Gilchrist, David G

Keen, Noel T. : Noel T. Keen, Professor, Plant Pathology, U.C., Riverside.

D . W . Rains examines a culture plate containing alfalfa cells growing on a high - salt medium in â?? cell city , â?쳌 where billions of cells are grown and selected for salt tolerance under light - and temperature - controlled conditions . development of recombinant DNA tech - salt . This selected line additionally displays of genetically niques opens the possibility other characteristicssuggestingthat the toler - engineering plants by introducing desired di - a true halo - ance results from a shift toward sease resistance genes from both intraspecific phytic nature . Besides tolerating high salts , and intergenic sources . Such techniques Genetic disease as evi - halophytes actually require some salt , would be immediatelyapplicableto thesetwo denced by poor growth in its absence and resistance problems and would also increase under - growth stimulation when salt is added . standing of the mechanisms of natural di - The capacity of a cell culture to regenerate David G . Gilchrist sease resistance and nonhost immunity . plants usually diminishes over time . This The major limitationto direct implementa - in cell selection pro - presents difficulties Noel T . Keen tion of this technology is that no single resis - grams , because , by the time the selection and tance mechanism has been characterized testing stages are completed , the selectedcells T h e most widely used plant disease control completely , nor have the interactive gene may fail to respond to standard regeneration method has been the incorporation of single , products been identified . Extensive physio - conditions . This was indeed the case with our usually dominant , genes for disease resis - logical information indicates that resistance 200 media alfalfa cell line . By testing some fall into two general classes , in - tance into cultivated plants . In some cases ; phenomena modifications , we developed a sequence of disease control also has been accomplished ducible and constitutive . media that enabled us to regenerate plants In the former case , resistant plant geno - by withdrawal from plants of certain domi - from the salt - tolerant cultures . The regener - types appear to recognize some feature of the nant alleles conferring vulnerability to attack ated plants could then be tested to determine invadingpathogen , which then initiatesan in - by pathogens that produce specifictoxins . By whether the salt tolerance of the cells in cul - either approach , genetic resistance affords duced defense response . This is typified by ture was carried through to the whole plant . hypersensitive necrosis.of plant cells at the the only practical control strategy in most Unfortunately , in our case , the regenerated infection site and is associated with the ac - major crops . plants were very weak and we were not able cumulation of antibiotic chemicals called Two current problemswith the use of resis - to demonstrate any increase in salt tolerance . ( 1 ) they are frequently over - phytoalexins . Recent evidence from Univer - tance genes are : Our evidence , and that from other laborator - come by new forms of the pathogen , which of California , Riverside , is consistent sity ies , indicates that our approach is sound , with the hypothesis that recognition involves appear to evolve from the original pathogen however , and we are confident that con - population by either genetic recombination resistancegene products in the form of recep - tinued efforts will produce salt - tolerant crop or mutation ; tor moleculesthat interact with specific com - ( 2 ) suitable genes conferring plants that transmit this valuabletrait to their plex cell surface carbohydrates ( elicitors ) of disease resistance often are not availablein a progeny . or genus , or they cannot be incor - the invading pathogen . The exact nature of plant species D . William Rains , Professor , Agronomy and porated into agronomically useful cultivars the pathogen genes that may determine this Range Science , and Director , Plant Growth La - boratory , U.C . , Davis . because of cross - fertility barriers . Recent unique carbohydrate structure is unknown , CALIFORNIA AGRICULTURE , AUGUST 1982 31 Enhancing nitrogen fixation Donald A . Phillips Larry E . Williams Eoduction of ammonia from atmospheric nitrogen by the Rhizobium - legumesymbiosis Lesions on tomato leaves and fruit offers opportunities for geneticimprovement caused by disease - determining toxins produced by the pathogen of both Rhizobium bacteria and host legume . A . alternata following infection of Root nodules formed by rhizobia are the or - plant . This disease was important gans responsible for nitrogen fixation . Cali - fresh - market tomato in - to the fornia crops that might benefit most directly dustry in San Diego and Ventura from such improvements are alfalfa , clover , 1960s to counties from the early Inheritance of a singlegene - the late 1970s . It is now controlled common beans , lima beans , garbanzos , and determined sensitivity to host - in these areas by genetic resis - blackeye peas . Additional