Submitted to: Planta
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/15/2007
Publication Date: 6/1/2007
Citation: Lopez, L., Camas, A., Sshivaji, R., Ankala, A., Williams, W.P., Luthe, D.S. 2007. Mirl-CP, a novel defense proteinase accumulates in maize vascular tissues in response to herbivory. Planta. 226:517-527. Interpretive Summary: Corn with resistance to leaf feeding by southwestern corn borer and fall armyworm has been developed by infesting whorl-stage plants with newly-hatched larvae and selecting plants that sustained the least leaf-feeding damage. Efforts to determine why some corn genotypes exhibit resistance and others do not have been in progress for several years. A cysteine protease was identified in leaves of the resistant plants. Further research indicated that this protein accumulated rapidly at the site of insect feeding on the leaves. The protease is synthesized in the vascular parenchyma and bundle sheath cells in the leaf. After 24 h of insect feeding, the cysteine protease in the root xylem increased and appeared to move from xylem parenchyma into root metaxylem. The accumulation of the cysteine protease in the vascular elements indicates that it may move through these tissues to protect against insect feeding.
Technical Abstract: When lepidopteran larvae feed on the insect-resistant maize genotype, Mp708, there is a rapid accumulation of a defensive cysteine protease, Mir1-CP, at the feeding site. Silver-enhanced immunolocalization visualized with both light and transmission microscopy was used to determine the location of Mir1-CP in the maize leaf. The results indicated that Mir1-CP is localized predominantly in the phloem of minor and intermediate veins. After 24 h of larval feeding, Mir1-CP increased in abundance in the vascular parenchyma cells and in the thick-walled sieve element. It was also found localized in bundle sheath and mesophyll cells. In situ hybridization of mRNA encoding Mir1-CP indicated the primary sites of Mir1-CP synthesis in the whorl are the vascular parenchyma and bundle sheath cells. In addition to the phloem, Mir1-CP was found in the metaxylem of the leaf and root. After 24 h of foliar feeding, the amount of Mir1-CP in the root xylem increased, and it appeared to move from xylem parenchyma into the root metaxylem elements. The accumulation of Mir1-CP in maize vascular elements suggests Mir1-CP may move through these tissues to defend against insect herbivores.