Insect Herbivory I and Cell Wall Composition

Worldwide, insect herbivores reduce maize yields and often cause secondary infestations with mycotoxin producing fungi. Cell wall composition and development as well as root morphogenesis and morphology play a key role in the response to above ground and soil borne herbivores and diseases. Guided by QTL studies, we identified a set of candidate genes for the resistance of maize against the European corn borer (ECB) – a species used as a model to represent other Lepidopteran species. All these candidate genes are involved in the lignin biochemical pathway. We are now in the process of evaluating the effect of these genes on the level of ECB resistance using gene sequences polymorphisms in linkage disequilibrium studies together with mutations at these genes. This work will also enable us to study the biochemical basis of quantitative genetic concepts, like dominance and epitasis.

Incidentally, this program will also yield insights in the potential use of maize stover as a feedstock for bio-refineries. I am an affiliate of the “Molecular Bioengineering of Biomass Conversion” theme at the Institute of Genomic Biology and within this context I will further develop this research line.

Status - We are performing a meta-analysis for QTL involved in insect resistance and cell wall composition and development. In addition, a set of diverse maize inbreds is evaluated for insect resistance and cell wall composition. We also started to sequence the same lines at putative candidate genes in order to perform an association study.

Insect Herbivory II

One of my long-term goals is to develop maize cultivars with improved host plant resistance against root feeding by larvae of the western corn rootworm (WCR), the most important insect pest in the U.S. Cornbelt. However, no germplasm sources with superior WCR resistance are known. Along with the lack of highly resistant maize cultivars, only limited information is available about the organized defense responses of maize against root feeding and wounding. To overcome these obstacles, we are investigating the genetic and biochemical basis of the response in maize against the WCR by integrating different approaches, i.e., phenotypic characterization, selection, genomics, and metabolomics, and developing material that allows for mapping of resistance QTL. In addition, available genomic tools for WCR will enable us to investigate the maize × WCR interactions on a gene-by-gene expression level. This knowledge about the genetic basis of the resistance trait(s) are also key requisites if we wish to increase the effectiveness of screening maize genetic resources for novel WCR resistance sources and to devise new methods for integrating this germplasm into elite breeding programs in the Midwestern Cornbelt.

Status - After two years of intensive germplasm screening, a pedigree breeding for improving WCR resistance in maize was established. In addition, a recurrent selection (RS) program was initiated. The RS base population was recombined and the selection process will start in the summer season of 2006. QTL mapping populations will be developed using maize inbreds with improved resistance identified in my program using the DH technique. The DHL will be developed by AgReliant. The genomic studies were initiated this summer and will continue.

Root Complexity

Few studies are available relating corn root architecture to yield, root lodging, and tolerance to stresses under field conditions. The stresses can be abiotic (e.g., drought, flooding, nutrient deficiencies) or biotic (e.g., competition among plants, diseases, pests). This lack of in-depth knowledge is mainly due to the labor intensive root digging required to obtain root samples, the destructive nature of this procedure, and the highly heterogeneous root systems within and among different corn cultivars as a response to a complex soil matrix. The objectives of this study are to (i) develop an imaging system capable of efficiently acquiring images of the primary (2 dimensional) and secondary (3 dimensional) corn root systems, (ii) quantify associations among parameters obtained from images - which estimate the complexity of primary and secondary maize root systems - and conventional root traits as well as important agronomic traits, (iii) create and maintain a database containing root morphology images featuring a wide spectrum of corn genotypes grown under diverse field conditions (e.g., different plant densities, altered nutritional status, drought, flooding, presence of soil borne diseases and pests), and (iv) identify QTL for root complexity and morphology.

Status - We gathered a set of more than 4000 maize root images from primary and secondary root systems grown in growth chambers and under protected conditions the field. We programmed a Matlab software package that automatically processes each image and determines among other parameters the fractal dimension of each root system. Fractal dimensions of primary root systems significantly different between RIL of the IBM-(B73×Mo17) population. We are now in the process of QTL mapping.

Herbivory III - Genetics of the Organ Replacement Program in Arabidopsis

This project will provide more basic and general information for research areas 1 and 2. As energy providers for terrestrial communities, plants contribute tissue to grazing animals occupying the upper trophic levels. A plant species courts extinction should it fail to compensate for the decrement in fitness incurred by these inevitable forfeitures of body parts. Herbivory may select for traits that allow plants to maintain fitness despite losing reproductive organs. Higher plants can usually replace organs lost to grazers by generating new reproductive axes from arrested axillary meristems. We propose that, as part of the plant’s compensatory response to herbivory, this organ replacement program (ORP) is constituted of a suite of traits and their cognate genes, varying combinations of which represent ORP strategies contributing to the success of differentially adapted plant populations. A genetic analysis of developmental and physiological aspects of the Arabidopsis thaliana ORP is the focus of this project. The long term goal of this project is to characterize the underlying genetic structure of the Arabidopsis ORP and the variation in its implementation among Arabidopsis accessions. Our specific objectives in the present proposal are to, (i) assess Arabidopsis germplasm for natural variation in components of the ORP, (ii) determine quantitative genetic parameters for these traits and develop a model for their roles in accessions of varying ORP strategies, (iii) estimate the number, chromosomal positions and genetic effects of quantitative trait loci (QTL) involved in the ORP using multiple crosses among Arabidopsis accessions displaying differential ORP strategies.