R. L. Brogin

Molecular mapping of two loci that confer resistance to Asian rust in soybean.

Asian soybean rust (ASR) is caused by the fungal pathogen Phakopsora pachyrhizi Sydow & Sydow. It was first identified in Brazil in 2001 and quickly infected soybean areas in several countries in South America. Primary efforts to combat this disease must involve the development of resistant cultivars. Four distinct genes that confer resistance against ASR have been reported: Rpp1, Rpp2, Rpp3, and Rpp4. However, no cultivar carrying any of those resistance loci has been released. The main objective of this study was to genetically map Rpp2 and Rpp4 resistance genes. Two F2:3 populations, derived from the crosses between the resistant lines PI 230970 (Rpp2), PI 459025 (Rpp4) and the susceptible cultivar BRS 184, were used in this study. The mapping populations and parental lines were inoculated with a field isolate of P. pachyrhizi and evaluated for lesion type as resistant (RB lesions) or susceptible (TAN lesions). The mapping populations were screened with SSR markers, using the bulk segregant analysis (BSA) to expedite the identification of linked markers. Both resistance genes showed an expected segregation ratio for a dominant trait. This study allowed mapping Rpp2 and Rpp4 loci on the linkage groups J and G, respectively. The associated markers will be of great value on marker assisted selection for this trait.

Soybean physiology and gene expression during drought.

Soybean genotypes MG/BR46 (Conquista) and BR16, drought-tolerant and -sensitive, respectively, were compared in terms of morphophysiological and gene-expression responses to water stress during two stages of development. Gene-expression analysis showed differential responses in Gmdreb1a and Gmpip1b mRNA expression within 30 days of water-deficit initiation in MG/BR46 (Conquista) plants. Within 45 days of initiating stress, Gmp5cs and Gmpip1b had relatively higher expression. Initially, BR16 showed increased expression only for Gmdreb1a, and later (45 days) for Gmp5cs, Gmdefensin and Gmpip1b. Only BR16 presented down-regulated expression of genes, such as Gmp5cs and Gmpip1b, 30 days after the onset of moisture stress, and Gmgols after 45 days of stress. The faster perception of water stress in MG/BR46 (Conquista) and the better maintenance of up-regulated gene expression than in the sensitive BR16 genotype imply mechanisms by which the former is better adapted to tolerate moisture deficiency.

Cloning and quantitative expression analysis of drought-induced genes in soybean.

We determined the expression levels of DREB transcription factor (Gmdreb1) and of the genes Gmgols, Gmpip1b, Gmereb, and Gmdefensin in drought-tolerant (MG/BR46-Conquista) and drought-sensitive (BR16) genotypes of soybean, during drought. The trial was carried out in a controlled-environment chamber, set up to provide drought conditions. Sequences of Arabidopsis thaliana DREB-family proteins were used to build a phylogenetic tree through the alignment of the conserved regions near the AP2 domain. We found that Gmdreb1 is similar to Atrap2.1, which is located near the AtDREB1 and AtDREB2 families. The amplified fragment was cloned and sequenced; alignment with the sequence available at Genbank showed total similarity. Expression analysis showed that under drought: a) Gmdreb1 expression increased in leaves and roots of both genotypes and expression level changes occurred that were correlated with the length of the water-deficit period; b) there were increased expression levels of Gmdefensin in roots of MG/BR46; c) expression of Gmgols increased in leaves and roots of the two genotypes; d) Gmpip1b expression generally increased, except in roots of BR16, and e) the same was found for Gmereb, except in roots of MG/BR46.

Morpho-anatomical and micromorphometrical evaluations in soybean genotypes during water stress.

In a greenhouse experiment, morpho-anatomical and micromorphometrical analyses of two soybean cultivars, MG/BR46 (Conquista) and BR16-tolerant and sensitive to drought, respectively-were used to study their water-deficit-tolerance strategies. Drought treatments were applied at reproductive stages from R2 to R7, where evaluations were conducted at 30 days and 45 days after stress started, respectively. The total length of Conquista plants (shoot + root) was greater than of BR16 plants. Pod dry weight was adversely affected due to the lack of moisture, decreasing productivity even of Conquista plants. Both the cultivars had normal development of root hairs; however, it was observed a decrease in the cortex:central cylinder ratio in BR16 stressed for 30 days, and they also showed similar leaflet thickness and stomata distribution. Differences in drought tolerance observed between the two cultivars seemed to be related to factors other than morphological traits since this species has a short lifecycle.