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«Biotechnology-Assisted Participatory Plant Breeding: Complement or Contradiction? PPB Monograph No. 3 Ann Mane Thro and Charlie Spillane 1 7 ...»

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Rushura. But they feel that Rushura cannot be recornmended for more widespread cultivation because it is susceptible to cassava mosaic disease (de Piter et al, 1997). Gene transfer would be an effective way of adding resistance to Rushura, greatly enhancing an already useful variety known to be in demand by small- scale farmcrs.

Two routes are open to [armcrs and formal breeders wishing to enhance existing varieties using transgenes: (i) genetic transformation of the variety or (ii) backcrossing the tran sgene from a transgenic variety into a non-transgenic one. While route (i) may be faster, it requires either that protocols for efficient transformation of the particular variety have been developed, which is unlikely to be the case for most landraces, or the use of a suitable gcnotype-independent transfer method. Route (ii) is more time-consuming, and is unlikely to be an endeavor that farmers would wish to undertake, because of the yield and other problems in early-generation progeny. The costs and benefits of each route would have to be worked out on a case by case basis.

Biotechnology-Assisted PPB: Complement or Contradiction?

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One option lhal m ight prove widely app licable would be to lra nsform a basic set of genotypes (perhaps those lhat can be grown with at least sorn e success in the broadest ran gc of environmen ts) with th e most useful tran sgen es. After biosafety testing, the sel could be made a vailable as donar parcnts for crossi ng or backcrossing according to s peciJic nccds (M.J. Sampaio, D. Duvick, pcrs. comm s.). This would be a 10w-lcc h ' method [ar delive rin g transgcnic innovations in a form rea dily usablc by national programs or even directly by fa rmers.

If farmc r s \Vere also provided with trait-linked selcction markers for use in iden tifying transgenic progeny at the field level, they could in Biotechnology as a Sel of Tools far Formal and Informal Plan! Breeding lheory shorten the amount of time spent on backcrossing, which might make them more willing to undertake it. In practlce, however, farmers are unlikely to wish to take this route without the assistance of formal researchers, who are more able to sus tain the risks of yield decline and quality deterioration associated with early·generation progeny.

As the lechnology progresses and more robust and efficient protocols become available, genetic transfer is likely to become applicable to a wider range of genotypes, as weU as faster and more reliable (e.g., Clough and Senl, 1998; Komari el al, 1998; Mazur el al, 1999). It may become the preferred approach for adding single-gene desired traits to otherwise popular varieties, since unlike sexual crossing it does not disrupt the complex genetic balance of other traits, especially quantitative traits. It may prove particularly useful in clonally propagated crops, in which conventiona! breeding is difficult.

Meeting biosafety requirements for containment in such crops is easier, because of the absence of natural seed dispersal. Efficient transformation systems may eventually become a service industry, in which varieties of a particular species can be transformed al core transformation facilities for that species.

MAS and transgenic techniques both have considerable potential for speeding up the 'upstream' germplasm enhancement or prebreeding stages of crop improvement. They can also allow the development of enhanced germplasm populations more precisely tailored lo !he needs of end users (Tanksley and McCouch, 1997). For a while at least, non-transgenic germplasm enhanced by MAS may prove more popular with formal breeders and farrocrs who do nol want or cannot afford the regulatory burdens and biosafety restrictions of working with transgenic material. But in the longer term it is clear tbat, used in corobination, these advanced biotechnologics could yield tangible benefits for farmers and consumers.

Fteld-level 4gene switch' technologies to increase farmers' control

DNA elements called promoter scquences can be used to control thc expression oí a transgene by directing it to certain tissues (e.g., to poUen ceUs) or to specific developmental stages (e.g., at dehiscence) or to respond to specific inducing or repressing agents (e.g., virus infection, herbicide treatment). Inducible promoter systems allow researchers to switch genes on or off at particular times in their laboratory work. In theory, farmers or formal breeders couId do the same thing at field level.

