Estudos relacionados à reprodução sexuada de Pyricularia oryzae

Abstract

Blast disease is caused by the Ascomycetous fungus, Pyricularia oryzae, which is the most important rice disease worldwide, also causing wheat blast in Brazil, Paraguay and Bolivia. High genetic-pathotypic variability is probably responsible for the low resistance durability to blast in cultures such as rice and wheat. Sexual reproduction is the most probable source of variability in population of the pathogens in Brazilian agro-ecosystems. Studies on the role of sexual reproduction over the genetic structure of P. oryzae populations, as well as on the reproductive biology of the pathogen involving morphogenesis and regulating factors are scarce. This work aimed conducting studies on induction, morphogenesis of the reproductive structures and gene expression during the sexual reproduction of P. oryzae. In a first assay, compatible P. oryzae isolates pathogenic to rice, were paired on Oatmeal agar and maintained for seven days at 25oC and continuous white light, followed by the transference to incubation under 18ºC with continuous white and dark light, for 21 days. Samples for qRTPCR and microscopy analyses were collected weekly. We identified three phases in the development of the ascocarp: initial hyphae folding phase, at the 14th day; proto-perithecia, at the 21 st day; perithecia maturation, at the 28th day. The expression in all MAT genes evaluated was higher when the compatible partner was present, and it showed the same pattern over time, with higher expression at the 14th day, during the mycelial folding to generate the protoperithecia, followed by the a reduction until the 28th day. In the second study, the objective was to evaluate the size and density of the P. oryzae wheat-infective perithecia in different Poaceae species, on different plant residue, and in culture medium, using light and scanning electron microscopy. We observed that the P. oryzae perithecia varied in size and density, according to the host, and varied in size according to the state of the plant and culture medium. The recognition of compatible individuals may occur externally or internally to plant tissues. In addition, we verified higher ascocarp development on tissues in necrosis, possibly indicating that the sexual cycle of the P. oryzae occurs in nature, at least preferentially, in plant residue. These results could orient works on morphogenesis and gene expression during the sexual reproduction of P. oryzae, in culture media and in plants, contributing for the description of the reproduction cycle of the pathogen, and possibly for the development of new blast disease management measures based on durable resistance.