Vol. 10, Issue 10 (2021)

Author(s):
Sariga R, Manonmani K, Akila R, Vellaikumar S, Ayyandurai M and Theradimani M

Abstract:
Huanglongbing (HLB) or Citrus greening (CG) disease is one of the destructive diseases causing severe crop decline in Citrus crops (Family: Rutaceae), which is caused by the pathogen, ‘Candidatus Liberibacter asiaticus’ (CLas). It is a systemic pathogen residing in the phloem tissues of Citrus crops. In India, HLB disease is emerging as a major disease in acid lime (Citrus aurantifolia Swingle.) crop. The pathogen and host interaction studies through metabolite profiling revealed the production of many terpene compounds, which are reported to have anti bacterial activity and induce signaling and defense responses in Citrus spp. The present study aiming at profiling the volatile composition of healthy and CLas infected C. aurantifolia leaf was carried out through Gas chromatography-Mass Spectrum (GC-MS). GC-MS chromatogram of the hexanal extract of C. aurantifolialeaves revealed the elution of two different pattern of volatiles in healthy and CLas infected acid lime leaf extract. Major volatiles with the remarkable peak area percentage observed in healthy acid lime leaves were, linalool (73.01%), ß- Sinensal (4.23%), Phytol (5.96%), α- Sinensal (1.15%), ß- Farnesene (1.61%), Germacrene B (1.51%), ß- elemene (1.29%). But distinct pattern of volatiles were observed in considerably higher amounts in CLas infected leaves. They were E- Citral (42.46%), ß- citral (25.58%), Geraniol/Lemonol (8.31%), Citronellal (3.84%), Phytol (2.56%) and Isocaryophyllene (2.22%). The outcome of the study revealed the major differences between these two profiles and also the dominance of linalool in healthy acid lime leaves. The pathogen is Gram negative and phloem residing one and transmitted by phloem feeding insect vector, Asian Citrus Psyllids (Diaphorina citri). Linalools were reported to have antibacterial activity against many Gram negatives including Xanthomonas citri pv. citri and insecticidal activity against phloem feeding insects. In Citrus plants, the oil glands are in the midribs and veins of the citrus leaves as well as in the phloem tissues, the direct effect of volatile compounds on the CLas pathogen and its vector is possible. An exhaustive cum successive volatile profiling studies in popular varieties, in different seasons and locations will be highly useful for using linalool changes in the volatile profile as an indicator for CLas infection in Citrus.