The root-knot nematode (Meloidogyne incognita) is one of most devastating pathogens that attack the common bean crop. Although there is evidence that some cultivars have race-specific resistance against M. incognita, these resistance sources have not proved effective, and nematodes are able to circumvent the host's defense system. We constructed RNA-seq based libraries and used a high-throughput sequencing platform to analyze the plant responses to M. incognita. Assessments were performed at 4 and 10 days after inoculation corresponding to the stages of nematode penetration and giant cell development, respectively. Large-scale transcript mapping to the common bean reference genome (G19833) resulted in the identification of 27,195 unigenes. Of these, 797 host genes were found to be differentially expressed. The functional annotation results confirm the complex interplay between abiotic and biotic stress signaling pathways. High expression levels of the wounding-responsive genes were observed over the interaction. At early response, an overexpression of the N gene, a TIR-NBS-LRR resistance gene, was understood as a host attempt to overcome the pathogen attack. However, the repression of heat shock proteins resulted in a lack of reactive oxygen species accumulation and absence of a hypersensitive response. Furthermore, the host basal response was broken by the repression of the ethylene/jasmonate pathway later in the response, resulting in a continuous compatible process with consequent plant susceptibility.

Autor para correspondência: Mário M. InomotoA introdução e adaptação do sistema plantio direto (SPD) às condições do cerrado brasileiro, nas décadas de 1980 e 1990, foi das mais relevantes transformações tecnológicas da nossa agricultura. Graças ao SPD, questões ligadas à atividade agrícola que até então afetavam negativamente sua sustentabilidade econômica, tais como perda de fertilidade natural do solo, assoreamento de rios e deficiência hídrica durante o veranico, foram em grande medida contornadas ou mesmo resolvidas. Entretanto, a rotação de culturas nas áreas produtoras sob o SPD criou uma questão técnica a mais: as conseqüências de várias culturas utilizadas para obtenção de palhada, as chamadas culturas de cobertura, serem hospedeiras de nematóides daninhos à cultura principal.Dentro desse contexto, as poáceas forrageiras Panicum maximum Jacq. (capim-colonião) e Brachiaria spp. (braquiárias) são novas e promissoras fontes de palhada para o SPD, devido à produção de grande quantidade de matéria seca (Kluthcouski et al., 2003) e à capacidade de suprimir os nematóides das galhas (Brito & Ferraz, 1987; Dias-Arieira et al., 2003) e o nematóide reniforme (Asmus & Cargnin, 2005). No entanto, pouco se conhece sobre a reação dessas plantas ao nematóide Pratylenchus brachyurus (Godfrey) INOMOTO Palavras-chave adicionais: cobertura vegetal, nematóide das lesões, plantio direto. ABSTRACT Host status of Brachiaria spp. and Panicum maximum to Pratylenchus brachyurusNo-tillage system has been used in many agricultural areas of Brazil, creating conditions that may increase the damage caused by phytonematodes in the main crops, depending on the host status of the plants used as cover crops. Guinea grass (Panicum maximum) and brachiaria grasses (Brachiaria spp.) are useful in areas infested by root-knot nematodes, because they are non-hosts or poor hosts of Meloidogyne incognita and M. javanica, but host status of these grasses for Pratylenchus brachyurus has scarcely been studied. Therefore, two greenhouse experiments were carried out in order to characterize the host status of five species and one hybrid of brachiaria grasses [Brachiaria decumbens, B. brizantha, B. humidicola, B. dyctioneura, B. ruziziensis, and Mulato grass (B. ruziziensis x B. brizantha)], and two cultivars of P. maximum ('Mombaça' and 'Tanzânia') for P. brachyurus. The experiments were similar to each other, except in the origin of the nematode population, Pb 20 coming from okra roots and Pb 24 from cotton roots. Final nematode populations from substrate and roots were estimated 118 days after inoculation for Pb 20 and 131 days for Pb 24 . Results showed that all the tested grasses host P. brachyurus, but at different degrees. Guinea grass and Mulato grass appeared to be good hosts of P. brachyurus, with a reproduction factor (RF = Pf/Pi) ranging from 4.96 to 12.17 for Pb 20 and from 10.38 to 13.18 for Pb 24 : thus they should be avoided as cover crops in fields infested with the nematode. Conversely, B. dyctioneura proved to be a poor host of P...

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