New concepts in whole-plant genetics

Thomas J. Orton, Assistant Professor, Vegetabl Srops, Davis

Inthe excitement surrounding accomplish- Synthetic (all potential genes) transfer ments in the manipulation of tissues, cells, Unrelated specles -somatic hybridization, and genes over the past ten years, one might gene isolation, and transfer get the impression that plant breeding re- IBarrIerto sexual hybridization search at the whole plant and population lev- els has been fied away and forgotten. On the contrary, several exciting developments in breeding technology have surfaced during this same period. One premise upon which the need for cel- lular and molecular approaches has been based is that plant breeders have exhausted existing pools of genetic variability, and pro- gress has consequently diminished. There is little evidence to substantiate the existence of such limitations for most crops. Moreover, it is premature to conclude that sexually acces- sible variability has been depleted when the existing collections are rarely, if ever, com- prehensive. New variability, for example, in soybean and rice introductions (among others) from the People’s Republic of China will more likely sustain performance in- creases in the foreseeable future than will new cellular/molecular approaches. As the following articles show, the idea of expanding the pool of accessible genes by overcoming natural isolating mechanisms be- tween species remains a possibility; hybrids between species can be synthesized and chro- mosomally altered to produce dramatic eco- nomic gains, as with strawberries; it is now possible to sexually engineer wheat lines con- taining desired genes from related species simply by adding or substituting whole chro- mosomes. In other research, crosses between the cultivated tomato and a salt-tolerant wild species have produced breeding lines with some salt tolerance. Preliminary observations suggest that cold tolerance is another possi- bility. To date, 14 genes for disease resistance have been transferred from wild species into Range of genetic Variability available to plant breeders for crop cultivated tomato varieties. Cell culture of in- improvement. Beginning from the inner circle, accesslble terspecific hybrids has been successfully ap- variablllty can be transmitted to the cultlvar from successively plied to facilitate the flow of genes among distant sources. While the overall range of Variability increases as the genetic distance increases, the difficulty of performing the species related to tomato, barley, and cab- transfer also increases. bage. Altogether, exploitation of genes from wild germplasm sources has barely begun.

22 CALIFORNIA AGRICULTURE, AUGUST 1982 To produce wheat-barley cross, embryo is excised from immature barley endosperm (A) and is replaced by frail hybrid embryo (B) produced by hand pollination. The endosperm serves as nurse tissue for the hybrid (C). About 70 percent of the embryo cultures may give rise to plants (D), but of 270 plants obtained by this process at Davis, only 20 were true wheat x barley hybrids (E, center; spike at left is wheat; right, barley).

A major factor limiting the rate of progress criterion in tomatoes. in plant breeding has been low heritability. One strength of cellular and molecular ge- This occurs frequently in complex traits such netic approaches is the possibility of perform- as yield and quality, which are controlled by a ing selection with chemicals or physical con- large number of genes and are environmen- ditions on extremely large populations of tally unstable. Plant breeders and physiolo- haploid individuals in a small space and over gists are cooperating in an effort to overcome a short time. Numerous schemes have been this problem by breaking a complex character devised to select desirable genetic variation down to components, and ultimately to the with respect to nutrition, stress tolerance, actual molecules that mediate the expression herbicide resistance, and disease resistance. of the character. By selecting for the compo- Until now, variant cells selected in vitro have nents of a character or for its underlying only rarely been translated into functionally molecular constitution, breeders hope to equivalent whole plants. Recent studies show reduce complexity and environmental im- that it may be possible to circumvent some of pact, allowing for faster progress. the problems associated with selection of cul- Areas currently being explored for applica- tured cells by substituting whole plants. For tion of this approach are yield, quality, salt example, chemicals have been used to screen and drought tolerance, water-logging toler- barley seedlings exhibiting increased nutri- ance, chilling tolerance, disease and insect re- tional value. Other experiments have shown sistance, and self-incompatibility phenotypes that pollen can be responsive to selection in (which are used in FIhybrid production). For vivo, opening up the possibility for screening example, in processing tomatoes the major large populations of haploid individuals. component of yield is volume of finished Plant breeding has brought about drastic product (sauce, paste, catsup) per unit of cul- changes in plant architecture or genetic tivated area. In breeding for harvest yield makeup, revolutionizing production and wt eat-ba Iey hybrids alone, much of the progress observed results marketing methods. Recent examples include from increased water content, which must, in determinate growth habit for mechanical har- Chao-Chien Jan turn, be eliminated by the processor. The so- vesting, disease-resistant cultivars, and hy- lution has been to maintain or increase the brid varieties. New ideas are constantly Calvin 0. Qualset economic yield of processing tomatoes (sol- emerging, such as techniques to permit the se- Jan Dvorak ids) while decreasing water content. The lection of plants with multiple tolerance genes alternative character used by breeders as a (“horizontal” resistance) as opposed to often selective criterion is sugar (soluble solids) unstable single gene (“vertical”) resistance. some crop plants have simple inherited concentration, which is negatively correlated Dwarf fruit trees are being developed that characters that would be desirable if trans- with water content. By combining this char- will produce more with lower management ferred to another crop. Our current work on acter with additional selection for gross yield, and harvesting costs. transfer of resistance to the barley yellow uniform maturity, thick skin, and fiiflesh, Although genetic engineering for crop im- dwarf virus (BYDV) from barley to wheat is it has been possible to make more rapid pro- provement holds great promise, plant breed- an example of a simple modification to the gress in improving this crop than in the past. ing and other aspects of whole plant genetics embryo culture method that produced hybrid Another approach to solving the problem are also becoming increasingly sophisticated. plants from two difficult-to-hybridizespecies. of low heritability, recently developed at the Advances in breeding technology guarantee BYDV is an aphid-transmitted virus that University of California, is the use of electro- that plant breeders will continue to make occurs on many grasses, including forage phoretic variation as an alternative selection valuable contributions to agriculture. grasses and cereal grains. Discovered in Cali-

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