Combined with the use of transgenics, promoters are powerful lools for broadening farrocrs' choices and increasing their control over key biological processes. The challenge posed by cyanogen toxicity provides a good example (Box 8l. The ability to control the exprcssion

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of selected genes in fi eld-grown plan ts by a pplying inducer compounds to them could co nfer substantial agronomíc benefits. Field-level intervention may be especially desira ble for controlling the expression of tran sge nes.

Box8 Seeking solutions to tbe paradolE oC cassava toxicity Convenlional plant breeding has been unable to produce cassava varieties lhal combine reduced labor requirements and reduced risk of toxicity with the a clvantagcs farroers require from toxicity. An altemative approach is to seek bctter processing methods, involving the dis tribution of improved (faste r) fe rmentation starter cultures. But this approach faces daunting logística! and educational challenges. A genetic solution would be easier to implemento Can biotechnology tools help achieve a genetic solution?





Vanous biotechnology approaches have been suggested. If beneficial trruts are linked to, but distinct Crom, the toxicity factors, then the Iinkage can be broken using precise new selection tools such as antibodies and m olecular markers.

However, it must be borne in mind that sorne benefits are conferred by the toxicity itself. These circumstances s uggest a transgenic approach designed to increase the oplions available, together with fanners' control over them. PossibJe

stratcgics inc1ude:

Tissue -specific or deveJopmentally controlled promoters inserted in front of the gene Cor cytochrome (P450), so as to limit synthesis of the toxin's precursor to certrun tissues or specific periods of plant growth A promoter ror the gene responsible Cor the brea kdown oC linamario lo toxic cyanide, lO increase lhe speed of cyanide release during processing.

Released cyanide would volatize rapidly and harmlessly in open-air processing arcas, before the cassava is consumed For situations where toxicity is not needed and 100% s afety is required, an antisense ar gene-silenced consttuct of cytochrome P450 under the control of a strong constitutive promoter could be in troduced. This would produce completely acyanogenic plants that lack the potentia! to become toxic under any circumstances.

Genetic tools are now available for pursuing these strategies. A cassava papulation on which to conduct the research to develop molecu lar markers for cyanogenic patential has been assembled at CIAT. Genes for cytochrome P450 and linamarase synthesis have been doncd. Constitutive and tissue-specific promote"s and the technology for the genetic transfonnation of cassava are available. The promoters a re patented, but free licensing is available to developing countries in the service of small -scale Carmcrs.

SOU RCES: Cassa va Safety Group (1994); Hughes el a! (1994,1997); Hughes and Hughes (1994); Whi te and Sayre (1 997); Lidd.1e et a! (1997);

Verdaguer et a! (1996); Sarria et al (1995); Schopke et al (1996);

Li el al (1996); Raemakers et al (1996); González et al (1 998) ; Arias and Sayre (1 998); R. Sayre, C. Iglesias, M. Frege ne (pers. comms.).

Biotechnotogy as a Set 01 Tools Jor Formal and Informal Plant Breeding

A number of'first-generation' inducible promoter systems have been developed (Table 2). Very few of these can be used on farmers' fields al presento Among Ihem are Ihe ethanol inducible promoter (Caddick et al,

1998) and Ihe sarener inducible promoter (de Veylder et al, 1997). More field-level systems wiU doubtless be developed ayer the next 5 years.

The ideal requirements for a farm-Ievel inducible promoter were outlined by Jefferson (1993a, 1993b). For example, in a subsistence cropping system, where commercial inputs are not practica!, the inducer would have to be an inexpensive, 10caUy available substance.

Jefferson et al (1999) suggests that no current systems meet aU the necessary criteria for farmer use, but that systems could easily be developed Ihat do.

