The Synthesis of Pentyl Leaf Volatiles and Their Role in Resistance to Anthracnose Leaf Blight

Volatiles are important airborne chemical messengers that facilitate plant adaptation to a variety of environmental challenges. Lipoxygenases (LOXs) produce a bouquet of non-volatile and volatile oxylipins, including C6 green leaf volatiles (GLVs), which are involved in a litany of plant physiological processes. GLVs are emitted by a diverse array of plant species, and are the best-known group of LOX-derived volatiles. Five-carbon pentyl leaf volatiles (PLVs) represent another widely emitted group of LOX-derived volatiles that share structural similarity to GLVs, however, relatively little is known about their biosynthesis or biological activity. In this study, we utilized PLV-deficient mutants of maize and Arabidopsis and exogenous PLV applications to elucidate the biosynthetic order of individual PLVs. We further measured PLVs and GLVs after tissue disruption of leaves by two popular methods of volatile elicitation, wounding and freeze-thawing. Freeze-thawing distorted the volatile metabolism of both GLVs and PLVs relative to wounding, though this distortion differed between the two groups of volatiles. These results suggest that despite the structural similarity of these two volatile groups, they are differentially metabolized. Collectively, these results have allowed us to produce the most robust PLV pathway to date. To better elucidate the biological activity of PLVs, we show that PLVs induce maize resistance to the anthracnose pathogen, Colletotrichum graminicola, the effect opposite to that conferred by GLVs. Further analysis of PLV-treated and infected maize leaves revealed that PLV-mediated resistance is associated with early increases of oxylipin α- and γ-ketols, and later increases of oxylipin ketotrienes, hydroxytrienes, and trihydroxydienes. Ultimately, this study has produced the most up-to-date pathway for PLV synthesis, and reveals that PLVs can facilitate pathogen resistance through induction of select oxylipins.

Tillage, Crop Rotation, and Hybrid Effects on Residue and Corn Anthracnose Occurrence in Wisconsin

Corn anthracnose (Colletotrichum graminicola) is an important disease of field corn (Zea mays). Two phases, leaf blight and stalk rot, can reduce yield through either premature leaf senescence or reduced grain harvest due to stalk lodging. Corn residue is an important source of primary inoculum and is increased through cultural practices such as no-tillage and continuous corn cropping, which are common practices in Wisconsin. Field studies conducted at the Arlington Agricultural Research Station (ARS) and the West Madison ARS showed that the incidence and severity of anthracnose leaf blight were higher in continuous-corn crop rotations than in soybean–corn rotations (91% higher incidence, 24 to 78% higher severity). Anthracnose stalk rot was marginally affected by tillage in 2008 (P = 0.09), with higher incidence in chisel-plowed treatments. There was a positive association between spring residue cover and anthracnose leaf blight but no association was found between residue and stalk rot. No association was found between anthracnose leaf blight and stalk rot. There was a negative association between anthracnose leaf blight and yield but not between anthracnose stalk rot and yield. Managing residue levels through crop rotation would help to reduce anthracnose leaf blight but further work is needed to elucidate factors that lead to stalk lodging prior to harvest.

Disruption of a Maize 9-Lipoxygenase Results in Increased Resistance to Fungal Pathogens and Reduced Levels of Contamination with Mycotoxin Fumonisin

Plant oxylipins, produced via the lipoxygenase (LOX) pathway, function as signals in defense and development. In fungi, oxylipins are potent regulators of mycotoxin biosynthesis and sporogenesis. Previous studies showed that plant 9-LOX-derived fatty acid hydroperoxides induce conidiation and mycotoxin production. Here, we tested the hypothesis that oxylipins produced by the maize 9-LOX pathway are required by pathogens to produce spores and mycotoxins and to successfully colonize the host. Maize mutants were generated in which the function of a 9-LOX gene, ZmLOX3, was abolished by an insertion of a Mutator transposon in its coding sequence, which resulted in reduced levels of several 9-LOX-derived hydroperoxides. Supporting our hypothesis, conidiation and production of the mycotoxin fumonisin B1 by Fusarium verticillioides were drastically reduced in kernels of the lox3 mutants compared with near-isogenic wild types. Similarly, conidia production and disease severity of anthracnose leaf blight caused by Colletotrichum graminicola were significantly reduced in the lox3 mutants. Moreover, lox3 mutants displayed increased resistance to southern leaf blight caused by Cochliobolus heterostrophus and stalk rots caused by both F. verticillioides and C. graminicola. These data strongly suggest that oxylipin metabolism mediated by a specific plant 9-LOX isoform is required for fungal pathogenesis, including disease development and production of spores and mycotoxins.