nitrogen fixed , but Alter - selective toxins produced by tance in what may have been the not used , by those plants would be bound in naria alternata f . sp . lycopersici , a first cloning of a gene for disease an organic form that could carry over to ben - fungal pathogen specific to resistance . tomato . Segregation in F , genera - efit subsequent crops . tion fits ratio of l resistant : 2 Recent genetic information indicates that intermediate : 1 susceptible plant leguminous plants vary in their capacity to from the original cross of homozy - use soilnitrogenand to fix atmosphericnitro - inhibition by the toxin than that from the sus - gous - resistant by homozygous - susceptible parental ( P , ) plants . gen with Rhizobium . Our understanding of ceptiblegenotype . Other host - selectivetoxins how efficiently nitrogen is fixed by common have been hypothesized to interfere with varieties of legumes grown in California is membrane integrity , although the exact limited , but significant work in this area is mechanism is unresolved . Clearly , cloning of being done by L . R . Teuber and K . W . Foster specific alleles lacking sensitivity to toxins at Davis . Genotypic variation for protein but : pecific glycosyl transferase enzymes are from the same or unrelated speciesand intro - concentration in alfalfa grown sequentially known to be involved in cell surface carbohy - ducing the cloned DNA into the cultivated on atmospheric nitrogen and ammonium ni - drate synthesis . Attempts to molecularly plant is a goal , along with establishinga valu - trate has been measured , and over 700 geno - clone the primary disease - determining genes able information base on resistance gene types of large - seeded grain legumes are being of certain plant pathogens , especially bac - products . assessed in the field for their capacity to use teria , are in progress in laboratories at U.C . , As assay systems for resistancegenes have atmospheric and soil nitrogen . Such studies Berkeley , Davis , and Riverside . If such genes become available at the level of gene action , will provide the information required to pro - can be cloned and their gene products more more laboratories are attempting to identify duce the next generation of nitrogen - efficient readily isolated , it will be possible to perform and clone disease resistance genes and are legumes for California . experiments to elucidate the molecular func - searching for suitable vehicles to introduce More immediate benefits will be reaped tion of such genes and to isolatethe predicted this DNA into desired plants . Although for - from genetic studies of the bacterial partner , resistance gene - coded receptors produced by eign DNA has been experimentally intro - Rhizobium . Laboratories at Davis are inves - the host plant . duced into plant cells using the crown gall Ti Other bases of resistanceappear to be con - tigating hydrogen uptake , a process affecting plasmid , we are not aware of the successful stitutive and , depending on the form of gene efficiency of nitrogen fixation ( see article by introduction of defined plant genesinto other expression , may involve the presence or R . C . Valentine in this issue ) , and D . N . plant cells by recombinant DNA technology . absence of host - plant sensitivity to chemical Munns at Davis is examining Rhizobium We predict , however , that this will happen disease determinants produced by the patho - strains for natural variation in tolerance to within the next few years . Disease resistance gen . In several diseases , alleles at a single acid and aluminum stress . Collaborative genes are indeed likely to be among the first genetic locus in the host control both suscep - work between the John Innes Institute in genes introduced into new plants by genetic tibility to the pathogen and sensitivity to Norwich , U.K . and our group in Davis has engineering , not only because of their consid - host - selectivetoxins produced by the patho - used conjugal plasmid transfer to produce erable economic importance , but because gen . Such toxins are useful as direct chemical Rhizobium strains significantly superior to they represent some of the few examples in probes of the genetic interaction . Recent either parent . The key plasmid , pIJ1008 , car - plants of naturally occurring and defined work at U.C . , Davis , with such a host - para - ries determinants for hydrogen uptake , single genes with definitive phenotypes . site interaction ( see photos ) indicates that the which are not found in Rhizobium strains Thus , their transfer can be detected easily and host - selective toxin inhibits a key enzyme in that nodulate alfalfa and clover . Although rapidly . the pyrimidine biosynthetic pathway , aspar - pIJ1008 is expressed most completely in pea David G . Gilchrist , Associate Professor , Plant tate transcarbamylase ; the enzyme from the rhizobia , it has been transferred to closely re - 7 : Keen , Pro - Pathology , U.C . , Davis , and Noel resistantgenotype is relativelyless sensitiveto fessor , Plant Pathology , U.C . , Riverside . lated clover and alfalfa bacteria , and those 32 CALIFORNIA AGRICULTURE , AUGUST 1982