NEWS AND VIEWS

http://biotech.nature.com • JUNE 2002 • VOLUME 20 • nature biotechnology

Though this is hard to conceive now, until150 years ago paper was manufactured notfrom trees but from rags. It took visionaryEnglish researcher Hugh Burgess to showthat wood pulp, which was cheaper and moreplentiful, was a suitable raw material forpapermaking. Today, the kraft pulpingprocess used in papermaking is not muchdifferent from that used by Burgess. But theadvent of genetic engineering has meant thattransgenic trees with altered lignin composi-tion could dramatically improve pulping effi-ciency and the environmental impact ofeffluent from paper mills. Before that canhappen, however, there is a dire need for dataconcerning the environmental impact andfield performance of transgenic trees. In thisissue, Halpin and colleagues1 report the find-ings of a four-year field trial of poplar trees inFrance and the United Kingdom intended toevaluate the ecological, agronomic, and pulping performances of lignin-modifiedtransgenic poplars. Their data indicate thatlignin-modified transgenic trees do not havedetrimental or unusual ecological impacts inthe areas of testing.

In 1851, after foreseeing a shortage of ragsfor papermaking, Burgess offered “a methodfor making a good pulp by boiling wood inalkali”2 to the UK government, which refusedit. Greatly disappointed, Burgess came to theUnited States, only to find more skepticismand more prejudice that wood could not besuperior to rags. In 1866, after years of strug-gle, he finally established the first pulp mill inthe United States to commercially producechemical pulp from wood, in Manayunk,Pennsylvania. The production output was 15tons per day.

Today, the United States produces over 80million tons of wood pulp annually3. Over80% of this pulp is produced by a chemicalmethod called the kraft process—one that issimilar to Burgess’s “boiling wood in alkali” toremove lignin, the phenolic polymer thatbinds cellulose fibers in wood (Fig. 1). Nearly30 million tons of lignin are removed from

wood annually, consuming enormousamounts of energy and chemicals and mak-ing the pulp and paper industry the secondmost energy-intensive industry group in theUS manufacturing sector3.

Although the worldwide demand for pulpcontinues to increase, the kraft process hasalready been optimized to minimize con-sumption of energy and chemicals4. Clearly,the key to improving pulp production is the

development of wood with novel properties,such as low lignin content or reactive lignin,that could sharply lower the kraft energy andchemical intensity limits. In fact, geneticallyengineered trees with these properties havealready been developed5–7. But these treeshave met with resistance, in part because ofunanswered questions about how they willperform in the field with respect to the stabil-ity of the wood properties, resistance to

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Vincent L. Chiang is the director of the PlantBiotechnology Research Center at the School ofForestry, Michigan Technological University,Houghton, MI 49931-1295(vchiang@mtu.edu).

Figure 1. Pathway of lignin biosynthesis in angiosperm trees. Wood is a composite of fibrouscellulose embedded in short-chain polysaccharides and polyphenolic lignin. During pulping, lignin ischemically degraded, with an unavoidable removal of a majority of the hemicelluloses, to releasecellulose fibers from wood for papermaking. When expression of the gene encoding CAD isdownregulated in hybrid poplar trees, lignin becomes more chemically reactive and transgenic woodcan be pulped with fewer chemicals. In contrast, suppression of COMT gene expression renderslignin more chemically resistant, requiring more pulping chemicals. But the field performances ofboth transgenic types are similar to those of wild-type poplars.

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From rags to richesTransgenic trees may improve the efficiency of pulp production without detrimental environmental andecological effects, according to new results from field trials.

Vincent L. Chiang

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nature biotechnology • VOLUME 20 • JUNE 2002 • http://biotech.nature.com

NEWS AND VIEWS

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insects and pathogens, potential ecologicalimpact, and most importantly, response tokraft pulping.

In this issue, Halpin and colleagues1

address these issues in a rigorous evaluationof the agronomic and pulping performancesof lignin-modified transgenic poplars grownin the field for four years in France and theUnited Kingdom. Lignin in angiosperm trees,such as poplar, is polymerized from coniferyland sinapyl alcohols, the guaiacyl (G) andsyringyl (S) monolignols, which are derivedfrom cinnamate (Fig. 1). Through phenylhydroxylation and methylation and side-chain carboxylic reductions, cinnamate ismetabolized into coniferyl alcohol.Coniferaldehyde is also converted into sinapylalcohol8.

In one of the two transgenic types evaluat-ed, the expression of a gene encoding caffeateO-methyltransferase (COMT), involved inthe biosynthesis of S monolignol (Fig. 1), wasdownregulated by the antisense approach,with the hope of reducing lignin content4. Butinstead it modified the lignin structure into atype that was deficient in the reactive S mono-lignol. A separate attempt to decrease ligninby antisense downregulation of the geneencoding cinnamyl alcohol dehydrogenase(CAD), which catalyzes the reduction ofconiferaldehyde (Fig. 1), was also unsuccess-ful. But, through unknown mechanisms, thelignin in this transgenic type contained agreater amount of free phenolic groups5.Phenotypically, these two types of transgenicpoplars were externally indistinguishablefrom the wild type, but the wood in CAD-transgenic trees was red in color, a character-istic of trees with CAD deficiency9.

