Elcio Perpetuo Guimaraes

Genetic variability of Brazilian rice landraces determined by SSR markers

The objective of this study was to evaluate the genetic variability of rice (Oryza sativa) landraces collected in Brazilian small farms. Twelve simple sequence repeat (SSR) markers characterized 417 landraces collected in 1986, 1987 and 2003, in the state of Goias, Brazil. The number of landraces with long and thin grain type increased in the evaluated period, probably due to market demand. Based on the molecular data, the genetic variability increased during this period and, as per to the factorial correspondence analysis, most of the accessions were grouped according to the year of collection. The incorporation of modern rice cultivars in landrace cultivation areas and the selection carried out by small farmers are the most probable factors responsible for increasing landrace genetic variability, during the evaluated period. Genotype exchange between farmers, selection practice and local environmental adaptation are able to generate novel adapted allele combinations, which can be used by breeding programs, to reinitiate the process.

Expression of resistance in rice hybrids to Pyricularia grisea

Thirty-nine rice (Oryza sativa) hybrids and their restorers were assessed for vertical resistance to Pyricularia grisea in the rice blast nursery, and in artificial inoculation tests with two pathotypes, under controlled greenhouse conditions. The hybrids were developed from cytoplasmic genetic male sterile lines 046I and IR 58025A, derived from WA cytoplasm. In the rice blast nursery all hybrids showed susceptible reaction varying from 5 to 9. Compatible and incompatible leaf blast reactions of hybrids to two pathotypes, IC-1 and IB-45, were observed in inoculation tests. A majority of the hybrids were resistant when the restorer was resistant. However, seven of the 25 F1 hybrids exhibited susceptible reactions even when one of the parents was resistant to a pathotype. The partial resistance of 11 hybrids and their parents that showed compatible reactions to two pathotypes was analyzed. Differential interaction between isolates and genotypes was observed for partial resistance in relation to both disease severity and lesion number indicating the specific nature of partial resistance.

Resistance to Blast in Hybrid Rice

Rice blast caused by Pyricularia grisea (Cooke) Sacc., is one of the most important diseases both in irrigated and upland rice ecosystems, in Brazil. Low durability of the blast resistance has been a common factor for all cultivars released so far to farmers. Therefore, durable resistance has been the main goal of our national rice breeding program. The National Research Center for Rice and Beans (Embrapa Arroz e Feijao), has been using different approaches to increase the durability of the cultivars for blast resistance. The normal procedures in the breeding projects consist, crosses between disease resistant sources and commercial rice cultivars widely adapted to local conditions, pedigree selection in hot spot sites and construction of a broad based population including well known sources of durable resistance using recurrent selection procedure (Prabhu and Guimaraes, 1990; Filippi and Prabhu, 1997). Recently, we have embarked on hybrid rice breeding for irrigated rice as a supplement to the ongoing breeding projects. High yielding lines improved by conventional breeding for pest resistance, grain quality, among other desirable agronomic traits, are being used as parents of the hybrid breeding project. Incorporation of blast resistance is one of the major objectives, because the hybrids are prone to disease outbreaks due to intensive cultural practices such as high fertilizer rates and the hypervariability of P.grisea.