Orton, Thomas J. : Assistant Professor, Vegetable Crops, Davis

New concepts in whole - plant genetics Thomas J . Orton , Assistant Professor , Vegetabl Srops , Davis I n the excitement surrounding accomplish - Synthetic ( all potentialgenes ) transfer ments in the manipulation of tissues , cells , Unrelatedspecles - somatic hybridization , and genes over the past ten years , one might gene isolation , and transfer get the impression that plant breeding re - I B a r r I e r to sexual hybridization search at the whole plant and populationlev - 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 substantiatethe existenceof such limitationsfor 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 Peopleâ??s Republic of China will more likely sustain performance in - creases in the foreseeable futurethan willnew cellular / molecular approaches . As the following articles show , the idea of expanding the pool of accessible genes by overcoming natural isolatingmechanismsbe - 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 - tolerantwild species have produced breeding lines with somesalt tolerance . Preliminaryobservations suggest that cold tolerance is another possi - bility . To date , 14 genes for disease resistance have been transferred from wild species into Rangeof genetic Variability available to plant breeders for crop cultivated tomato varieties . Cell cultureof in - improvement . Beginning from the inner circle , accesslble be transmittedto the cultlvar from successively variabllltycan terspecific hybrids has been successfully ap - distant sources . While the overall range of Variability increasesas plied to facilitate the flow of genes among the geneticdistance increases , the difficulty of performingthe 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 nurse tissue for the hybrid ( C ) . About 70 percent of the embryo cultures may give serves as 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 ) . criterion in tomatoes . A major factor limitingthe rate of progress One strength of cellular and molecular ge - in plant breeding has been low heritability . netic approaches is the possibilityof perform - This occurs frequently in complex traits such ing selection with chemicals or physical con - as yield and quality , which are controlled by a large number of genes and are environmen - ditions on extremely large populations of tally unstable . Plant breeders and physiolo - haploid individualsin a small space and over gists arecooperatingin an effort to overcome a short time . Numerous schemes have been by breaking a complex character this problem 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 - in vitro have Until now , variant cells selected nents of a character or for its underlying only rarely been translated into functionally molecular constitution , breeders hope to equivalent whole plants . Recent studiesshow reduce complexity and environmental im - that it may be possibleto circumvent some of pact , allowing for faster progress . the problems associated with selection of cul - Areas currentlybeing 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 - incompatibilityphenotypes that pollen can be responsive to selection in ( whichare 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 of cul - product ( sauce , paste , catsup ) per unit 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 examplesinclude fromincreasedwater content , which must , in determinategrowth habit for mechanicalhar - Chao - Chien Jan so - turn , be eliminated by the processor . The vesting , disease - resistant cultivars , and hy - Calvin 0 . Qualset lution has been to maintain or increase the brid varieties . New ideas are constantly Jan Dvorak of processing tomatoes ( sol - economic yield emerging , such as techniquesto permit the se - ids ) while decreasing water content . The lection of plants with multipletolerancegenes alternative character used by breeders as a as opposed to often ( â?? horizontal â?쳌 resistance ) selective criterion is sugar ( soluble solids ) unstable single gene ( â?? vertical â?쳌 ) resistance . s o m e crop plants have simple inherited concentration , which is negativelycorrelated 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 selectionfor gross yield , and harvesting costs . transfer of resistance to the barley yellow f i i flesh , uniform maturity , thick skin , and 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 . of whole plant genetics ing and other aspects embryoculture method that produced hybrid Another approach to solving the problem are also becoming increasinglysophisticated . plants from two difficult - to - hybridizespecies . of low heritability , recently developed at the BYDV is an aphid - transmitted virus that Advances in breeding technology guarantee University of California , is the use of electro - occurs on many grasses , including forage that plant breeders will continue to make phoretic variation as an alternative selection valuable contributions to agriculture . grasses and cereal grains . Discovered in Cali - CALIFORNIA AGRICULTURE , AUGUST 1982 23