CELLULAR AND MOLECULAR GENETIC IMPROVEMENT OF CEREALS
INDRA K. VASIL Laboratory of Plant Cell and Molecular Biology 1143 Fifield Hall University of Florida Gainesville, FL 32611-0690, USA
ABSTRACf. Efficient methods for the regeneration of plants from cultured cells of gramineous species were developed during the 1980's, based on the culture of tissue/organ explants comprised largely of undifferentiated cells on media containing high concentrations of 2,4-dicblorophenoxyacetic acid (2,4-D). These advances, combined with the later development ofmethods for the direct delivery of DNA, led to the production of the fIrst transgenic cereals in 1988. Since then, in the relatively short period of six years, all major cereal crops have been transformed, often with agronomically useful genes that confer resistance to non-selective herbicides, or viruses and insects. Such improved crops can reduce or even eliminate the huge losses in crop productivity caused by weeds, pathogens and pests. Further molecular improvement of cereals will depend on the availability of genes that determine the quality and productivity of cereal crops, and protect them from biological and environmental stresses.
Plants account for 93% of the human diet, and even the remaining 7% is indirectly contributed by plants through animal products (Borlaug 1983, Vasil 1990). Of the more than 250,000 species of angiosperms, a mere 29 provide most of our calories and proteins. The most important of these are the eight cereal grains (wheat, rice, maize, barley, oats, rye, sorghum, pearl millet) of the family Gramineae (poaceae), which together constitute the centerpiece of world agriculture by providing 52% of the total food calories. The primitive forms of many cereals were amongst the handful of plants that were first domesticated during the Neolithic age nearly 10,000 years ago, when agriculture gradually replaced hunting and gathering. In retrospect, the choices made by the Neolithic man have not only proven to be wise and remarkably durable, but have also helped to sustain the development of the human civilization by providing a rich and dependable source of nutrition for the ever increasing world population.
The dramatic increases in the yields and production of cereal grains achieved by breeding and selection during 1950-1984, kept pace with the rapidly growing world population and prevented widespread hunger and famine. Such increases can not be realistically sustained indefmitely, and indeed during the past ten years grain yields and production have begun to decline. Yet by various estimates world population will double to nearly 11 billion some time between 2030 to 2050, of which more than 90010 will reside in the already overpopulated and undernourished countries of Africa, Asia and Latin America. This huge increase in population will cause even more rapid degradation of the already fragile environment, and will further decrease per capita supplies of crop land, grain and water,
M. Terzi et al. (eds.), Current Issues in Plant Molecular and Cellular Biology, 5-18. 1995 Kluwer Academic Publishers.
all at a time when food supplies should be actually more than doubled just to meet the basic needs of human nutrition.
Plant improvement by breeding is severely restricted by the availability of a rather limited gene pool owing to natural incompatibilities, even between related species, and by the time scale of most breeding programs. Therefore, much attention has been directed recently to the newly emerging and novel technologies of plant cell and molecular biology (biotechnology), which provide a powerful means to supplement and complement the traditional methods of plant improvement, by permitting access to an unlimited gene pool through the transfer of desirable genes between any two species of interest, irrespective of their evolutionary or taxonomic relationships.
Advances in plant cell culture research, especially of major crop species, have played an increasingly critical role in the development of modern plant biotechnology (Vasil 1990). Widely used protocols for the regeneration of plants from cultured cells of dicotyledonous species (dicots) were already available in the early 1980's, when Agrobacterium-based methods for DNA delivery and integration were developed, leading to the production of the frrst transgenic plants in 1983 (Fraley et al. 1983, Zambryski et al. 1983). The difficulties faced with the adaptation of these procedures for the transformation of cereals led to the development of alternative strategies and technologies for the improvement of cereals. In this brief review, I describe the key components of these strategies, and highlight the significant advances in the molecular improvement of cereal crops.
PLANT REGENERATION FROM CULTURED CELLS
Prior to 1980, there were only scattered reports of plant regeneration from tissue cultures of gramineous species. In most instances regeneration was sporadic and transient, limited to obscure genotypes of a few species, and was based either on the de novo formation of shoot meristems in callus cultures or the 'microtillering' of preexisting meristems (Vasil 1987). Four significant discoveries were made in 1980-1981 that form the basis of much of the modem work on tissue culture of cereals: (a) That the culture of immature embryos, and segments of young inflorescences and bases of immature leaves at defined stages of development, (b) on simple nutrient media containing high concentrations of strong auxins like 2,4-D, (c) gives rise to long-term embryogenic callus cultures in which plant regeneration takes place via the formation of somatic embryos, and that (d) embryogenic cell suspension cultures derived from the embryogenic calli yield totipotent protoplasts (Dale 1980, Vasil and Vasil 1980, 1981, Wernicke andBrette1l1980, Haydu and Vasil 1981, Lu and Vasil 1981). During the past 15 years, these strategies have been successfully used to develop reliable and efficient methods for the regeneration of plants from all of the important species of cereals as well as grasses (Vasil and Vasil 1994).
