Feeding under the radar: A gall-inducing pest of brassica vegetables may reprogram host transcriptomes to manipulate defense

The invasive swede midge (Contarinia nasturtii, Diptera: Cecidomyiidae) is a specialist herbivore of the crucifer family (Brassicaceae), including all economically important varieties of Brassica oleracea, such as broccoli, cabbage, and kale. Swede midge larvae feed on the host apical meristem by tearing cells and secreting digestive saliva, which causes distortion of tissues called ‘galls’. However, little is known about plant defense against swede midge at the molecular level. Preliminary findings in canola (B. napus) suggest swede midge infestation greatly augments host salicylic acid (SA) levels, a primary defense hormone in pathogen-triggered defense and antagonist of jasmonic acid (JA), a key hormone in anti-herbivore defense. We will investigate the chemical and molecular response of three B. oleracea crops to swede midge feeding: broccoli (B. oleracea var. italica), cabbage (var. capitata), and collards (var. acephala), which represent three major lineages of B. oleracea domestication. We hypothesize that (a) damaged host tissues in all varieties will be high in SA and will increase transcription of SA biosynthesis and SA-mediated defense genes; and (b) heading varieties (i.e., broccoli, cabbage) will exhibit transcriptional signatures of cell fortification within the meristem, while the non-heading collards will express growth and branching genes, as compensatory growth has been observed in infested canola. We will analyze phytohormones and mRNA from swede midge-damaged tissue by GC-MS and Illumina RNAseq, respectively. Our combined phytohormone and transcriptome analyses will establish a much-needed molecular model of the response of Brassica crops to this emerging pest.