Stink bug nezara viridula sustains late MAPKs phosphorylation status and induces expression of genes related with cell wall rearrangement in developing soybean seeds

Abstract

The southern green stink bug (Nezara viridula) is a serious invasive pest in United States and South America that decreases the quantity and quality of soybean seeds. Plants respond to insect attack recognizing cell injury and oral secretions, triggering mitogen-activated protein kinases (MAPK) pathway and inducing defenses against herbivores. Our field studies were conducted to evaluate late MAPKs involvement in defense modulation and the transcriptional response of soybean genes implicated in cell wall modification after stink bugs fed on developing seeds. We observed an induction in MPK3 and MPK6 transcription at 24 and 72 h after insects attacked soybean pods, while MPK6 was the only gene up-regulated after mechanically damaging the seeds. Exposure to JA and SA stimuli increased MPK3 and MPK6 levels. While SA triggered MPK4 activation, MPK3 and MPK6 were phosphorylated after both JA and SA treatments. Stink bugs feeding and SA treatment specifically increased the expression of expansine (EXP), xyloglucan endo-transferase (EXT), pectate lyase (PL), and polygalacturonase (PG) genes, all involved in the relaxation and restructuration of the cell wall. Moreover, examination of safranin-stained seed sections revealed that stink bug damage resulted in thickening of cell walls even on distal undamaged areas of cotyledons. Our study shows that stink bug damage elicits activation of MAPK signal in soybean seeds and induced SA that may induce genes related with cell wall restructuration, and could increase resistance to new insect attack by hardening cell walls.