrolytic enzyme synthesis and activity [112], and the induction of processes leading to host colonization [113] and tissue necrosis [16, 27]. OA can be synthesized from a variety of precursors, but in fungi the most common mechanism is through the hydrolysis of oxaloacetate to form OA and carbon dioxide.…”

Rhamnolipids From Pseudomonas aeruginosa Are Elicitors Triggering Brassica napus Protection Against Botrytis cinerea Without Physiological Disorders

“…Data were from one representative experiment among three independent repetitions. Aniline blue colorations of mycelium were adapted from Grace and Stribley (1991) and Davidson et al (2016) . Foliar disks were fixed and stored in 1/3 lactoglycerol (10 mL lactic acid/20 mL glycerol/10 mL water)/96% ethanol (v/v).…”

Ssams2, a Gene Encoding GATA Transcription Factor, Is Required for Appressoria Formation and Chromosome Segregation in Sclerotinia sclerotiorum

“…Additionally, OA as a virulence factor has been demonstrated by studies, the full virulence requires a dynamic control of OA accumulation through multiple mechanisms (Liang and Rollins, 2018). OA accumulation lowers the ambient pH and the production of OA is critical for host colonization (Cessna et al, 2000; Rollins and Dickman, 2001; Davidson et al, 2016). Ssrhs1 -silenced strains showed a defect in compound appressoria formation, OA secreted decreased and virulence defect (Yu et al, 2017).…”