At Plant Biology 2022, a varied group of speakers presented the latest advances from one of the oldest wars that takes place in our world: Plants versus insects, during the “Plants Versus Insects” concurrent symposium on Tuesday, July 12, 2022. Attendees heard news from multiple front lines: cotton, cowpea, and Arabidopsis, and I served as a correspondent to share the details of this gathering with the global plant science community. Let’s go!
Cotton vs Cotton Boll Weevil
During the first talk, Chair Dr. Marcio Alves-Ferreira, from the universidade federal do Rio de Janeiro, Brazil, talked about his latest paper in Current Plant Biology. They aimed to identify molecular players mediating the defense response of cotton (Gossypium spp) to the Cotton Boll Weevil (CBW, Anthonomus grandis), an insect pest that attacks reproductive structures causing severe loss in cotton fiber production. Plants identify herbivores through Herbivore-Associated Molecular Patterns (HAMPs), such as molecules present in insect oral secretions (a combination of regurgitated material from the gut and saliva). HAMPs are recognized by membrane receptors that can activate Mitogen-Activated Protein Kinases (MAKPs), that participate in the transduction of the signal leading to the establishment of the defense response (see Snoeck et al., 2022). The series of events that take place from the recognition of patterns to the set-up of the inducible defenses is collectively known as Pattern-triggered immunity (PTI). PTI has been deeply studied in the interaction between plants and pathogens (for more information and the newest research on the topic, see more from Plant Biology 2022 via Plant Biology EXTENDED, coming soon!). Dr. Alves-Ferreira and collaborators found that different CBW extracts, such as oral secretions or egg extracts, were able to activate MAPKs in cotton and in Arabidopsis. Interestingly, the activation of MAPKs was independent of previously characterized receptors required for the defense against bacteria or fungus. Together, their results showed that HAMPs from CBW activate PTI, although the receptors involved remain unknown (de Moura et al., 2022). Dr. Alves-Ferreira also mentioned that they are working on the analysis of a RNAseq data from cotton leaves infested with CBW in order to keep dinging in the molecular pathways behind the cotton-CBW battle. Stay alert for news!
Know Your Weapons: Not All Jasmonates are the Same!
The next talk was about one of my favorite topics: Jasmonates. For those who haven’t had the chance to talk to me (maybe the lucky ones, he!), jasmonates are a group of lipid-derived compounds that regulate the balance between growth and defense (Wasternack and Feussner, 2018). Ariel Sorg, a PhD student from the Gilroy lab (University of Wisconsin-Madison, USA), found that specific jasmonates are required for different responses, i.e., some jasmonates trigger defenses against herbivores, while others are required for growth repression. By the way, I must mention that they have designed a robot that regularly touches plants to induce stress responses. Besides being super cool, the robot could mimic signals derived from flying insects casually touching leaves.
Cowpea vs Lepidoptera
(with some help from Nicotiana benthamiana and Manduca sexta)
Inceptin, a HAMP present in Lepidoptera oral secretions (Schmelz et al., 2006; Schmelz et al., 2007), enhance the expression of defense genes in cowpea (Vigna unguiculata), such as Kunitz trypsin inhibitors (KTI). KTI are anti-insect proteins that affect the digestion of leaf tissues in the larvae guts, and therefore, are detrimental for growing. KTIs contain a variable number of cysteines that form disulfide bonds required for protein structure and stability (Blow et al., 1974). PhD student Natalia Guayazan Palacios presented her work with the Steinbrenner lab (University of Washington, USA) where they designed a heterologous system to study whether the number of cysteines can also impact KTI anti-herbivore function. They expressed different versions of KTIs in N. benthamiana and performed bioassays with M. sexta. After letting the caterpillars feed on the leaves, the researchers recorded the growth of the caterpillars, and then extracted proteins from different sections of the digestive system of the insects. They followed the presence of KTIs and, as control, peroxidases (inceptin-induced defense proteins with a different activity) by western blot. Only KTIs were found in the guts, which is consistent with their anti-digestive function. Therefore, the N. benthamiana-M. sexta system can be a powerful tool to test protease inhibitors as potential direct defenses. And, with respect to the role of the cysteines in KTI activity, I think we may have interesting news soon!
Plants vs Aphids vs Ladybugs
To keep in line with the multiple advantages of using N. benthamiana as tool, I will continue with the talk of Dr. Georg Jander from the Boyce Thompson Institute, USA. RNA interference (RNAi) technologies are emerging as a powerful tool to control pests. They rely on engineering a plant to express a RNAi that targets insect genes needed for growth or development. However, if the RNAi is not species-specific, it may damage beneficial insects. Dr. Jander and collaborators used N. bentamiana plants expressing a RNAi against green peach aphids (Myzus persicae), and evaluate if the RNAi was transmitted to, and could negatively affect ladybugs (Coccinella septempunctata) that prey on the aphids. For those like me who love ladybugs: don’t worry! Even if RNAi was found in the ladybugs, it can be designed in a way that is only detrimental to aphids. Phew!
Arabidopsis vs Aphids
Finally, also belonging to the aphid world, there was the presentation of Dr. Keyan Zhu-Salzman (Texas A&M University, USA), where she explained a recent paper from her lab about how plants coordinate defenses with their daily rhythm. Circadian clock-regulated defenses allow plants to anticipate pest attacks and allocate resources at the most beneficial time of the day, thus minimizing metabolic cost. A previous report found that CIRCADIAN CLOCK-ASSOCIATED1 (CCA1), a well-known central circadian clock regulator, link daily cycles with defenses against Trichoplusia ni caterpillars (Goodspeed et al., 2012). Dr. Zhu-Salzman and her team found that although a functional circadian clock confer resistance to green peach aphids, CCA1 over expression lines, that lack circadian rhythm, were more resistant to aphid feeding. To solve the mystery behind this apparent contradiction, they performed in-depth data mining using published transcriptomic data sets and found that CCA1 regulates indolic glucosinolates (iGS) biosynthesis. Their results showed that CCA1 has a role in both circadian dependent and independent defenses (Lei et al., 2019).
Guada Fernández-Milmanda is a postdoc in the Specialized Metabolism group in the VIB-UGent Center for Plant Systems Biology, Ghent, Belgium, and an Assistant Features Editor for Plant Physiology. Since her PhD, she has pursued her passion for jasmonates, hormone regulators of plant defenses.