Genetic Analysis Reveals Three Novel QTLs Underpinning a Butterfly Egg-Induced Hypersensitive Response-Like Cell Death in Brassica Rapa

Background Cabbage white butterflies (Pieris spp.) can be severe pests of Brassica crops such as Chinese cabbage, Pak choi (Brassica rapa) or cabbages (B. oleracea). Eggs of Pieris spp. can induce a hypersensitive response-like (HR-like) cell death which reduces egg survival in the wild black mustard (B. nigra). Unravelling the genetic basis of this egg-killing trait in Brassica crops could improve crop resistance to herbivory, reducing major crop losses and pesticides use. Here we investigated the genetic architecture of a HR-like cell death induced by P. brassicae eggs in B. rapa. Results A germplasm screening of B. rapa 56 accessions, representing the genetic and geographical diversity of a B. rapa core collection, showed phenotypic variation for cell death. An image-based phenotyping protocol was developed to accurately measure size of HR-like cell death and was then used to identify two accessions that consistently showed weak (R-o-18) or strong cell death response (L58). Screening of 160 RILs derived from these two accessions resulted in three novel QTLs for Pieris brassicae-induced cell death on chromosomes A02 (Pbc1), A03 (Pbc2), and A06 (Pbc3). The three QTLs Pbc1-3 contain cell surface receptors, intracellular receptors and other genes involved in plant immunity processes, such as ROS accumulation and cell death formation. Synteny analysis with A. thaliana suggested that Pbc1 and Pbc2 are novel QTLs associated with this trait, while Pbc3 contains also LecRK-I.1, a gene of A. thaliana previously associated with cell death induced by a P. brassicae egg extract. Conclusions This study provides the first genomic regions associated with the Pieris egg-induced HR-like cell death in a Brassica crop species. It is a step closer towards unravelling the genetic basis of an egg-killing crop resistance trait, paving the way for breeders to further fine-map and validate candidate genes.

Hydrocotyle ranunculoides (floating pennywort).

H. ranunculoides is a perennial, aquatic plant native to the Americas. It was introduced outside of its native range through the aquatic nursery trade and has since naturalized in many countries around the world. Like many aquatic weeds, H. ranunculoides possesses a number of characteristics which contributes to its invasiveness: high growth rates, adaptability to prevailing nutrient conditions, very effective vegetative propagation, plasticity in growth response, overwintering to avoid low temperature stress, resistance to herbivory, resistance to chemical control, and absence of specific pests and diseases in introduced environments. Its rapid growth means that H. ranunculoides can produce dense, interwoven floating mats across slow-flowing waters. These mats restrict the light available for submerged macrophytes, decreases oxygen levels and therefore decrease the overall biodiversity of an area. It can also increase the risk of flooding and block channels. H. ranunuloides is considered a serious invader in Belgium, the Netherlands, and the UK in particular and was added to the EPPO alert list in 2004 (EPPO, 2004) and Schedule 9 of the Wildlife and Countryside Act in the UK. It is also banned from sale in the Netherlands. It has spread into water bodies in a number of other European countries including France, Belgium, Germany and Italy. In 2016, the European Commission's Implementing Regulation (2016/1141) was published, which includes H. ranunculoides among the list of 14 invasive alien plant species of Union concern.

Interspecific and intraspecific variation in leaf toughness of Arctic plants in relation to habitat and nutrient supply

Leaf toughness is an important functional trait that confers resistance to herbivory and mechanical damage. We sought to determine how species composition, climate, seasonality, and nutrient availability influence leaf toughness in two types of tundra in northern Alaska. We measured leaf toughness as force to punch for 11 species of Arctic plants in tussock tundra and dry heath tundra at 17 sites distributed along a latitudinal gradient. Rubus chamaemorus and the graminoids occupied opposite ends of the leaf toughness spectrum, with R. chamaemorus requiring the least force to punch, while one of the graminoids, Eriophorum vaginatum, required the most. Leaf toughness increased with mean summer temperature for E. vaginatum and Betula nana, while it declined with warmer temperatures for the other species. Toughness of mature leaves of E. vaginatum did not vary through the growing season but declined significantly after senescence. Application of N and P fertilizer in an experimental site decreased leaf toughness in three species but had no effect on four others. Leaf toughness of four out of five species in dry heath was greater than for the same species in tussock tundra, but there was no difference in community-weighted mean toughness between tussock tundra and dry heath.

A latitudinal gradient in herbivore resistance in common sunflower, Helianthus annuus (Asteraceae)

Background and aims – The intensity of herbivory is expected to decline with increasing latitude. As herbivory varies spatially and over time, a reliable method of assessing the intensity of herbivory is to examine the degree of herbivore resistance in the plant community. Latitudinal gradients in resistance to herbivory were examined in wild populations of common sunflower, Helianthus annuus. Materials and methods – Seeds from 23 different latitudes, ranging from 20 to 44°N, were obtained from the USDA’s Germplasm Resources Information Network. Plants were grown in a greenhouse for nine weeks. At that time, the size (height, leaf length, number of leaves) and resistance of each plant to herbivory (determined through a bioassay using a generalist herbivore, Helicoverpa zea was assessed. Key results – Resistance to herbivory decreased significantly with latitude, while plant size, as indicated by height, was positively correlated with latitude and negatively correlated with both temperature and resistance to herbivory.Conclusion – Populations from lower latitudes exhibited elevated resistance to herbivory and slower growth, suggesting first, that herbivory is more intense at lower latitudes and second, that there is a tradeoff between growth and defense.

Evaluation of Selection Methods for Resistance to a Specialist Insect Pest of Squash (Cucurbita pepo)

Plant varieties resistant to insect pests are a critical component of integrated pest management, but challenges associated with plant breeding for insect resistance, such as a long breeding cycle duration and low trait heritability, slow progress in the field. In this study, we tested two novel selection schemes to improve genetic gain for resistance to the major pest, the striped cucumber beetle (Acalymma vittatum), in squash (Cucurbita pepo, e.g., zucchini). First, we tested an indirect selection scheme using a proxy insect with correlated resistance phenotypes, Trichoplusia ni, in place of the seasonally available A. vittatum. We found that while resistance to herbivory by T. ni was heritable, there was no reciprocal benefit for resistance to A. vittatum. Second, we tested genomic selection, a method that allows for selection without phenotyping every generation, for both resistance to A. vittatum directly and resistance to the proxy T. ni. Although there was moderate genomic predictive ability, we did not observe realized gains from selection in field trials. Overall, strategies that minimize investment in direct phenotyping, leverage efficiencies from phenotyping correlated traits, and shorten breeding cycle duration are needed to develop insect resistant varieties, and this study provides examples and empirical data of two such approaches deployed in an applied breeding program.