VARIABILITY OF VOLATILE ORGANIC COMPOUNDS EMITTED BY SEEDLINGS OF SEVEN AFRICAN MAIZE VARIETIES WHEN INFESTED BY ADULT CICADULINA STOREYI CHINA LEAFHOPPER VECTORS OF MAIZE STREAK VIRUS

Abstract

Maize ( Zea mays L.) is an important source of energy for the people in sub-Saharan Africa. Unfortunately, maize production is under the threat of Maize streak virus (MSV) (Geminiviridae: genus Mastrevirus), an endemic pathogen of native African grasses. This virus is acquired and transmitted in a persistent manner, by Cicadulina spp. leafhoppers (Homoptera: Cicadellidae) .One possibility for controlling the virus and the vectors is to exploit volatile semiochemicals released by plants following insect herbivory. A study was conducted on herbivoreinduced volatile organic compounds (VOCs) of seven African maize (Zea mays L.) varieties in the presence of the leafhopper ( Cicadulina storeyi China, Homoptera: Cicadellidae) an important vector of maize streak virus (MSV) with a view of developing novel leafhopper control strategies in sub-Saharan Africa. VOCs are known to repel herbivory and also serve as cues for parasitoids to locate their hosts. Volatiles were collected from young maize seedlings of 7 varieties after entrainment with or without infestation of leafhoppers in 0 - 24, 24 - 48, 48 - 72 and 72\u201396 hr periods and combined and subsequently analysed by gas chromatography. Analysis of the VOCs collected from the seven elite African varieties shows high interspecific VOC variability. Gas chromatography (GC) and coupled GC-mass spectrometry (GC-MS) led to the identification of (z)-3-hexen-1-ol, heptanal, (z)- 2-pentenyl acetate, MHO, octanal, myrcene, (z)-3-hexen-1-yl acetatate, (E)-3-hexen-1-yl acetatate, nonanal Linalool, DMNT, (E)-2-decen-1-ol, Methyl salicylate, DMNT, Phenethyl acetate, Indole, cis-jasmone, \u3b1- copaene, Geranyl acetone, \u3b2-caryophyllene, \u3b1-bergamotene, (E)-\u3b2-farnesens, TMTT, and Humulene from infested seedlings. Of these, methyl salicylate, (E)-caryophyllene, (E)-\u3b2-farnesene and TMTT were identified previously as volatile semiochemicals involved in plant defense against other sucking insect pests. TZB-SR seedlings emitted 22 compounds in the presence of leafhoppers, compared with 5 by uninfested seedlings; while BR9943-DMR.SR emitted only seven compound in the presence of leafhoppers compared with 6 by uninfested seedlings. The compounds emitted by the seven maize varieties after herbivory were in the following descending order: TZB-SR > TZBR.ELD3.C3 > AMA.TZBR = 8338-1> GUSAU 81.POOL16-SR = 9021-18.STR > BR9943-DMR.SR. Although these varieties were bred for resistance to MSV and other production constraints like downy mildew and stem borers, the variability being reported would have significant ecological implications in utilising plant induced volatiles as repellents for leafhoppers in controlling MSV and underscores the need to breed for varieties that produce volatiles which are very attractive to biological control agents and repellent to leafhoppers. This is the first report of variability of maize genotypes on herbivores-induced volatiles from an African pest/pathogen complex and is in conformity with previous studies that maize shows relatively high genetic and VOC variability, both intrinsically and in herbivore-induced emissions.Le ma\uefs ( Zea mays L.) est une importante source d\u2019\ue9nergie du peuple d\u2019Afrique sub-saharienne. Malheureusement, sa production est affect\ue9e par la striure brune du ma\uefs (MSV) (Geminiviridae: