Controlling elemental composition is critical for plant growth and development as well as the nutrition of humans who utilize plants for food. Uncovering the genetic architecture underlying mineral ion homeostasis in plants is a critical first step towards understanding the biochemical networks that regulate a plant's elemental composition (ionome). Natural accessions of Arabidopsis thaliana provide a rich source of genetic diversity that leads to phenotypic differences. We analyzed the concentrations of 17 different elements in 12 A. thaliana accessions and three recombinant inbred line (RIL) populations grown in several different environments using high-throughput inductively coupled plasma- mass spectroscopy (ICP-MS). Significant differences were detected between the accessions for most elements and we identified over a hundred QTLs for elemental accumulation in the RIL populations. Altering the environment the plants were grown in had a strong effect on the correlations between different elements and the QTLs controlling elemental accumulation. All ionomic data presented is publicly available at www.ionomicshub.org.

Summary• Breeding for resistance to Striga in maize (Zea mays), with paucity of donor source and known mechanisms of resistance, has been challenging.• Here, post-attachment development of S. hermonthica was monitored on two maize inbreds selected for field resistance and susceptibility reactions to Striga at the International Institute of Tropical Agriculture. Haustorial invasion of the parasite into roots of these inbreds was examined histologically.• Morphological differences were observed between roots of the susceptible and the resistant inbreds. The resistant maize had fewer Striga attachments, delayed parasitic development and higher mortality of attached parasites compared with the susceptible inbred. Striga on the susceptible inbred usually penetrated the xylem and showed substantial internal haustorial development. Haustorial ingress on the resistant inbred was often stopped at the endodermis. Parasites able to reach resistant host xylem vessels showed diminished haustorial development relative to those invading susceptible roots.• These results suggest that the resistant inbred expresses a developmental barrier and incompatible response against Striga parasitism.

Breeding for durable Striga resistance requires identifi cation and stacking of multiple genetic barriers to parasitism in crops. Th is task would be greatly aided by having screening tools for identifying in parent sources and breeding populations resistance traits that function at various stages in the parasitic association. In this study, we describe a root observation system called the sand packed titer plate assay (SPTPA) used to study early postattachment development of Striga hermonthica (Del.) Benth. on two maize (Zea mays L.) and two sorghum [Sorghum bicolor (L.) Moench] inbred lines selected for fi eld resistance and susceptibility to Striga. Th e resistant maize and sorghum lines had fewer Striga attachments and these showed delayed parasitic development, and higher mortality compared to those on susceptible cultivars. Th e SPTPA was useful in identifying specifi c resistance reactions to S. hermonthica in maize and sorghum and refl ected results from fi eld studies of these varieties. Th is assay would complement fi eld testing during development of Striga-resistant varieties.