Alice L. Pilgeram

Elevating optimal human nutrition to a central goal of plant breeding and production of plant-based foods

High-yielding cereals and other staples have produced adequate calories to ward off starvation for much of the world over several decades. However, deficiencies in certain amino acids, minerals, vitamins and fatty acids in staple crops, and animal diets derived from them, have aggravated the problem of malnutrition and the increasing incidence of certain chronic diseases in nominally well-nourished people (the so-called diseases of civilization). Enhanced global nutrition has great potential to reduce acute and chronic disease, the need for health care, the cost of health care, and to increase educational attainment, economic productivity and the quality of life. However, nutrition is currently not an important driver of most plant breeding efforts, and there are only a few well-known efforts to breed crops that are adapted to the needs of optimal human nutrition. Technological tools are available to greatly enhance the nutritional value of our staple crops. However, enhanced nutrition in major crops might only be achieved if nutritional traits are introduced in tandem with important agronomic yield drivers, such as resistance to emerging pests or diseases, to drought and salinity, to herbicides, parasitic plants, frost or heat. In this way we might circumvent a natural tendency for high yield and low production cost to effectively select against the best human nutrition. Here we discuss the need and means for agriculture, food processing, food transport, sociology, nutrition and medicine to be integrated into new approaches to food production with optimal human nutrition as a principle goal.

Transformation ofGaeumannomyces graminis to benomyl resistance

SummaryGaeumannomyces graminis var.graminis andtritici were transformed to benomyl resistance using pBT3, a plasmid encoding fungicide-resistant β-tubulin. Either circular or linear plasmid DNA producedG. graminis var.graminis transformants in which plasmid DNA was integrated into the fungal genome. There was no evidence for autonomous plasmid replication in any of the transformants examined. 4/11 linear DNA transformants had a single plasmid copy, whereas 8/9 circular DNA transformants had multiple copies of the plasmid. Integration of transforming DNA occurred by nonhomologous recombination in all (20/20) of these transformants.

Methods for selecting hypervirulent biocontrol agents of weeds: why and how.

A considerable number of plant pathogens have been studied for their possible use in weed control. Some have proven virulent enough to control weed species and to compete commercially with chemical herbicides. However, most pathogens of weeds are not useful in their wild form because they are not sufficiently host-specific and/or virulent. The authors believe that these barriers can be overcome. The present research has focused on the inhibitory effects of certain amino acids on the growth and development of specific plants. Pathogens that overproduce these selected amino acids can be easily selected from a pool of spontaneous mutants. Such mutants can have increased pathogenicity to their target weed and enhanced field performance as biocontrol agents. Enhancement of biocontrol efficacy in three separate pathogen-host systems, two with Fusarium and one with Pseudomonas, has already been reported. It is proposed to use the same technology to enhance the biocontrol efficacy of the two species of Fusarium that are host-specific pathogens of the broomrape group of parasitic weeds. The stepwise approach outlined can lead to obtaining enhanced biocontrol agents capable of producing inhibitory levels of selected amino acids in situ. It is proposed that these approaches, in combination with other methods of virulence enhancement, will lead to sustainable systems of biological control of parasitic weeds.

Sexual crosses of the homothallic fungusGaeumannomyces graminis var.tritici based on use of an auxotroph obtained by transformation

Abstract Transformation of Gaeumannomyces graminis , a homothallic ascomycete, to benomyl resistance (BenR) or phleomycin resistance (PhleoR) enabled us to isolate crossed perithecia and analyze phenotypic and genotypic segregation of the selectable markers in ascospore progeny. One BenR transformant required nicotinic acid for growth and the relationship between the auxotrophy and BenR was examined. Normal distributions of both benomyl resistance and phleomycin resistance were observed in ascospore progeny from crosses between the auxotroph and a wild-type strain or between the auxotroph and a PhleoR transformant. Benomyl resistance always segregated with nicotinic acid auxotrophy in the progeny, indicating that the two genes encoding these traits were closely linked. A simple method for faster production of G. graminis perithecia is also described.

