Game of 'Mones: Comprehending Bemisia tabaci MEAM1 Nymph-Based Resistance and Defense Phytohormone Signaling in Alfalfa
Hemipteran insects are a significant threat to food security in California and worldwide; their piercing-sucking mouthparts make them difficult for host plants to perceive. Of these insect pests, the whitefly (Bemisia tabaci) is a cosmopolitan pest which stunts plant growth and development, vectors viruses, and also secretes honeydew which can result in sooty mold growth on host plants. Whiteflies are extant on all continents except Antarctica and climate change increases the propensity superabundant whitefly populations will be more common worldwide over time. Whiteflies are difficult to control as the release of natural enemies in crops has limited effectiveness and whiteflies circumvent the pesticides by rapidly adapting by developing insecticide resistance. For this reason, integrated pest management (IPM) programs centered around host plant resistance (HPR) may be the most effective means of controlling whiteflies. An effective, nymph-based whitefly-resistance mechanism was identified in alfalfa. While this mechanism may greatly inhibit whitefly population over generations, the polyploid nature and limited genomic resources in alfalfa make elucidating this resistance a challenge. Here, we describe whitefly-resistance found among three alfalfa populations. Upon screening 84 individual lines from the three populations (two resistant and one susceptible), we concluded whitefly resistance was multigenic as a continuous spectrum of phenotypes from highly resistance to highly susceptible in each population. We identified several highly resistant (R1, R2 and R3) and susceptible lines of alfalfa (S1) for further studies. Through a series of experiments exploring B. tabaci MEAM1, MED1 and NW1 behaviors, we determined the whitefly-resistance displayed in R1, R2 and R3 lines were distinct and both antibiosis and antixenosis were detected for all three whitefly species. MEAM1 nymph mortality was displayed in all R lines, while the nymph mortality mechanisms did not impact MED whitefly. In addition, differences in host-choice, adult longevity and fecundity on R1, R2, R3 and S1 lines were whitefly-species specific. To gain insights into the mechanisms of resistance deployed in R1 plants vs S1 plants, B. tabaci MEAM1 whitefly-infestation time courses in R1 and S1 plants were performed. To understand the phytohormone underpinnings of alfalfa’s defense response to whitefly, S1 alfalfa’s response to salicylic acid (SA) and jasmonic acid (JA) treatments was also evaluated. De novo transcriptomic assembly of these libraries led us to postulate alfalfa’s whitefly resistance mechanism is independent of SA, JA and abscisic acid (ABA) signaling and is ET-dependent. In addition, the downregulation of several pattern-triggered immunity receptors, suggests defense signaling in R1 plants is distinct for S1 plants and unique in the resistance responses reported to date in Hemipteran literature. In addition, R1 alfalfa have substantial difference in the expression of cutin, wax and suberin biosynthesis transcripts implicating the role of cuticle/cell wall alterations in R1’s whitefly resistance. Analysis of phytohormone-response libraries led us to conclude alfalfa’s SA and JA responses are distinct from Arabidopsis as there was no evidence for reciprocal regulation of SA and JA responses and a substantial number of genes are responsive to both hormones. Finally, unlike the previously characterized basal immunity response Arabidopsis to whiteflies, where JA has an important role in deterring nymph development, and there is little correlation between alfalfa’s whitefly response and SA- and/or JA-regulated genes. Collectively, these data provide the first insights into the alfalfa’s mechanism of resistance to the global pest Bemisia tabaci.