Aboveground to belowground herbivore defense signaling in maize

Abstract

Insect pests that attempt to feed on the caterpillar-resistant maize genotype Mp708 encounter a potent, multipronged defense system that thwarts their invasion. First, these plants are on "constant alert" due to constitutively elevated levels of the phytohormone jasmonic acid that signals the plant to activate its defenses. The higher jasmonic acid levels trigger the expression of defense genes prior to herbivore attack so the plants are "primed" and respond with a faster and stronger defense. The second defense is the rapid accumulation of a toxic cysteine protease called Mir1-CP in the maize whorl in response to caterpillar feeding. When caterpillars ingest Mir1-CP, it damages the insect's midgut and retards their growth. In this article, we discuss a third possible defense strategy employed by Mp708. We have shown that foliar caterpillar feeding causes Mir1-CP and defense gene transcripts to accumulate in its roots. We propose that caterpillar feeding aboveground sends a signal belowground via the phloem that results in Mir1-CP accumulation in the roots. We also postulate that the roots serve as a reservoir of Mir1-CP that can be mobilized to the whorl in response to caterpillar assault.

Induced resistance in maize is based on organ‐specific defence responses

“…Mir1–CP is highly toxic for caterpillars as it damages the insect mid‐gut. Root‐derived Mir1–CP is translocated to the shoot, confirming the crucial role of roots in both local and systemic defence systems (Luthe et al ., ).…”

Nonsensical choices? Fall armyworm moths choose seemingly best or worst hosts for their larvae, but neonate larvae make their own choices

“…Three maize inbred lines, B73, Mp708, and B73- lox10 , selected on the basis of known resistance or susceptibility to fall armyworm, were used in this study, unless noted otherwise. B73- lox10 ( lox10-3 mutant allele) is a jasmonic acid- (JA), terpene-, and green leaf volatile (GLV)-deficient mutant line in B73 background (backcrossed seven times to B73, > 99% similarity to B73), and was included as a susceptible host, relative to B73 [ 57 ]; B73, is a reference inbred line, and was included as an intermediately resistant host, relative to B73- lox10 , and Mp708 [ 57 , 58 ], and; Mp708 is an inbred line bred for resistance to fall armyworm, and was included as an overall resistant host [ 36 , 58 – 60 ]. B73- lox10 and B73 seed were produced by MVK; Mp708 seed was obtained from USDA National Plant Germplasm System, North Central Regional PI Station, Ames, Iowa (Plant ID PI 536520).…”

More beneath the surface? Root versus shoot antifungal plant defenses

“…However, leaf infection of fungal pathogens were recently shown to only trigger minor transcriptional and metabolomic adaptations of roots (Balmer et al, 2013). Hence, it remains highly debatable if there is a root-to-shoot and vice versa communication, a phenomenon notably observed in herbivore defense signaling in maize, were aboveground caterpillar feeding induces the belowground accumulation of the cysteine protease Mir1-CP (Luthe et al, 2011). …”