Dvorak, Jan

Jan, Chao-Chien : Chao-Chien Jan, formerly Postgraduate Research Agronomist (now Research Geneticist, USDA-ARS, Davis, Calif.)

Qualset, Calvin O

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 fornia in 1951 , BYDV causes serious yield chromosomes can occur . One approach em - ly into a wheat chromosome . Techniquesare losses in barley , wheat , and oats . Resistance ploys hybrids between such species and already available for this step , but the key to to BYDV was found only in barley from wheat . When an alien set of chromosomes is in making the difficult hybrid was the success Ethiopia and was subsequentlytransferred to present in the hybrid along with the donor rescue of the hybrid embryos with the endo - several California barley varieties . A single chromosome carrying the desired trait , the sperm nurse tissue . gene , Yd2 , in barley confers resistance . An chances of pairing between the donor and Chao - Chien Jan , formerly Postgraduate Re - extensive search among wheats from around wheat chromosomes are enhanced . The sec - search Agronomist ( now Research Geneticist , USDA - ARS , Davis , Cat $ ) , Calvin 0 . Qualset , so the world failed to find a resistant variety , ond approach manipulates dosages of genes Professor , and Jan Dvorak , Associate Professor , it appeared necessary to transfer known to regulate chromosome pairing in Yd2 from Agronomy and Range Science , Davis . This work barley to wheat . wheat . was supported by USDA grants 801 - 15 - 42 and 50 - 2063 - 01402 . The wheat x barleyhybrid is very difficult A recent successfulexampleof the first ap - proach involves the wild diploid species to makebecausethe two species are not close - Aegilops speltoides , which is native to the 7 pairs of chromo - ly related . Barley has Middle East . This species has a genotypecap - somes , and bread wheat has 21 pairs . When of inducing pairing between homoeo - able pollen from barley is applied to wheat , fertili - Induced chromosome logues in hybrids with bread wheat . It also zation can take place , but at a very low fre - possesses resistance to the leaf rust fungus quency . The resulting embryos abort unless pairing Puccinia recondita.Followinginitial hybridi - a sterile growth excised and cultured on zation between the susceptiblewheat cultivar medium ; even then few embryos survive ( less Jonathan lrvine Pitic 62 and resistant Ae . speltoides plants , 1 percent ) . than Patrick McGuire the hybrids were backcrossed to Pitic 62for In the early 1970sAnthon Kruse , working several generations . The F , and first back - in Denmark , discovereda simpleembryo res - crossplants were male sterile , but by the third cue technique . He placed the hybrid embryo Success in hybridizing wild with cultivated backcross , pollen fertility was substantially on immature endosperm excised from the de - of species ultimately depends on the ability restored in the derivative lines . When pro - of one of the parent species . veloping seed geny of the third and fourth backcrosses were the alien chromosomesto pair and recombine The endospermservesas a nurse tissuefor the with the chromosomes of the crop species at screened for resistance to the fungus , resis - hybrid . Australian workers found that barley of meiosis . In bread wheat , pairing and thus re - tant plants were found . Observations endosperm was the best nurse tissue and re - combination normally occur only between chromosomes at meiosis in resistantplants of ported that about 70 percent of the cultured the fourth backcross revealed some 42 - chro - identical or homologous chromosomes and hybrid embryos produced plants . not , unfortunately , between a wheat chromo - mosome plants with normal wheat - likemeio - We adopted these techniques at Davis in a some and an alien chromosome . However , sis . Thus , the required portion of anAe . spel - Yd2 program that will transfer to wheat the toideschromosome increasing knowledge of the components of conferringresistancehad gene , which is on barley chromosome 3 . Even the genetic system controlling pairing has been translocated to a wheat chromosome . with the method described , it is difficult to made it possible to manipulate them to F , The exchange occurred in plants of the produce the hybrid plant . In two years , more achieve pairing between nonhomologous generation following initial hybridization 50,000 individual wheat ovaries were than chromosomes . A major advantage of such through homoeologous chromosome pairing pollinated by barley , and of those , onlyabout Ae . spel - induced pairing is that it usually occurs be - permitted by the interaction of the 440 showed embryo development . These genotype with the wheat genotype . tween closely related but not homologous toides were excised and transferred to nurse tissue The desired exchange was only one of many chromosomes - termed homoeologues . Re - 270 plants on sterile culture medium . About that may have occurred . Backcrossing and combination of genetic material between ho - were obtained , but only 20 were true wheat screening were necessary to exclude un - moeologues is likely to result in genetically x barley hybrids . wanted exchanges and retain plants with more balanced chromosomes than if recom - The hybrid plants were completely male - bination were between unrelated chromo - sterilebut did produce seeds when pollinated wheat characteristics and resistance . After of theAe . speltoides the first backcross , some somes . with wheat . These seeds , called backcrossF , , 7chromosomes Two approaches can be used to transfer produced plants that had the chromosomes were lost , since they had no of barley and 21 wheat chromosome pairs . genes from related species into wheat . There pairing partner at meiosis . Normal meiosis 14chromosomes ) re - We have backcrossed and self - pollinated are diploid species ( with to the derivatives as backcrossing returned lated to wheat ( 42chromosomes ) that modify several more generations and now have continued . the chromosome pairing control system wheat - like plants with 44chromosomes - all of This technique is efficient , because in one ( 42 ) and one wheat so that pairing between homoeologous of the wheat chromosomes step it incorporates resistance and provides chromosome pair from barley . of an opportunity for the recombination homoeologous chromosomes . From these re - Seven types of wheat plants have indivi - combinants , backcrossing and screening se - dual barley chromosomes , one type for each lect any plants that possess the desired por - 7barley chromosomes.Our task now is of the tion of the alien chromosome translocated to to identify the wheat plant with barley chro - wheat without interfering with normal chro - 3 . We can then expose those plants mosome mosome pairing and fertility once the wheat to BYDV to see if the Yd2 gene functions in background is restored . wheat . The next step will be to induce recom - a wheat chromosome with the bination of Jonathan Zrvine , Research Associate , Znterna - tional Plant Research Institute , Son Carlos , Cali - barley chromosome , so that we eventually Reactions to leaf rust on leaves of ( from fornia , and Patrick McGuire , Postgraduate 21 pairs of chromo - have a wheat plant with left ) : susceptible Pitic 62 wheat cultivar ; Research Agronomist , Agronomy and Range somes with the Yd2gene incorporated direct - susceptible derivative ; resistant derivative . Science , U.C . , Davis . 24 CALIFORNIA AGRICULTURE , AUGUST 1982