Rains, D. William : D. William Rains, Professor, Agronomy and Range Science, and Director, Plant Growth Laboratory, U.C., Davis.

JKC Suzanne Stavarek . I . . . . . . . . as yeia , stressresistance , or numtiona qua - ity , but it may present methods to releasepre - E viously unavailable variability for such traits . F MPGE does not stand alone in plant Clearly , improvement ; integration with conventional methods is absolutely necessary . In considering agricultural applications of genetic engineering , some requirements should be mentioned that are the subjects of current and future research . ( 1 ) Cell or tissue culture systems are needed to select for the ( 2 ) Systemsmust target characteristic ( gene ) . be developed to regenerate plants from cell ( 3 ) cultures rapidly and in large numbers . Gene expression in cell cultures must also be sustained at the stage of growth in which the Selecting for salt tolerance : ( A ) Alfalfa cotyledon produces callus from cut portion . ( 6 ) A ( 4 ) Good vectors and plants will be utilized . single alfalfa cell has potential to regenerate into a complete alfalfa plant . ( C ) Cells are probes must be developed to transmit and challenged with salt in nutrient medium ; light - colored areas are living and presumably salt . identifythe desired genes . These are substan - resistant ; brown and black areas are dead . ( D ) Resistant cells , on new medium , begin to MPGE , but they are tial criteria for utilizing differentiate into plantlets . ( E ) Regeneratedalfalfa plant , when hardy enough , will be certainly not beyond the scope of plant biol - transplanted into soil for further study . ogy research . Plant cell culture techniques may be used Developing salt of to obtain salt - tolerant cell lines . This use Some opportunities lor using molec - cell cultures is based on the application of tolerance ular plant genetic engineering in mutant selectiontechniques developed in mi - plant improvement crobial and fungal systems ( see articles by Z . OTo make transfers of genes from one D . William Rains R . Sung and G . E . Jones in this issue ) . The species to another that would not be a cell general selective strategy is to produce possible with nonmolecular culture of a crop plant and to challengethese methods . OTo transfer genes at a single step , cells with an inhibitory salt concentration by Salinityand its potential influenceon plant ratherthan through repeated crosses incorporating the salt into the nutrient productivity can be managed either by physi - or backcrosses . medium in which the cells are grown . The cally manipulating the environment in which OTo transfer only the target gene , with - rare cells that can survive and divide under the plant grows or by biologically manipu - out detrimental genes linked or other - wise associated with it . these conditions are candidates for further lating the plant to reduce the harmful effects OTo transfer genes rapidly in species of biologicalman - study . By inducingthe survivorsto regenerate of excess salt . The concept with long generation times . ipulation is based on the observation that salt whole plants , it is possible to determine 0To transfer genetic information to tolerance appears to be geneticallycontrolled whether the salt tolerance observed in the plants that will facilitate new end - products . vary widely in their sensitivity and that plants selected cells is stable and transmitted geneti - OTo conserve plant genes in cloned to high levels of salt . cally . DNA gene banks . Sometimes traditional breeding methods a line In our laboratory we have obtained OTo assess genetic variation and ge - of salt - tolerant alfalfa cells by this means . cannot be used to develop salt - tolerant crops . netic relationships among species by molecular methods . The salt - selected line grows better than un - Natural genetic variability in tolerance to sa - 0 To capitalize on " spinoff " technology at a high level of salt ( 1 percent selected cells linity may be lacking or not accessible by for use in conventional gene transfer sodium chloride ) , indicating that the selec - usual breeding methods . In these cases , it is systems - for instance , tissue , cell tion isolated variant cells with an increased appropriate to apply new techniques to or protoplast culture methods . capacity for growth in the presence of high achieve this goal . 30 CALIFORNIA AGRICULTURE , AUGUST 1982 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