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Abstract
Maize (Zea mays L.) is one of the major cereal crops cultivated throughout the world with the U.S. producing over 40% of the crop annually. Although several studies have shown the role of maize defense against chewing herbivores, very little is known about how maize mount direct defenses against aphid attack. Corn leaf aphid [(CLA, Rhopalosiphum maidis (Fitch)], is one of the major piercing-sucking pests of maize, which reduces the plant nutritional quality and decreases the yield. The interaction between maize and CLA provides an excellent system to enhance our understanding of innate defense mechanisms deployed by maize against the phloem-feeding insects. The maize genotype, Mp708, developed by classical plant breeding confers enhanced resistance to CLA compared to B73 and Tx601 genotypes. Feeding by CLA on Mp708 triggers the rapid accumulation of the maize insect resistance1 (mir1) transcripts, which encodes a cysteine protease. Feeding behavior studies using Electrical Penetration Graph (EPG) technique revealed that CLA spent less time in the sieve elements of Mp708, thus confirming Mp708’s resistance to CLA is phloem-localized. Benzoxazinoids (BXs), secondary metabolites found in maize have shown to provide resistance to insects. BXs or BX-derived metabolites were insufficient to trigger the maize callose deposition, an important plant defense mechanism against aphids. However, this study indicates that Mp708-mediated resistance to CLA is independent of BXs. The enhanced callose deposition in Mp708 genotype was regulated by the constitutive levels of 12-oxo-phytodienoic acid (OPDA), an intermediate in the jasmonic acid (JA) biosynthesis pathway. Furthermore, OPDA activates ethylene (ET) signaling and mir1 transcript accumulation, suggesting that CLA-triggered callose response in maize requires the concomitant induction of ET signaling pathway and mir1 accumulation. This work also shows that the aboveground feeding by CLA rapidly sends as yet unidentified signals to the roots that trigger belowground accumulation of mir1. The results indicate that aboveground feeding by CLA provides enhanced resistance to subsequent herbivory by belowground feeding of western corn rootworm (WCR), suggesting that roots may also act as a site for toxin synthesis in response to aboveground herbivory. Results from this work will help us to better understand the maize defense signaling mechanisms against phloem-feeding insects.