Importantly, these engineered lignin prop-erties were unchanged in the field-growntrees, as were growth and development char-acteristics. Halpin and colleagues furtherdemonstrated that the engineered ligninstructure variations had no adverse effect onthe trees’ resistance to insects and pathogens.The rate of root decomposition over fivemonths, however, was greater for transgenictrees than for wild-type trees. For most forestmanagement purposes, this could be desir-able. But it may not be advantageous forbelow-ground carbon sequestration. A greatvariety of vegetation grew normally aroundthe transgenic trees for four years, suggestingthat the trees were not invasive—a centralconcern discussed in a recent conference onthe introduction of transgenic trees to forestecosystems10. Indeed, these first comprehen-sive field evaluations by Halpin and colleagues provide strong evidence thatlignin-modified transgenic trees do not dam-age or perturb the ecosystems around them .

The practical significance of CAD-trans-genic trees is their potential to improve the

energy and chemical efficiency and the envi-ronmental performance of the world’s kraftindustry. Use of these trees could save the USkraft industry $200 million annually in sodi-um hydroxide consumption alone. Thereduction in chemical use would also havecascade effects on all associated operations,leading directly to reduced environmentalimpacts. And, just as importantly, CAD-transgenic wood offers a 3% gain in pulpyield, a considerable improvement. However,wood from COMT transgenics is not a suitable raw material for pulping, as morechemicals are needed to degrade the S mono-lignol–deficient lignin.

All of these field and pulping evaluationsare exciting and certainly encouraging, butthey are not likely to result in the immediatecommercialization of transgenic trees.Rather, they are a first step in the long processof developing transgenic wood as an alterna-tive to traditional wood. Transgenic trees withother altered lignin properties need to beexplored with respect both to the science itselfand to field and pulping performances.

Given that gymnosperm trees are usedmore commonly than angiosperm species aswood-pulp raw material, it is remarkable thatlignin genetic engineering has not beendemonstrated in gymnosperms. Could weengineer, in both angiosperms and gym-nosperms, desired levels of lignin and certainsubunits (such as the reactive syringyl moi-eties) in a cell-specific manner? Are these

specificities needed? Could we modify morethan one lignin trait simultaneously? Mostimportantly, could we engineer lignin itself,rendering it completely removable duringpulping without costly and environmentallydamaging bleaching processes? Findinganswers to these questions will require anintimate collaboration between industry andthe research community. Industry, whichunderstands the process constraints and end-product specifications (and society’s needs),must tell scientists exactly what characteris-tics are needed in transgenic wood.

Beyond these scientific challenges, we stillface an uphill battle with respect to socialacceptance of transgenic trees and consumerreaction to products derived from transgenicwood. But a similar battle was fought afterBurgess’s discovery that a superior pulp couldbe made from wood rather than from rags.

1. Pilate, G. et al. Nat. Biotechnol. 20, 607–612 (2002).2. Burgess, H. UK patent 1,942 (August 19, 1853).3. American Forest & Paper Association. U.S. Pulp and

Paper Industry’s Energy Use (American Forest &Paper Association, Washington, DC, 1994).

4. Nilsson, L.J., Larson, E.D., Gilbreath, K.R. & Gupta,A. Energy efficiency and the pulp and paper indus-try (Report no. IE962) (American Council for anEnergy-Efficient Economy, Washington, DC, 1995).

5. Van Doorsselaere, J. et al. Plant J. 8, 855–864(1995).

6. Baucher, M. et al. Plant Physiol. 112, 1479–1490(1996).

7. Hu, W.J. et al. Nat. Biotechnol. 17, 808–812 (1999).8. Li, L. et al. Plant Cell 13, 1567–1585 (2001).9. Ralph, J. et al. Science 277, 235–239 (1997).

10. http://www.pewtrusts.com/pdf/hhs_biotech_forests.pdf.

The saying that “the ripest fruit falls first”may no longer be true. In this issue, Mattooand colleagues1 have used a transgenicapproach to modify polyamines in tomatofruit and have demonstrated a clear effect onthe ripening process. The transgenic fruithave longer vine lives, suggesting thatpolyamines do indeed have a function indelaying the ripening process. Moreover, the

fruit have increased levels of the antioxidantlycopene.

Ripening is a developmental processunique to plants, and the biochemical andmolecular basis of this process are not fullyunderstood. Three key areas under investiga-tion include elucidating the role of ethylenein fruit ripening, identifying major regulato-ry components involved in the ripening ofall fleshy fruits, and modifying fruit quality,especially with respect to nutrition andhealth. Polyamines long have been suspectedof having a controlling role in ripening, asthey are known to decline throughout theprocess.

The model plant Arabidopsis thaliana isproviding insights into the regulation offruit development2,3, but it is difficult to do

Greg Tucker is a professor of plantbiochemistry, University of Nottingham,School of Biosciences, Loughborough, UnitedKingdom and Graham Seymour is a researchleader, Horticulture Research International,Wellesbourne, Warwick, United Kingdom(gregory.tucker@nottingham.ac.uk).

Life on the vineTomato plants engineered with increased levels of polyaminesshow delayed ripening and increased levels of the antioxidantlycopene.

Greg Tucker and Graham Seymour

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