GENETICALLY ENHANCING THE EFFICACY OF PLANT PATHOGENS FOR CONTROL OF WEEDS

There are many plant pathogens that attack weeds, but only a few have proven virulent enough to control weed species and compete with chemical herbicides (R. E. McFayden, Annu. Rev. Entomol. 43, 369-393, 1998). One might surmise that there has been strong selection against highly virulent host-specific pathogens, as survival of the pathogen depends upon survival of the host. Total eradication of the host weed would not benefit the pathogen, an impasse that challenges researchers to develop innovative strate- gies using formulation, genetics, and synergy to enhance the effectiveness of biocontrol pathogens. Our research has capitalized on the inhibitory effects of certain amino acids on plant growth and development. Biocontrol pathogens that overproduce selected amino acids have increased virulence to the target weed and enhanced field performance. We report enhancement of virulence in three separate pathogen-host systems, two with Fusarium and one with Pseudomonas.

Striga Biocontrol on a Toothpick: A Readily Deployable and Inexpensive Method for Smallholder Farmers

Striga hermonthica (witchweed) is a parasitic weed that attacks and significantly reduces the yields of maize, sorghum, millet, and sugarcane throughout sub-Saharan Africa. Low cost management methods such as hand weeding, short crop rotations, trap cropping, or conventional biocontrol have not been effective. Likewise, Striga-tolerant or herbicide-resistant maize cultivars are higher yielding, but are often beyond the economic means of sustenance farmers. The fungal pathogen, Fusarium oxysporum f.sp. strigae, has been the object of numerous studies to develop Striga biocontrol. Under experimental conditions this pathogen can reduce the incidence of Striga infestation but field use is not extensive, perhaps because it has not been sufficiently effective in restoring crop yield and reducing the soil Striga seed bank. Here we brought together Kenyan and US crop scientists with smallholder farmers to develop and validate an effective biocontrol strategy for management of Striga on smallholder farms. Key components of this research project were the following: (1) Development of a two-step method of fungal delivery, including laboratory coating of primary inoculum on toothpicks, followed by on-farm production of secondary field inoculum in boiled rice enabling delivery of vigorous, fresh inoculum directly to the seedbed; (2) Training of smallholder farmers (85% women), to produce the biocontrol agent and incorporate it into their maize plantings in Striga-infested soils and collect agronomic data. The field tests expanded from 30 smallholder farmers to a two-season, 500-farmer plot trial including paired plus and minus biocontrol plots with fertilizer and hybrid seed in both plots and; (3) Concerted selection of variants of the pathogen identified for enhanced virulence, as has been demonstrated in other host parasite systems were employed here on Striga via pathogen excretion of the amino acids L-leucine and L-tyrosine that are toxic to Striga but innocuous to maize. This overall strategy resulted in an average of >50% increased maize yield in the March to June rains season and >40% in the September to December rains season. Integration of this enhanced plant pathogen to Striga management in maize can significantly increase the maize yield of smallholder farmers in Kenya.

Enhancing the Efficacy of Bioherbicides

Plant pathogenic pseudomonads are inhibited by certain amino acids due to feedback inhibition or repression of key biosynthetic enzymes in amino acid biosynthesis pathways. As it turns out, plants are similarly inhibited by certain amino acids. These inhibitions can play a large role in plant pathology. For instance, Frenching disease of tobacco is caused by rhizosphere bacteria that overproduce the amino acid isoleucine. We found that overproduction of an inhibitory amino acid would greatly enhance the virulence of a plant pathogen greatly increasing its efficacy as a bioherbicide for control of noxious weeds.

Pseudomonas syringae: Prospects for Its Use as a Weed Biocontrol Agent

Pseudomonas syringae is found on numerous weeds, occasionally causing serious disease. The challenge lies in developing this ubiquitous bacterium into a safe biocontrol agent with consistent knockdown, host-specificity, and cost efficacy for weed control in environments where chemical herbicides are not an option. We have demonstrated that an overabundance of certain amino acids will have a nega- tive impact on certain plants presumably due to feedback inhibition of branched pathways of amino acid biosynthesis. Strains of P. syringae that overproduce a specific amino acid can be selected using appropriate amino acid analogs. Such amino acid overproducing strains of P. syringae retain the ability to colonize plants and, importantly, are inhibitory to plants sensitive to the overproduced amino acid. The host-specificity of the amino acid excreting pathogens might be narrowed by selection for one or more amino acids that are abundant in the target weed.