genus Mastrevirus), un pathog\ue8ne end\ue9mique originaire d\u2019herbes africaines. Ce virus est acquis et transmis de mani\ue8re persistante par la Cicadelle (Homoptera: Cicadellidae). Pour contr\uf4ler ce virus et ses vecteurs, il est possible d\u2019exploiter les produits semiochimiques \ue9mis par les plantes infest\ue9es par les insects. Une \ue9tude \ue9tait effectu\ue9e sur des compos\ue9s organiques volatiles \ue9mis par sept vari\ue9t\ue9s de ma\uefs (Zea mays L.) losrqu\u2019infect\ue9es par la cicadelle ( Cicadulina storeyi China, Homoptera: Cicadellidae), un vecteur important du virus de la striure de ma\uefs (MSV) afin de d\ue9velopper des strat\ue9gies de contr\uf4l de la cicadelle en Afrique sub-saharienne. Les VOCs servent comme signaux pour les parasito\uefdes afin de localiser leurs h\uf4tes. Les volatiles \ue9taient collect\ue9s des jeunes plants des sept vari\ue9t\ue9s apr\ue8s introduction ou pas d\u2019infestation de la cicadelle \ue0 des p\ue9riodes de temps allant de 0 - 24, 24 - 48, 48 - 72 and 72\u201396 heures et combin\ue9e, plutard analys\ue9s \ue0 l\u2019aide du gaz chromatographique. L\u2019analyse des VOCs issus de sept vari\ue9t\ue9s africaines \ue9lites montre une variabilit\ue9 intersp\ue9cifique des VOCs. Le gaz chromatographique (GC) et la combinaison GC- masse sp\ue9ctrom\ue9trie (GC-MS) ont permis l\u2019identification de (z)-3-hexen-1-ol, heptanal, (z)-2-pentenyl acetate, MHO, octanal, myrcene, (z)-3-hexen-1-yl acetatate, (E)-3-hexen-1-yl acetatate, nonanal Linalool, DMNT, (E)-2-decen-1-ol, Methyl salicylate, DMNT, Phenethyl acetate, Indole, cisjasmone, \u3b1-copaene, Geranyl acetone, \u3b2-caryophyllene, \u3b1-bergamotene, (E)-\u3b2-farnesens, TMTT, et Humulene dans des plants infest\ue9es. De tous ces produits pr\ue9cit\ue9s, seuls methyl salicylate, (E)-caryophyllene, (E)-\u3b2- farnesene et TMTT \ue9taient auparavant identifi\ue9s comme volatiles semiochemiques impliqu\ue9s dans la d\ue9fence des plants contre d\u2019autres pestes d\u2019insectes suceurs des plantes. Les plants TZB-SR ont \ue9mis 22 compos\ue9s en pr\ue9sence de la cicadelle en comparaison avec 5 plants non infest\ue9s; alors que BR9943-DMR.SR ont \ue9mis seulement sept compos\ue9s en pr\ue9sence de la cicadelle en comparaison de 6 plants non infest\ue9s. Les compos\ue9s \ue9mis par les sept vari\ue9t\ue9s de ma\ubfs apr\ue8s infestation de la cicadelle \ue9taient classes comme suit par ordre descendant: TZB-SR > TZBR.ELD3.C3 >AMA.TZBR = 8338-1> GUSAU 81.POOL16-SR = 9021-18.STR > BR9943- DMR.SR. Bien que ces vari\ue9t\ue9s \ue9taient dot\ue9es de r\ue9sistance au MSV et autres contraintes \ue0 la production comme le mildew et le stem borers, la variabilit\ue9 manifest\ue9e pourrait avoir des implications \ue9cologiques significatives dans l\u2019utilisation des compos\ue9s volatiles \ue9mis des plantes comme moyen de repousser les cicadelles dans le contr\uf4l du MSV. Ceci souligne le besoin d\u2019entreprendre des \ue9tudes d\u2019am\ue9lioration g\ue9n\ue9tique des vari\ue9t\ue9s pouvant \ue9mettre des compos\ue9s volatiles comme agent de contr\uf4l biologique et repulsif de la cicadelle. Ceci constitue le premier rapport sur la variabilit\ue9 des g\ue9notypes de ma\ubfs eu \ue9gard aux compos\ue9s volatiles organiques \ue9mis \ue0 partir du complexe de peste/pathog\ue8ne africain. Ceci est en conformit\ue9 avec des \ue9tudes ant\ue9rieures qui montrent une relative variabilit\ue9 g\ue9n\ue9tique et des VOCs, tous intrins\ue9quement dans des \ue9missions induites des herbivores