8/10/2019 Cervantes 2001 Trends in Plant Science

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Alcohol dehydrogenase (ADH) is a keyenzyme in fermentation and anaerobicmetabolism where it contributes to ethanolproduction from acetaldehyde derivedfrom pyruvate. During the anaerobicresponse in plants, transcription of theADH gene is induced, transcripts are morestable and ADH protein levels increase.Indeed, ADH was identified as ananaerobic-induced polypeptide in severalscreenings. Furthermore, it has beenknown for some time that ethylene is animportant mediator of the response tohypoxia in many plant systems.

Hsiao-Ping Peng et al. 1 have nowreached a crossroads in the investigation of

the role of ethylene in the induction of ADH.Treatment with amino-oxacetic acid (AOA),an inhibitor of ethylene synthesis, resulted ina notable decrease in the hypoxic inductionof an ADH::GUS transgene after 28 hoursof hypoxia treatment; this effect could bereversed by the addition of ACC (ethyleneprecursor). Similar results supportingethylene induction where observed whenADH mRNA was analysed in northern blots.Interestingly, these effects of AOA and ACCneed several hours of the hypoxia treatmentto occur. For example, after four hours inhypoxia, AOA treatment does not decreasethe amount of mRNA for ADH; on the contrary,at these shorter incubation times, AOA seems

to increase the amount of mRNA for ADHand ACC decreases it, which is the oppositeeffect of what happens after 28 hours.

Results with ethylene mutants come tothe same conclusions: ethylene is involved

in late induction of ADH, but somethingelse is responsible for the earlierinduction. The case is similar to recentlyreported examples of gene inductionduring germination, where ethylene isknown to regulate the expression of several genes, such as genes encodingcysteine proteinase or -glucanaseenzymes. However, ethylene is producedat the time of radicle emergence orthereafter, when these genes are alreadyinduced. Now the question remains: whatother signals contribute to the inductionof gene t ranscription during the hypoxicresponse?

1 Peng, H-P. et al. (2001) Signaling events in thehypoxic induction of alcohol dehydrogenasegene in Arabidopsis . Plant Physiol. 126, 742749

Emilio Cervantesecervant@gugu.usal.es

TRENDS in Plant Science Vol.6 No.10 October 2001

http://plants.trends.com 1360-1385/01/$ see front matter 2001 Elsevier Science Ltd. All rights reserved.

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A distinctive voice for ethylene signaling in hypoxia

ethylene is involved in late induction of

ADH, but something else is responsible forthe earlier induction

Studying the historic migrations of the Irish potato faminepathogen using ancient DNAPhytophthora infestans , the causal agent of

potato and tomato late blight, is one of themost devastating crop pathogens knownbecause it spreads quickly and the infectioncannot be controlled effectively usinggenetic resistance or fungicides. Althoughits center of origin is believed to be in theAmericas, a series of migrations has led toa worldwide distribution. The firstdocumented migration occurred in the1840s, and led to the Irish potato famine.Disease reports suggest that the pathogenreached the eastern USA in the early 1840s,from where it migrated to Western Europe.Previous work concerning this migration hasinvolved drawing inferences based ongenotyping isolates collected primarily fromthe late twentieth century. Now, JeanRistaino and colleagues 1 have usedgenotyping of P. infestans from infectedpotato leaves collected as early as 1847 todefine the history of the pathogen better.

Outside of its center of diversity, centralMexico, P. infestans is primarily asexuallyreproductive, owing to the requirement of two mating types for the formation of sexualspores. As a result, in most locales, one or a

few clonal lineages exist. Genotyping of extant

isolates collected worldwide has revealed thatbefore the second known round of migration,which occurred in the 1970s1990s, one clonallineage, US-1, predominated outside of Mexico. Based on this distribution and theabsence of other documented migrationssince the 1840s, it was suggested that US-1was probably responsible for the Irish potatofamine. To test this hypothesis, Ristaino et al.isolated DNA from P. infestans -infectedherbarium samples and sequenced a PCR-amplified 167 bp variable mitochondrial DNA(mtDNA) fragment to genotype the ancientisolates. Four major mtDNA haplotypes areknown, type Ib being unique to US-1.Surprisingly, none of the ten herbariumsamples successfully analyzed (collected inEurope and in the USA from 18471928)displayed the Ib haplotype, suggesting thatUS-1 was not solely responsible for theearly migration. Additional analysis isnecessary to identify more fully the mtDNAhaplotype(s) present in these samples.

This is the first time that herbariumspecimens have been successfully used totrack the migrations of a plant pathogen, and

the results reveal that the past of P. infestans

is more sordid than previously believed.

The real significance of this work is that itimplies that additional evolutionary eventsmust have occurred between this earlymigration and that in the 1970s1990s.Although disease reports do not suggest asmuch, it is possible that US-1 gained itsworldwide distribution by the 1970s via anintermediate migration event and successfuloutcompetition of the previously distributedisolate(s). Alternatively, US-1 might havebeen distributed with several other lineagesat the time of the famine, and over time itoutcompeted the others. Further work withthese and additional herbarium samplesshould shed more light on these migrations.

1 Ristaino, J.B. et al. (2001) PCR amplification of the Irish potato famine pathogen from historicspecimens. Nature 411, 695697

Matthew R. Willmannwillmann@fas.harvard.edu

' the past of P. infestans is more sordidthan previously believed'