Diversidade e ecofisiologia de leveduras em plantio orgânico de cana-de-açúcar

Abstract

Soil is traditionally known as a repository for yeasts from diverse habitats and not an environment where they can establish autochthonous populations. Because of the scant data available, this widely distributed group of fungi is not frequently included in general discussions about soil microbiology. Recently, some authors have shown a potential use of yeasts in agriculture for mycorrhyzal and rhizobia/leguminous symbioses and pathogen antagonism. The growth of filamentous fungi on isolation media has contributed to the low number of studies on yeasts in soil communities. Therefore, a key factor for increasing and improving soil yeast ecology studies was development of a protocol that could inhibit mycelial growth, but permit good recovery of yeasts. A protocol was developed for isolating yeasts from samples with many mold propagules in order to survey populations on rhizoplane, phyloplane, and in bulk soil of an organically cultivated sugar cane field. It consisted of a plating methods to access fast growing prevalent populations, and enrichment media for smaller populations or those inhibited on plates. This was followed by a screening step with differential media, to diminish the redundant isolates. Relatively large composite samples were used to include populations distributed in different microhabitats. Samples were taken during both the dry and wet seasons. Different yeast community structures were found for bulk soil, root and leaf surface. The yeasts prevalent in soil were Torulaspora globosa, Pichia caribbica and Cryptococcus podzolicus in both seasons. Aureobasideum pullulans and basideomicetes yeasts of Cryptococcus spp., Rhodotorula marina and Pseudozyma spp. with P. aff. pruni and P. jejuensis, were prevalent on leaves. Whereas on roots community dominance was shared by ascomicetic yeasts, specially comprised by P. caribbica and Candida aff. azyma, and by basidiomicetes species Cryptococcus laurentii and C. podzolicus. When plant roots were metabolically active the ascomicetous yeast guild was increased by several fermentative species, indicating that there was a rhizosphere effect on them. Cultivation by various methods allowed the isolation over 700 cultures including 24 ascomycetes and 45 basidiomycetes species of yeasts and yeast like organisms from sugar cane and associated soil. These isolates were identified by conventional phenotypic characterization and rDNA partial sequencing methods. More than 40 % of the species had D1/D2 rDNA sequences with less than 99 % of similarity to the type cultures of known species suggesting that they are new species. Of special note were probable new species similar to Candida tsuchiyae and C. azyma among the ascomycetes and various Cryptococcus, Rhodotorula and Pseudozyma species among the basidiomycetes. A probable new species similar to the dimorphic fungus Lecythophora decumbens was also discovered. Our data showed soil, especially when associated with roots of actively growing plants, to be a rich source of yeast biodiversity. This diversity can be studied by application of appropriate cultivation methods and could contribute as a source for novel cultures for application in biotechnology.