Jasmonic acid is associated with resistance to twospotted spider mites in diploid cotton (Gossypium arboreum) Junji Miyazaki A,D , Warwick N. Stiller A , Thy T. Truong C , Qian Xu B , Charles H. Hocart C , Lewis J. Wilson A and Iain W. Wilson B A CSIRO Plant Industry, Locked Bag 59, Narrabri, NSW 2390, Australia. B CSIRO Plant Industry, Black Mountain Laboratories, Clunies Ross Street, Black Mountain, ACT 2601, Australia. C Research School of Biology, Mass Spectrometry Facility, The Australian National University, ACT 0200, Australia. D Corresponding author. Email: junji.miyazaki@csiro.au Abstract. The twospotted spider mite (Tetranychus urticae Koch) is capable of dramatically reducing the yield of cotton crops and is often difficult and expensive to control. This study investigated and compared two important plant hormones, jasmonic acid (JA) and salicylic acid (SA), as constitutive and/or induced defence response components in a mite susceptible commercial cotton cultivar, Sicot 71 (Gossypium hirsutum L.) and a resistant diploid cotton BM13H (Gossypium arboreum L.). Foliar application of JA and methyl jasmonate (MeJA) reduced the mite population and leaf damage but application of other potential elicitors, SA and methyl salicylate (MeSA) did not. The concentrations of JA and SA in leaf tissues of induced and non-induced Sicot 71 and BM13H were quantified by liquid chromatography coupled to electrospray ionisation tandem mass spectrometry (LC-ESI-MS/MS). The JA content was constitutively higher in BM13H than Sicot 71 and also highly induced by mite infestation in BM13H but not in Sicot 71. However, SA was not significantly induced in either BM13H or Sicot 71. The expression levels of JA related genes, LOX, AOS and OPR were measured by quantitative PCR and elevated expression levels of JA related genes were detected in mite-infested BM13H. Therefore, JA and MeJA were implicated as key biochemical components in both the constitutive and induced defence responses of BM13H to spider mites. Additional keywords: cotton, jasmonic acid, liquid chromatography tandem mass spectrometry, quantitative PCR, salicylic acid, twospotted spider mites. Received 12 November 2013, accepted 23 January 2014, published online 26 March 2014 Introduction The twospotted spider mite (Tetranychus urticae Koch) is an important secondary pest of cotton (Leigh 1963). T. urticae (hereafter referred to as ‘mites’) pierce the underside of leaves using their mouthparts and suck out the contents of the mesophyll cells (Warabieda et al. 1997), reducing the photosynthetic capacity of damaged leaves (Reddall et al. 2007). Thus, infestation of cotton by mites can cause a reduction of both yield and fibre quality (Wilson 1993). Transgenic Bt-cottons have been commercially available since the mid 1990s. The use of the most recent strain, Bollgard II (Monsanto Co., St Louis, MO, USA), has resulted in reduced insecticide use against Helicoverpa armigera and Helicoverpa punctigera, with kg active ingredient (ai) ha –1 declining by 80% in Australia (Constable et al. 2011). However, spraying against other pests, such as the green mirid (Creontiades dilutus Stal.) is still required. Insecticides targeting green mirids are mostly broad-spectrum and consequently their use reduces beneficial invertebrate populations (predatory insects, spiders and predatory mites) allowing mite populations to increase rapidly to levels that require control (Wilson and Sadras 1998). Further, mites have short life cycles (7–10 days generation time at 30  C) (Miyazaki et al. 2013b), which means they have the capacity to rapidly develop resistance to pesticides targeted against them, such as bifenthrin (Herron et al. 2001), chlorfenapyr (Herron et al. 2004) and organophosphates (Herron et al. 1998). It is therefore highly desirable that cotton cultivars resistant to mites be developed to reduce risks of yield loss and reduce or eliminate pesticide use. A novel source of host plant resistance to mites was reported in genotypes of the diploid cotton species, Gossypium arboreum L. and a Gossypium hirsutum L. race line (Miyazaki et al. 2012). The magnitude of resistance in the G. arboreum genotype was larger than the G. hirsutum race line, in terms of mite density and feeding leaf damage. In addition, G. arboreum genotypes indicated resistance to other pests, such as silverleaf whitefly (Miyazaki et al. 2013a). Hence, the G. arboreum genotype has been targeted for transfer of mite- resistance into commercial G. hirsutum. To expedite this process it would be valuable to have a method that can rapidly assess the resistance against mites of breeding lines that are developed, rather than having to undergo a time consuming bioassay. CSIRO PUBLISHING Functional Plant Biology, 2014, 41, 748–757 http://dx.doi.org/10.1071/FP13333 Journal compilation Ó CSIRO 2014 www.publish.csiro.au/journals/fpb