The importance of the physiological condition as well as the developmental state of the explant has been found to be related to the endogenous levels of plant growth regulators (Rajasekaran et al. 1987a,b, Carnes and Wright 1988, Wenck et al. 1988). Although regeneration in vitro is a function of the genotype of the donor plant, regeneration has been obtained by a judicious choice of explant and nutrient media from a wide variety of germplasm in most species, including those which were once considered to be recalcitrant.
Somatic embryogenesis is the predominant mode of regeneration in cereal species (Vasil and Vasil 1986, 1994). This is significant, because the somatic embryos, like their zygotic counterparts, are derived directly or indirectly from single cells, are non-chimeric in nature and possess a root-shoot axis which is critical for their transfer to, and survival in, soil. Another useful characteristic of plants derived from somatic embryos is their uniformity and genetic fidelity. These are attributed to a
stringent selection during somatic embryogenesis, and the inability of somatic embryos derived from genetically aberrant cells to develop into mature viable embryos (Swedlund and Vasil 1985, Hanna et al. 1984, Rajasekaran et al. 1986, Cavallini et al. 1987, Kobayashi 1987, Breiman et al. 1989, Cavallini and Natali 1989, Morrish et al. 1990, Gmitter et al. 1991, Shimron-Abarbanell and Breiman 1991, Shenoy and Vasil 1992, Chowdhury and Vasil 1993, Isabel et al. 1993, Valles et al. 1993, Chowdhury et al. 1994). This unique characteristic of plants derived from somatic embryos makes them ideally suited not only for clonal propagation but also for genetic transformation.
Until 1987 , the only possibility to obtain transgenic cereals was by the direct delivery of DNA into protoplasts. Yet, exhaustive attempts made during 1975-1980 to induce sustained divisions in protoplasts isolated from leaves of cereal species proved to be entirely unsuccessful (Potrykus 1980, Vasil 1983, Vasil and Vasil 1992). Even today, no reliable and reproducible method is available for the recovery of plants from mesophyll protoplasts of any grass or cereal species. Therefore, based on the induction of sustained divisions in protoplasts isolated from non-morphogenic cell suspension
Table 1. Regeneration of plants from protoplasts isolated from embryogenic cell suspension cultures.
Pennisetum americanum Panicum maximum Pennisetum purpureum Saccharum sp. Oryza sativa Polypogon fugax Dactylis glome rata Festuca arundinacea Lolium perenne Lolium multiflorum Zeamays
Triticum aestivum Setaria italica Agrostis alba Sorghum vulgare Hordeum vulgare Oryza rufipogon Agrostis palustris Paspalum dilatatum Festuca pratensis Oryza granulata Poa pratensis Triticum durum Hordeum murinum Festuca rubra
Vasil and Vasil 1980 Lu et al. 1981 Vasil et al. 1983, Wan and Vasil 1994 Srinivasan and Vasil 1986, Chen et al. 1988 Yamada et al. 1986, Kyozuka et al. 1987, Datta et al. 1990a Chen and Xia 1987 Hom et al. 1988a Dalton 1988 Dalton 1988, Creemers-Molenaar et al. 1989 Dalton 1988, Wang et al. 1993a Rhodes et al. 1988a, Prioli and Sondahll989, Shillito et al. 1989, Morocz et al. 1990 Vasil et al. 1990, Ahmed and Sagi 1993 Dong and Xia 1990 Asano and Sugiura 1990 Wei and Xu 1990 J ahne et al. 1991, "Holm et al. 1994 Baset et al. 1991 Terakawa et al. 1992 Akashi and Adachi 1992 Wang et al. 1993b Baset et al. 1993 Nielsen et al. 1993 Yang et al. 1993 Wang and Lorz 1994 Spangenberg et al. 1994
"From mechanically isolated fertilized egg cell protoplasts.
