The schizotrophic lifestyle of Sclerotinia sclerotiorum

Abstract

Sclerotinia sclerotiorum is a cosmopolitan and typical necrotrophic phytopathogenic fungus that infects hundreds of plant species. Because no cultivars highly resistant to S. sclerotiorum are available, managing Sclerotinia disease caused by S. sclerotiorum is still challenging. However, recent studies have demonstrated that S. sclerotiorum has a beneficial effect and can live mutualistically as an endophyte in graminaceous plants, protecting the plants against major fungal diseases. An in-depth understanding of the schizotrophic lifestyle of S. sclerotiorum during interactions with plants under different environmental conditions will provide new strategies for controlling fungal disease. In this review, we summarize the pathogenesis mechanisms of S. sclerotiorum during its attack of host plants as a destructive pathogen and discuss its lifestyle as a beneficial endophytic fungus.

Keywords: Sclerotinia sclerotiorum; Sclerotinia disease; endophytic fungus; lifecycle; schizotrophism.

FIGURE 1

Morphology of Scelortinia sclerotiorum and its pathogenicity in rapeseed. (a) The hyphal tips of S. sclerotiorum (left) and mycelia stained with DAPI (right, arrows point to septa). (b) The phenotype of sclerotia on potato dextrose agar (left) and the internal structure of sclerotia (right). (c) The apothecia produced on germinated sclerotia (left) and ascospores in asci (right, arrow points to ascospores). (d) The infection cushions of S. sclerotiorum that formed on Arabidopsis thaliana leaves as observed by scanning electron microscopy at 4 h post‐inoculation (hpi) (left, arrows point to the surface and the bottom of infection cushions), and numerous infectious hyphae formed on leaves centred around the infection cushion at 16 hpi (right, IC indicates the infection cushion). (e) Disease symptoms caused by S. sclerotiorum on rapeseed plants. Petals were dropped onto the leaves as inocula, which caused necrosis on the leaves. Sclerotia appeared on the infected pod. The white colour of the stem indicates rapeseed stem rot (arrows point to the infected stem) and numerous sclerotia in the stem.

FIGURE 2

Schizotrophic lifestyle of Sclerotinia sclerotiorum. The formation of sclerotia allows S. sclerotiorum to survive in soil. Under the proper conditions, sclerotia may germinate mycelogenically to produce hyphae or germinate carpogenically to apothecia. Ascospores are released from apothecia and long‐distance spread as the primary source of inoculum to infect petals or senescent host plant tissues. A few S. sclerotiorum individuals may produce microconidia, but the function of these microconidia is unknown. Mycelia can form infection cushions, leading to infection of the host plant. Sometimes, mycelia can be infected by mycoviruses. The DNA mycovirus SsHADV‐1 can cause S. sclerotiorum to switch from being a pathogen to being an endophyte. However, whether other mycoviruses have a similar ability has not yet been determined. The mycelia of S. sclerotiorum can also undergo endophytic growth in nonhost plants, such as wheat, rice and maize. Upon maturation of the apothecium, numerous ascospores are released from the ascus and can disseminate via the air, ultimately leading to infection of the host plants. Ascospores may also act as inocula of S. sclerotiorum when growing endophytically in nonhost plants (such as rice, wheat and maize), but additional experiments are needed to verify this. After crop harvest, the pathogen S. sclerotiorum can survive in soil or plant residue to prepare for the next life cycle. However, as an endophyte, it may survive in plant residues. The dashed lines indicate the possible predicted survival routes of S. sclerotiorum. The red arrows represent the lifestyle of S. sclerotiorum as an endophyte, and the blue arrows represent the lifestyle of S. sclerotiorum as a destructive fungal pathogen.