Genetic resistance is an important component of integrated strategies used to control problematic diseases in common bean (Phaseolus vulgaris L.). Molecular linkage maps have been used to identify, tag, and map disease resistance genes and QTL in common bean, leading to improved breeding strategies and implementation of marker‐assisted selection. Most widely used marker types, random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphisms (AFLP), for linkage mapping in bean are located randomly throughout the genome and associate with particular traits by chance. We sought to determine the potential application of a new marker system, TRAP, which uses expressed sequence information and a bioinformatics approach to generate polymorphic markers around targeted candidate gene sequences. TRAP markers were amplified by fixed primers designed against sequenced expressed sequence tag (EST) associated with disease resistance in the Compositae Genomics database or against sequenced resistance gene analog (RGA) from common bean. Seventeen of 85 TRAP markers located in the BAT 93/Jalo EEP558 core mapping population mapped in the vicinity of R genes. Six of 21 TRAP markers generated in the Dorado/XAN 176 mapping population were linked with newly identified QTL, two conditioning resistance to ashy stem blight (14% and 16% of the phenotypic variation explained, R2), and one each conferring resistance to Bean golden yellow mosaic virus (BGYMV) (15%) and common bacterial blight (30%). The TRAP marker system has potential for mapping regions of the common bean genome linked with disease resistance.
Breeding for genetic resistance to white mold [Sclerotinia sclerotiorum (Lib.) de Bary] in dry bean (Phaseolus vulgaris L.) is difficult because of low heritability. To facilitate breeding, researchers have sought to identify QTL underpinning genetic resistance to white mold. We identified two QTL conditioning ICA Bunsi‐derived resistance to white mold in a pinto × navy bean (Aztec/ND88–106–04) recombinant inbred line (85 RILs) population. ND88–106–04 is a navy breeding line with resistance to white mold derived from ICA Bunsi navy. Aztec pinto is susceptible. The QTL were located to linkage groups B2 and B3 of the core map. The B2 QTL expressed in three of four field environments explaining 24.7, 9.0, and 8.7% of the phenotypic variation for disease severity score. The B3 QTL expressed in two of four environments, explaining 15.7 and 5.3% of the phenotypic variation. The B2 QTL was identified previously in ICA Bunsi × navy and ICA Bunsi × black bean RIL populations. The resistance conferred by the B2 QTL has a physiological basis due to association with stay green stem trait and lack of association with disease avoidance traits. The B3 QTL, undetected in previous studies, was associated with disease avoidance traits (canopy porosity, plant height), stay green stem trait, and maturity. The B2 QTL with stable expression in multiple environments and across genetic backgrounds will be most amenable to manipulation by breeders.
Gibberellins (GA) Al, A19, and A20 were identified in shoot cylinders containing the apical meristems from sorghum (Sorghum bicolor L.). Extracts were purified by sequential SiO2 partition chromatography and reversed-phase C18 high performance liquid chromatography and biologically active (dwarf rice cv Tan-ginbozu microdrop assay) fractions were subjected to gas chromatography-selected ion monitoring. Based on the use of PHIGA and 12Hj(d2)GA internal standards, amounts of GA,, GA1,, and GA2n were estimated to be 0.7, 8.8, and 1.5 namograms per gram dry weight of tissue, respectively.Gibberellins characteristic of the early 13-OH biosynthetic pathway have been previously identified from a number of C4 and tropical grasses. GA192 and GA20 have been identified from bamboo (Phyllostachys edulis) (13); GA, and GA19 from rice (Oryza sativa) (9); GA1, GA19, and GA29 from sugarcane (Saccharum spp.) (8); and eight GA from the early 13-OH pathway have been identified from maize (Zea mays) (3, 4). Further, GA1 has also been identified from a number of other cereal grasses (6). The physiological similarities between maize and sorghum (Sorghum bicolor) and the evolutionary relationship between sorghum and other C4 and tropical grasses suggests that sorghum might also contain GA characteristic of the early 13-OH metabolic pathway. The present study was initiated to identify the endogenous GA of sorghum, a commercially important C4 cereal for which GA-like substances have been previously reported (1) but not characterized. (12) were added to the extract (i.e. 0.5 ng GA, and 13.3 ng GA20). MATERIALS AND METHODSThe methanol was removed in vacuo at 35°C after the addition of0.5 M phosphate buffer (pH 8.0). The buffered aqueous extract was slurried with poly-N-PVP and filtered. The pH was raised to 9.0 with NaOH and Chl was removed by two extractions with diethyl ether. The pH was then reduced to 3.0 with HC1 and the sample extracted 3 times with equal volumes of H20-saturated ethyl acetate. The ethyl acetate was frozen at -40C the ice removed by filtering, and the ethyl acetate was subsequently removed in vacuo at 35TC. The acidic, ethyl acetate-soluble extract was purified on columns of charcoal:celite (1:1) eluted with acetone:water (80:20). This was followed by stepwise-elution Sio2 partition chromatography (2, 16), and detection of GA-like activity using the dwarf rice cv Tan-ginbozu microdrop assay (11) modified by using 0.5 gl application droplets and 48 h of incubation. Biologically active Sio2 fractions were then chromatographed on reversed-phase C18 HPLC (7,15). Flow and solvent parameters were as previously described (7,15), although the gradient from 0 to 70% MeOH was run over 60 rather than 30 min. Eighty 1-min fractions were collected and subsequently bioassayed at 3 dilutions (1/200, 1/400, 1/800 aliquots). Bioassay results after SiO2 partition chromatography (see Fig. 1) are expressed as moving three-point averages to reduce experimental "noise," although the chromatographic peaks are broadened in so doin...
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