cultures, attempts were made to establish embryogenic suspension cultures as a source of totipotent protoplasts (Vasil and Vasil 1980, 1981). This strategy proved to be most useful, as plants have been regenerated from protoplasts isolated from embryogenic suspension cultures of a large number of gramineous species, including all the major cereal crops (Table 1). It should be noted, however, that the establishment of regenerable embryogenic suspension cultures is still a difficult and tedious procedure. Fmthermore, in some species (eg. maize and wheat), such cultures can be established only from a special type of callus. Nevertheless, once established, such cultures can be cryopreserved to provide a permanent and reliable source of totipotent protoplasts (Shillito et al. 1989, Gnanapragasam and Vasil 1990, Meijer et al. 1991, Fretz et al. 1992, Lu and Sun 1992, Wan and Vasil 1994).
For almost twenty years, somatic hybridization has been billed as a potentially powerful tool for creating new and useful hybrids that can not otherwise be obtained by breeding. Unfortunately, with a few exceptions, this has generally not proven to be true. In cereals, few somatic hybrids have been obtained, as of Oryza sativa (+) Echinochloa oryzicola (Terada et al. 1987), or Festuca arundinacea (+)Lolium multiflorum (Takamizo et al. 1991). Perhaps the best use of this procedure is in the transfer of cytoplasmic male sterility, as was demonstrated in rice (Kyozuka et al. 1989, Yang et al. 1989). Also, in vitro fusion of egg and sperm cell protoplasts of maize, leading to the generation of fertile plants (Kranz and Lorz 1993), provides a powerful new tool for the study of cell to cell interaction and the process of fertilization.
TRANSFORMATION OF CEREALS
Genetic transformation of dicot species has been greatly aided by the natural transformation system provided by Agrobacterium tumefaciens. It has become a preferred and popular method because it requires minimal exposure of cells to tissue culture conditions, and provides high frequencies of transformation with single copy insertions which appear to be less prone to methylation and silencing. It is not surprising, therefore, that extensive attempts have been made to transform cereals with Agrobacterium. However, in spite of many attempts and a few reports (Hess et al. 1990, Gould et al. 1991, Chan et al. 1993), no unambiguous and clear evidence for Agrobacterium-mediated transformation of cereals has ever been presented. Indeed, it has been suggested that the reported claims of transformation may be actually artifacts caused by the transformation of an endophytic organism associated with the host plants (Langridge et al. 1992, Chen et al. 1994). It is known, nevertheless, that Agrobacterium can deliver viral genomic sequences to cereal cells resulting in systemic viral infection (Grimsleyet al. 1987, Dale et al. 1989, Shen et a!. 1993). This process, termed "agroinfection", does not result in the integration of the viral genes into the plant genome. It is thus likely that the difficulties faced with Agrobacterium-mediated transformation of cereals lie in the integration, rather than the delivery of genes, and may be eventually overcome with further understanding of the process of transformation.
In the mean time, other useful methods, based on the direct delivery of DNA into protoplasts as well as intact cells and tissues, have been developed for the transformation of cereals. All of the early successes, including the fIrst transgenic cereal (maize, Rhodes et al. 1988b) and grass (Orchard grass, Hom et al. 1988b) plants, were based on DNA delivery into protoplasts by osmotic (polyethylene glycol treatment) or electric (electroporation) shock. Although this procedure has proven to be very useful for rice, maize and some grass species (Table 2), its use is limited because of dependence on totipotent protoplasts isolated from embryogenic suspension cultures, both of which are difficult to obtain and maintain in culture.
The development of the biolistics procedure, based on the high velocity bombardment of DNA-coated microprojectiles into intact cells and tissues (Sanford et al. 1987, 1991, Klein et al. 1992), has provided an important alternative to the use ofprotoplasts for cereal transformation. Within a short period of four years, from 1990 to 1994, it was used to produce transgenic plants of maize, rice, wheat, oats, barley, sorghum, rye, and important grass species, like sugarcane (Table 2). The delivery of DNA directly into regenerable cells or tissues substantially reduces the time that the cells must be maintained in culture. The immature embryo, which is the most widely used explant for plant regeneration in cereals, has proven to be particularly useful for the biolistic delivery of DNA and the production of transgenic plants.
More recently, transgenic maize and rice plants have been also obtained by electroporating DNA into enzymatically or mechanically wounded suspension culture cells, callus or embryos (D'Halluin et al. 1992, Laursen et al. 1994, Xu and Li 1994).
Table 2 lists all transgenic cereal and grass species produced to date. It includes many examples where agronomically useful...