Geneticists and breeders are positioned to breed plants with root traits that improve productivity under drought. However, a better understanding of root functional traits and how traits are related to whole plant strategies to increase crop productivity under different drought conditions is needed. Root traits associated with maintaining plant productivity under drought include small fine root diameters, long specific root length, and considerable root length density, especially at depths in soil with available water. In environments with late season water deficits, small xylem diameters in targeted seminal roots save soil water deep in the soil profile for use during crop maturation and result in improved yields. Capacity for deep root growth and large xylem diameters in deep roots may also improve root acquisition of water when ample water at depth is available. Xylem pit anatomy that makes xylem less “leaky” and prone to cavitation warrants further exploration holding promise that such traits may improve plant productivity in water-limited environments without negatively impacting yield under adequate water conditions. Rapid resumption of root growth following soil rewetting may improve plant productivity under episodic drought. Genetic control of many of these traits through breeding appears feasible. Several recent reviews have covered methods for screening root traits but an appreciation for the complexity of root systems (e.g., functional differences between fine and coarse roots) needs to be paired with these methods to successfully identify relevant traits for crop improvement. Screening of root traits at early stages in plant development can proxy traits at mature stages but verification is needed on a case by case basis that traits are linked to increased crop productivity under drought. Examples in lesquerella (Physaria) and rice (Oryza) show approaches to phenotyping of root traits and current understanding of root trait genetics for breeding.

A stable QTL that may be used in marker-assisted selection in wheat breeding programs was detected for yield, yield components and drought tolerance-related traits in spring wheat association mapping panel. Genome-wide association mapping has become a widespread method of quantitative trait locus (QTL) identification for many crop plants including wheat (Triticum aestivum L.). Its benefit over traditional bi-parental mapping approaches depends on the extent of linkage disequilibrium in the mapping population. The objectives of this study were to determine linkage disequilibrium decay rate and population structure in a spring wheat association mapping panel (n = 285-294) and to identify markers associated with yield and yield components, morphological, phenological, and drought tolerance-related traits. The study was conducted under fully irrigated and rain-fed conditions at Greeley, CO, USA and Melkassa, Ethiopia in 2010 and 2011 (five total environments). Genotypic data were generated using diversity array technology markers. Linkage disequilibrium decay rate extended over a longer genetic distance for the D genome (6.8 cM) than for the A and B genomes (1.7 and 2.0 cM, respectively). Seven subpopulations were identified with population structure analysis. A stable QTL was detected for grain yield on chromosome 2DS both under irrigated and rain-fed conditions. A multi-trait region significant for yield and yield components was found on chromosome 5B. Grain yield QTL on chromosome 1BS co-localized with harvest index QTL. Vegetation indices shared QTL with harvest index on chromosome 1AL and 5A. After validation in relevant genetic backgrounds and environments, QTL detected in this study for yield, yield components and drought tolerance-related traits may be used in marker-assisted selection in wheat breeding programs.

Interpretation of quantitative trait locus (QTL) studies of agronomic traits is limited by lack of knowledge of biochemical pathways leading to trait expression. To more fully elucidate the biological significance of detected QTL, we chose a trait that is the product of a well-characterized pathway, namely the concentration of maysin, a C-glycosyl flavone, in silks of maize, Zea mays L. Maysin is a host-plant resistance factor against the corn earworm, Helicoverpa zea (Boddie). We determined silk maysin concentrations and restriction fragment length polymorphism genotypes at flavonoid pathway loci or linked markers for 285 F2 plants derived from the cross of lines GT114 and GT119.Single-factor analysis of variance indicated that the p1 region on chromosome 1 accounted for 58.0%o of the phenotypic variance and showed additive gene action. The pi locus is a transcription activator for portions of the flavonoid pathway. A second QTL, represented by marker umclO5a near the brown pericarpi locus on chromosome 9, accounted for 10.8% of the variance. Gene action of this region was dominant for low maysin, but was only expressed in the presence of a functional pi allele. The model explaining the greatest proportion of phenotypic variance (75.9%) included pi, umclOSa, umcl66b (chromosome 1), ri (chromosome 10), and two epistatic interaction terms, pi x umclO5a and pi x ri. Our results provide evidence that regulatory loci have a central role and that there is a complex interplay among different branches of the flavonoid pathway in the expression of this trait.Development of molecular-marker linkage maps in many species facilitates the identification of chromosome regions associated with variation in quantitative traits (1). By dissecting the continuous phenotypic variation typical of many traits into contributions from discrete genetic factors, quantitative trait locus (QTL) studies provide insights into trait inheritance and genome organization, and often are sufficient to initiate marker-assisted selection. However, the biological interpretation of QTL data is generally limited by lack of knowledge of the genetics, biochemistry, and physiology underlying trait expression. To advance the level of QTL interpretation, we analyzed variation in an economically important trait that is determined by a well-characterized genetic and biochemical pathway.The corn earworm (CEW) is a major silk-and kernelfeeding insect pest of maize in the United States and parts of Latin America (2, 3). Host-plant resistance to CEW results from both antibiosis due to chemical factors in silks (stylar/ stigmatic tissue), and morphological features such as tight covering of the ear by husk leaves (4). Understanding of the nature of antibiosis to CEW was advanced when maysin (Fig. 1), a C-glycosyl flavone that inhibits CEW larval growth, was isolated from silks of the Mexican maize landrace "Zapalote Chico" (5). Later, Wiseman et al. (6) found a highly significant relationship between increased silk maysin concentration and reduced earworm ...

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