Segregation of Hydroxycinnamic Acid Esters Mediating Sweetpotato Weevil Resistance in Storage Roots of Sweetpotato.

Resistance to sweetpotato weevils (Cylas spp.) has been identified in several sweetpotato (Ipomoea batatas) landraces from East Africa and shown to be conferred by hydroxycinnamic acids that occur on the surface of storage roots. The segregation of resistance in this crop is unknown and could be monitored using these chemical traits as markers for resistance in F1 offspring from breeding programs. For the first time in a segregating population, we quantified the plant chemicals that confer resistance and evaluated levels of insect colonization of the same progeny in field and laboratory studies. We used a bi-parental mapping population of 287 progenies from a cross between I. batatas 'New Kawogo,' a weevil resistant Ugandan landrace and I. batatas 'Beauregard' a North American orange-fleshed and weevil susceptible cultivar. The progenies were evaluated for resistance to sweetpotato weevil, Cylas puncticollis at three field locations that varied climatically and across two seasons to determine how environment and location influenced resistance. To augment our field open-choice resistance screening, each clone was also evaluated in a no choice experiment with weevils reared in the laboratory. Chemical analysis was used to determine whether differences in resistance to weevils were associated with plant compounds previously identified as conferring resistance. We established linkage between field and laboratory resistance to Cylas spp. and sweetpotato root chemistry. The data also showed that resistance in sweetpotato was mediated by root chemicals in most but not all cases. Multi-location trials especially from Serere data provided evidence that the hydroxycinnamic acid esters are produced constitutively within the plants in different clonal genotypes and that the ecological interaction of these chemicals in sweetpotato with weevils confers resistance. Our data suggest that these chemical traits are controlled quantitatively and that ultimately a knowledge of the genetics of resistance will facilitate management of these traits, enhance our understanding of the mechanistic basis of resistance and speed the development of new sweetpotato varieties with resistance to sweetpotato weevil.

Distributions, ex situ conservation priorities, and genetic resource potential of crop wild relatives of sweetpotato [Ipomoea batatas (L.) Lam., I. series Batatas].

Crop wild relatives of sweetpotato [Ipomoea batatas (L.) Lam., I. series Batatas] have the potential to contribute to breeding objectives for this important root crop. Uncertainty in regard to species boundaries and their phylogenetic relationships, the limited availability of germplasm with which to perform crosses, and the difficulty of introgression of genes from wild species has constrained their utilization. Here, we compile geographic occurrence data on relevant sweetpotato wild relatives and produce potential distribution models for the species. We then assess the comprehensiveness of ex situ germplasm collections, contextualize these results with research and breeding priorities, and use ecogeographic information to identify species with the potential to contribute desirable agronomic traits. The fourteen species that are considered the closest wild relatives of sweetpotato generally occur from the central United States to Argentina, with richness concentrated in Mesoamerica and in the extreme Southeastern United States. Currently designated species differ among themselves and in comparison to the crop in their adaptations to temperature, precipitation, and edaphic characteristics and most species also show considerable intraspecific variation. With 79% of species identified as high priority for further collecting, we find that these crop genetic resources are highly under-represented in ex situ conservation systems and thus their availability to breeders and researchers is inadequate. We prioritize taxa and specific geographic locations for further collecting in order to improve the completeness of germplasm collections. In concert with enhanced conservation of sweetpotato wild relatives, further taxonomic research, characterization and evaluation of germplasm, and improving the techniques to overcome barriers to introgression with wild species are needed in order to mobilize these genetic resources for crop breeding.

Methodology for Inoculating Sweetpotato Virus Disease: Discovery of Tip Dieback, and Plant Recovery and Reversion in Different Clones.

