Enrico Biancardi

The origin of rhizomania resistance in sugar beet

In the last 35 years, breeding has greatly reduced the damages caused by rhizomania in sugar beet crops. After the first encouraging results using the Alba genotypes, the cultivar Rizor represented a substantial step forward and has given good yield improvement in diseased fields in many parts of the world. The original variety and subsequent improved versions continued to offer good performances for about a decade, after which it was surpassed by other hybrids derived in part from the Rizor itself. Further progress in terms of sugar production became possible in 1986, when the Holly monogerm lines were released in USA and Europe. In spite of the incomplete information about the genealogy of the first resistant materials,many evidences and the molecular analyses on the different genotypes suggest a possible common progenitor and lineage. The resistant cultivars have kept the yield at an adequate level, allowing cultivation to continue in countries where the disease has reached epidemic proportions. The case of rhizomania resistance in sugar beet can therefore be considered as one of the most important achievements in plant breeding.

Sugar beet resistance to rhizomania: state of the art and perspectives.

The productivity of sugar beet is strongly limited by several biotic stresses, among them rhizomania (induced by Beet necrotic yellow vein virus, BNYVV) which causes yield losses of 20–50% or more. The only way to control this disease is the use of resistant varieties. Sources of resistances have been found in the ancestor of the cultivated beets Beta vulgaris L. ssp. maritima (L.) Arcang. Rz1 is the major resistance gene present within commercial sugar beet varieties. This resistance was recognized as monogenic and dominant. Experimental evidence highlighted that Rz1 originated from sea beets belonging to Munerati’s genetic pool. The development of molecular markers linked to Rz1 allows the use of marker assisted selection (MAS) to introgress this gene into pollinator lines, to assess the trueness of hybridity and to remove off-type individuals. MAS speeds variety development and reduces production costs guaranteeing at the same time a high qualitative standard in variety development. Recent studies have shown an emergence of new BNYVV strains with increased virulence that could overcome Rz1 resistance. Therefore, exploitation of new genetic sources of resistance and the pyramiding of several resistance genes into new breeding lines is becoming a main priority for sugar beet breeding companies to maintain an adequate resistance level to rhizomania.

Breeding for Biotic Stress Resistance/Tolerance in Plants

The long-term goal of crop improvement for biotic stress tolerance in plants is a traditional objective of breeders. Plants must continuously defend themselves against attacks from bacteria, viruses, fungi, invertebrates, and even other plants. This chapter will therefore summarize the benefits and drawbacks of resistance versus chemical protection. Attempts will be made to provide a description on the effective genetic and molecular mechanisms that plants have developed to recognize and respond to infection by a number of pathogens and pests, such as non-host resistance, constitutive barriers and race-specific resistance, including recent advances in elucidating the structure and molecular mechanisms used by plants to cope with pathogens and pest attacks. This chapter also covers the most relevant problems in breeding for resistance to parasites and will include aspects related to specificity of defense mechanisms, specificity of parasitic ability, inheritance of resistance, gene-for-gene interaction, and durability of resistance. Major considerations in breeding for resistance to parasites, conventional sources of resistance and possible alternatives, namely mutation breeding, genetic manipulations, tissue cultures, and molecular interventions to develop plants resistant to pests and pathogens will also be dealt.

Range of Distribution

Sea beet is the most widespread taxon within genus Beta, and can be found quite easily along the seashores of Mediterranean Sea and the European Atlantic Ocean. On these coasts, countless localizations have been reported in the literature beginning in the early 1700s. The frequency of sea beet populations decreases as one goes inland, where the origin of the populations is more likely due to hybridization between sea beet and cultivated beet crops. Although rare, the presence of sea beet has been reported on the shores of the Middle East North Sea, India, China, Japan, and California. In North America, wild populations of Beta maritima, Beta macrocarpa, and respective hybrids (with cultivated beet) likely originated from contaminated seed imported from Europe during colonization of California by the Europeans.

Morphology, Physiology, and Ecology

The traits of Beta maritima have been reviewed and summarized from a number of recent and classical publications dealing with the ecology, morphology, and whole-plant physiology of the species. Because few papers have been written only on sea beet, B. maritima, most information comes from cultivated forms of Beta vulgaris. A striking feature of B. maritima gleaned from this review is how variable and adaptive it is. The species is fairly plastic allowing it to live in many different environments. This capacity for adaptation has been correlated with its breeding system, which allows sea beet to rapidly change reproduction systems, flowering time, life span, etc. according to the local environmental conditions. This is most evident to the observer in the differences between the Mediterranean populations with easy bolting, short life cycles, and those from the sea coasts of northwest Europe. This chapter provides the reader with a comprehensive overview of the plant and populations to answer the question: “What is B. maritima?”

Source of Useful Traits

In the late 1800s, there already was speculation that Beta maritima might provide a reservoir of resistance genes that could be utilized in sugar beet breeding. European researchers crossed B. maritima and sugar beet and observed many traits in the hybrid progeny. It is impossible to estimate how widely B. maritima was used in the production of commercial varieties, because most of the germplasm exchanges were informal and are difficult to document. Often these crosses of sugar beet with sea beet germplasm contained undesirable traits, e.g., annualism, elongated crowns, fangy roots, high fiber, red pigment (in root, leaf, or petiole) and much lower sucrose production. It is believed that lack of acceptance of B. maritima as a reservoir of genes was because most of the evaluations of the progeny were done in early generations: The reactions of the hybrids vulgaris × maritima were not impressive, and it is clear now that they were not adequately studied in the later generations.

Molecular progress in sugar beet breeding for resistance to biotic stresses in sub-arid conditions-current status and perspectives

The yield of sugar beet depends mainly on the effective protection to a number of pests and diseases affecting the crop. In the absence of actual methods of management, as in the case of viral diseases, the availability of genetic resistance sometimes allows the survival of the crop in the affected areas. Integration of molecular markers in conventional breeding procedures has provided a reliable means for improving the efficiency of selection methods. The present review summarizes the evolution, thanks to the application of molecular techniques, of traditional breeding for resistance to some biotic stresses in sub-arid environments.

Control of the Disease

This chapter describes the preventative systems adopted for limiting the spread and the damage caused by rhizomania. However, the area affected by the disease is still expanding, notwithstanding the quarantine measures tried in several countries. Since the first observations, the new disease has appeared unusually dangerous for the beet crop, mainly due to the easy spread and the severe effects on sugar yield. When the etiology of rhizomania was discovered, the development of genetic resistances appeared among the few options available against the disease. Because the genetic control is incomplete and it is currently being overcome by new strains of the virus in some areas, new systems of agronomic and biological control are under evaluation for potential integration with the genetic resistances and have shown some evidence of antagonistic ability in depressing the development of Polymyxa betae. A genetically modified strain of Pseudomonas fluorescens was employed for evaluating the possible effect of this organism on lowering the spread of rhizomania. Using some species belonging to the genus Trichoderma, positive results were obtained in the glasshouse.

History and Current Importance

Sea beet is known from prehistory for food and above all for medicinal uses. After domestication, beet became more and more important, especially after its most recent use as a sugar crop. But also the cultivation for leaves and root to be used as vegetables and cattle feed retains its economic value. Beta maritima has become crucial as source of useful traits, which disappeared in the crop during domestication. This research, which has led to important results, especially in the field of resistances to severe diseases, continues today. The activity of some involved scientists is recounted. An increasing amount of publications are dedicated to sea beet because the species also fits well into studies concerning population genetics, natural breeding systems, colonization, speciation, gene flow, etc.