Controversy has becn aroused by the development of inducible promotor-based systems to restrict transgenic phenotypes to a single

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ge neration (e.g., Moore et a l, 1998). These systcrns, devcloped by Delta a nd Pine and th e United S tates Department of Agricultu re (USDA), were pa tented on the basis of their u sefu lness in protecting proprietary technology. They use a combinatío n of inducible a nd growth stagespecific promote rs in conju nction with orber transgenes to limit aceess to proprietary 'embcclcled technology' to the firsl eommercialized gencra tion only (Jefferson et a l, 1999). If seeond-gen eration seed is sown it does no t germinate, leadi ng to crop failu re. Th is is the teeh nology that was du bbed the 'termin a tor ' by the Rural Advaneeme n t Found a tion Intcrnational (RA FI, 1998). During the Biosafety Protocol negotiation s of 1999-2000, severa! developin g eountries expressed concern that secondgen eration transgenic seed carrying th e technology could aecidentally be sown, especially by resource-poor farmers, who often divert some of th eir food sup ply LO this purpose a t the start of the growing season.

However, su eh seed could be a high -value product for speeialized uses (J. Blalock, pers. comm.), in which case it would be too valu ab le to handle as a bulk commodity and would therefore be u nlikely to become avail a ble fo r sowing.

A reeently posited variant of th ese systems is the one in which farmers would be a ble to apply a specific compound to 'switch on ' an agronomic tran sge ne if h e ar s he wished to do so. One corn mentator noted that this technology could give rise to food security concerns, since it could m ake farmers su sceptible to gene warfare (J. J iggins, pers.

cornm.). The authors feel that this concern is u n likely to materia lize, partiy for logistical reasons (replacin g a major part of the seed of a wh ole region is a high ly visible activity) and partIy because sowings that \Vere not exposed to the compound \Vould still produce the basie erop. Only the value-added trait would be lacking.

Compounds and inducible promoter systems produced by the private sector are proprie tary and a vaila ble to farmers and researchers only on a commercial basis. However, such systems could in th eory al so be developed by the public sector foc n on- or less commercial a pplications, such as those in basie research or those directed lo meetin g the needs of resource- poor farmers (Jefferson, 1993a, 1993b). Publicly funded systcm s would u se n on-proprietary indu ce r compounds which, ideal ly would be relatively abundant and inexpensive in ru ral areas. If made widcly available, su eh systems could be useful in transgenic approaches to facili tating the process of farrner PPB. However, it remains a n open question whether or not th ey will actually be developed.

Needs assessments with farmers wiU help identify the priority traits over which farmers mi ght wi s h to h ave grea ter control. The

following is a possible lisr:

The ability to switch apomixis gen e{s) on and off could have major empowering implications for resouree poor breeding situations (see Tools fo/" Selecting Germplasm, p. 47) Biotechnology as a Set 01 Tools lor FormoJ. and 11110nnal Plan! Breedíng

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There may be cases in which the additionallabor implied by increased farmer control over biological processes and products may prove a disincentive. For example, in IPM, rather lhan continually monitoring a field of erops for the emergence of inseet pests before manually inducing Bt expression in infested plants, sorne farrners may wish to rely on promoters induced by feeding insects, which would enable them to devote their labor to other activities.

Tools for Dellverlng Planting Materials

The shorlage of high -quality, healthy seeds and other planti ng material s is among the most widely expressed concems of resource-poor farmers.

Shortages are both chronic, caused by a variety of factors including poorly developed systems for multiplication and dissemination, and acute, caused by natural and man-made disasters, such as droughts and war.

Farmers everywhere are almast invariably keen to try out new crap varieties. Their planting material wishes are nearly always expressed in tenns of a specific variety or varieties of interest. This may be a variety seen in a fonnal breeder's demonstration plot, or a local selectian, or a sample carried home from a trip to distant relatives. Farmers often find their opportunities to grow desirable new varieties limited by a cce ss to planting material, whether from formal or informal breeding programs.

An example is cassava in Tanzania (Thro et aJ, 1994). In other cases, the performance of an already widely adopted variety may have Biorechnology-Assisted PPB: Complernent or Contradiction?

deteriorated due to th e infestation of planting material with system ic pathogens. Other quality-related problems in planting material s include poor ge rmination, slow maturation, and low yie ld potential. It is nol uncommon to find all th ese constraints together.



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