Evaluating sweetpotato (Ipomoea batatas) genotypes for resistance to sweetpotato virus disease (SPVD) has been slow and inefficient. Ipomoea setosa plants, normally used as the source of scions for graft-infecting sweetpotatoes with viral diseases, are often severely stunted and their mortality is 10 to 30% when infected with SPVD, making them unsuitable as scions. Tanzania, a landrace of I. batatas widely grown in East Africa, was found to be a superior host for maintaining and increasing SPVD inoculum (scions) for mass grafting. Modifications to a cleft-grafting technique also increased survival of grafted SPVD-affected scions from 5 to 100%. These modifications, coupled with an efficient SPVD scoring technique, allowed rapid screening of large sweetpotato populations for SPVD resistance. Plant recovery from SPVD is reported here as a component of SPVD resistance. Differences in recovery from SPVD were detected among progenies, indicating its genetic basis. Plant tip dieback, a hypersensitivity response, was observed only in families with cv. Wagabolige as a parent. These findings may open up new opportunities for improved understanding and control of this devastating disease.

QTL mapping of internal heat necrosis in tetraploid potato.

Internal heat necrosis (IHN) is a physiological disorder of potato tubers. We developed a linkage map of tetraploid potato using AFLP and SSR markers, and mapped QTL for mean severity and percent incidence of IHN. Phenotypic data indicated that the distribution of IHN is skewed toward resistance. Late foliage maturity was slightly but significantly correlated with increased IHN symptoms. The linkage map for 'Atlantic', the IHN-susceptible parent, covered 1034.4 cM and included 13 linkage groups, and the map for B1829-5, the IHN-resistant parent, covered 940.2 cM and contained 14 linkage groups. QTL for increased resistance to IHN were located on chromosomes IV, V, and groups VII and X of 'Atlantic', and on group VII of B1829-5 in at least 2 of 3 years. The QTL explained between 4.5 and 29.4% of the variation for mean severity, and from 3.7 to 14.5% of the variation for percent incidence. Most QTL detected were dominant, and associated with decreased IHN symptoms. One SSR and 13 AFLP markers that were linked to IHN were tested in a second population. One AFLP marker was associated with decreased symptoms in both populations. The SSR marker was not associated with IHN in the second population, but was closely linked in repulsion to another marker that was associated with IHN, and had the same (negative) effect on the trait as the SSR marker did in the first population. The correlation between maturity and IHN may be partially explained by the presence of markers on chromosome V that are linked to both traits. This research represents the first molecular genetic research of IHN in potato.

Applications of tagging and mapping insect resistance loci in plants.

This review examines how molecular markers can be used to increase our understanding of the mechanisms of plant resistance to insects and develop insect resistant crops. We provide a brief description of the types of molecular markers currently being employed, and describe how they can be applied to identify and track genes of interest in a marker-assisted breeding program. A summary of the work reported in this field of study, with examples in which molecular markers have been applied to increase understanding of the mechanistic and biochemical bases of resistance in potato and maize plant/pest systems, is provided. We also describe how molecular markers can be applied to develop more durable insect-resistant crops. Finally, we identify key areas in molecular genetics that we believe will provide exciting and productive research opportunities for those working to develop insect-resistant crops.

Leaf surface extracts ofSolanum berthaultii hawkes deter colorado potato beetle feeding.

Leaf rinses ofS. berthaultii PI 473334 with methylene chloride were deterrent to feeding by the Colorado potato beetle when applied toS. tuberosum tuber and leaf disks. When the leaf rinse was separated into its nonvolatile and volatile fractions and applied to tuber disks, the nonvolatile fraction was highly deterrent, while the volatile fraction reduced consumption, but not significantly compared to the controls. A hexane leaf rinse was not deterrent to feeding, while an acetone rinse was approximately twofold more deterrent than the methylene chloride rinse when applied to leaf disks. Three cycles of bioassay-guided, reversed-phase open-column fractionation of an acetone leaf rinse yielded a relatively polar fraction with low deterrent activity, and two nonpolar fractions exhibiting higher specific activity. Reversed-phase preparative HPLC of these fractions yielded seven active fractions among the 10 assayed. Subsequent analytical HPLC indicated that two fractions each contained a single UV-absorbing compound, while another represented a mixture of at least four compounds. The remaining fractions were composed of complex mixtures of possibly ionic or polymeric compounds that were poorly resolved by HPLC.