Novel approaches to implementation of pumpkin resistance in control of viral diseases

Abstract

As there is a growing frequency of viral plant diseases in epidemic proportions, the possibilities for successful control are constantly being explored. Despite the fact that integral and simultaneous employment of numerous control measures may contribute to the decreasing amount of yield losses, especially concerning non-persistently aphid-transmitted viruses, these measures are often not efficient enough. Research into the basis of resistance to viral infection and principles of its inheritance, introduction of sources of resistance in susceptible genotypes, by conventional or genetic manipulations, are very intensive for cucurbit crops, especially pumpkins. Pumpkin crops are being endangered by a great number of different viruses, among which the Zucchini yellow mosaic virus, (ZYMV), Watermelon mosaic virus (WMV) and Cucumber mosaic virus (CMV) are present every year in Serbia, frequently causing epidemics. The majority of pumpkin cultivars are not resistant or tolerant to viral infections, but sources of resistance have been identified in various related species. So far, the identified sources of resistance to the ZYMV are found in Cucurbita moschata and Citrullus lanatus var. lanatus genotypes and consist of one or several major dominant genes of resistance. It is a similar case with WMV, although the sources of dominant major genes are identified in C. lanatus and C. colocynthis. The sources of resistance to CMV in the form of one dominant gene have been identified in the genotype C. moschata, although the introduction of this gene by conventional means proved to be very difficult. Besides the aforementioned, substantial efforts are being made in developing genotypes with multiple resistance against several viruses and even other pathogens, as well as genotypes with resistance to the most significant plant aphid species, through mechanisms of antixenosis or antibiosis. The other way of obtaining resistant genotypes includes genetic manipulation. Genetically modified resistant pumpkins have been among the first successfully developed crops. Genotypes with pathogen derived resistance can already be found in commercially grown pumpkins in some parts of the world, and they have been developed by introducing the coat protein gene of one, two or all three viruses which are the most frequent, ZYMV, WMV and CMV. Yet, this approach to the control of pumpkin viral diseases is related to possible negative consequences, mostly through the already detected gene transfer to wild plants and development of resistant transgenic weeds of unpredictable impact on the environment. Improved host plant genetic resistance to viral infections or biological vectors, developed by conventional or genetic engineering methods, represents the most dynamic and prominent field of research. It is economically and ecologically the most justified approach to the control of pumpkin and other plant diseases caused by viruses non-persistently transmitted by aphids.

Kako je pojava brojnih viroza gajenih biljaka u epidemijskim razmerama sve češća, mogućnosti za uspešnu kontrolu stalno se ispituju. Integralna i istovremena primena brojnih pojedinačnih mera kontrole, naročito u slučaju virusa koji se prenose biljnim vašima na neperzistentan način, mogu da doprinesu smanjenju šteta, ali često nisu dovoljno efikasne. Proučavanja osnova otpornosti i načina nasleđivanja, unošenje izvora otpornosti u osetljive genotipove, konvencionalnim ili genetičkim manipulacijama, vrlo su intenzivna kod vrežastih kultura, posebno tikve. Tikve ugrožava veliki broj raznorodnih virusa od kojih se u Srbiji svake godine javljaju virus žutog mozaika cukinija (Zucchini yellow mosaic virus, ZYMV), virus mozaika lubenice (Watermelon mosaic virus, WMV) i virus mozaika krastavca (Cucumber mosaic virus, CMV), a pojedinih godina izazivaju epidemije. Većina sorti gajenih tikava ne poseduje otpornost ili tolerantnost na virusne zaraze, ali izvori otpornosti identifikovani su u različitim srodnim vrstama. Do sada identifikovani izvori otpornosti na ZYMV nalaze se u genotipovima Cucurbita moschata i Citrullus lanatus var. lanatus i obuhvataju jedan ili nekoliko major gena rezistentnosti koji se nasleđuju dominantno. Slična situacija je i sa WMV, mada su izvori dominantnih major gena identifikovani u C. lanatus i C. colocynthis. Izvor otpornosti na CMV u vidu jednog dominantnog gena identifikovan je u genotipu C. moschata, mada je unošenje ovog gena konvencionalnim putem bilo veoma teško. Pored toga, veliki napori ulažu se u dobijenje genotipova sa istovremenom otpornošću na više virusa čak i drugih patogena, kao i genotipova koji ispoljavaju otpornost na najznačajnije vrste biljnih vaši, kroz mehanizme antiksenoze ili antibioze. Drugi pravac dobijanja otpornih genotipova podrazumeva genetičke manipulacije. Genetski modifikovane otporne tikve su među prvima, od svih gajenih biljaka, uspešno razvijene. Genotipovi sa PD (pathogen derived) otpornošću već se nalaze u komercijalnoj proizvodnji tikava u nekim delovima sveta i dobijeni su unošenjem gena za CP (coat protein) jednog, dva ili sva tri u svetu najraširenija virusa, ZYMV, WMW i CMV. Ipak, ovaj prilaz kontroli virusnih oboljenja tikava vezan je za uočene moguće negativne posledice, pre svega kroz već registrovan transfer gena u biljke spontane flore i nastanak otpornih transgenih korova nepredvidljivog ponašanja i značaja u prirodi. Poboljšana genetička otpornost biljke domaćina bilo na infekciju virusima, bilo na biološke vektore, dobijena konvencionalnim ili metodama genetičkog inženjeringa predstavlja najdinamičnije i najperspektivnije polje istraživanja kao ekonomski i ekološki najopravdaniji pristup kontroli oboljenja tikava i drugih biljaka koje izazivaju virusi koji se neperzistetno prenose biljnim vašima.