Abstract
Oxalic acid is an important virulence factor for disease caused by the fungal necrotrophic pathogen Sclerotinia sclerotiorum, yet calcium oxalate (CaOx) crystals have not been widely reported. B. carinata stems were infected with S. sclerotiorum and observed using light microscopy. Six hours post inoculation (hpi), CaOx crystals were evident on 46% of stem sections and by 72 hpi on 100%, demonstrating that the secretion of oxalic acid by S. sclerotiorum commences before hyphal penetration. This is the first time CaOx crystals have been reported on B. carinata infected with S. sclerotiorum. The shape of crystals varied as infection progressed. Long tetragonal rods were dominant 12 hpi (68% of crystal-containing samples), but by 72 hpi, 50% of stems displayed bipyramidal crystals, and only 23% had long rods. Scanning electron microscopy from 24 hpi revealed CaOx crystals in all samples, ranging from tiny irregular crystals (< 0.5 μm) to large (up to 40 μm) highly organized arrangements. Crystal morphology encompassed various forms, including tetragonal prisms, oval plates, crystal sand, and druses. Large conglomerates of CaOx crystals were observed in the hyphal mass 72 hpi and these are proposed as a strategy of the fungus to hold and detoxify Ca2+ions. The range of crystal morphologies suggests that S. sclerotiorum growth and infection controls the form taken by CaOx crystals.
Highlights
Sclerotinia sclerotiorum has been widely studied due to its ability to cause serious disease on many important food crops [1]
S. sclerotiorum successfully infected stems of B. carinata inoculated with filter paper discs impregnated with hyphal fragments of S. sclerotiorum
This study has clearly shown that calcium oxalate (CaOx) crystals are present on the surface of B. carinata stems infected by S. sclerotiorum as early as 6 hpi
Summary
Sclerotinia sclerotiorum has been widely studied due to its ability to cause serious disease on many important food crops [1]. The involvement of oxalic acid in pathogenesis by S. sclerotiorum was noted by de Bary in 1886 [2] and since many studies have definitively identified it as an important virulence factor for the fungus [3,4,5,6,7,8]. One of the most convincing demonstrations of its role is the fact that isolates of S. sclerotiorum which have lost the ability to produce oxalic acid have lost their ability to infect [4]. Oxalate Crystals in Sclerotinia/Brassica Pathosystem acknowledges the financial assistance of an Australian Postgraduate Award. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript
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