OTTAWA, CANADA
University of Ottawa Variations in levels of dehydrodimers of ferulic acid in the pericarp of maize infected by Fusarium graminearum -- Bily, AC, Reid, LM, Regnault-Roger, C, Arnason, JT, Philogène, BJR Phenylpropanoids have been suspected of implication in plant resistance in many different plant-pathogen interactions (Bell, Annu. Rev. Plant Physiol. 32:21-81, 1981). These secondary metabolites are generally induced after elicitation. In maize, different flavones have been found to increase significantly during silk infections in resistant genotypes (Reid et al., Can. J. Bot. 70:1697-1702, 1992). Some other phenolics implicated in resistance are constitutively present in tissues, like 4-ABOA in grain (Miller et al., J. Agric. Food Chem. 45:4456-4459, 1996). We are investigating the implication of cell wall phenylpropanoids in maize resistance to Fusarium graminearum. Ferulic acid and p-coumaric acid are the two predominant constitutive forms of phenolics in the cell wall of grasses. These compounds in free forms are known to be fungistatic for different fungi. In vivo, these compounds are ester-linked to cell wall arabinoxylan residues. A positive correlation has been observed between resistance to Gibberella ear rot and levels of these compounds at harvest (Assabgui et al., Phytopathology, 83(9):949-53, 1991). Ferulic acid has the ability to be dimerized forming cross-links strengthening the primary cell wall (Fry, Ann. Rev. Plant Physiol. 37:165-186, 1986). Maize genotypes with high levels of diferulates present high levels of resistance to leaf borers (Bergvinson et al., Environ Entomol 23:1516-1523, 1994).

During the 2000 field season, we monitored the level of cell wall phenolics and particularly diferulates in inbred lines of maize developed by Agriculture and Agri-Food Canada with improved resistance to Gibberella ear rot (CO432 and CO433). Twelve days after silk emergence, plants of this inbred plus susceptible inbreds were artificially inoculated with a macroconidial suspension of F. graminearum. Control plants were inoculated with distilled water. Ears were collected at 0, 2, 4, 6, 8, 10, 15, 20, 30 days and harvesting time. Ergosterol levels were monitored by HPLC (Young, J. Agric. Food Chem., 43(11), 2904-2910, 1995) in whole grains. Pericarp and aleurone layers were separated from the rest of the grain. Cell wall bound phenylpropanoids were analyzed after NaOH digestion of crude cell wall using a HPLC and LC-MS that we developed.

The levels of diferulates were significantly different among genotypes. More interestingly, in CO433, a resistant genotype, diferulates/ferulate ratio was significantly higher in inoculated pericarps than in the controls during the first six days of infection and after the twentieth day (Figure 1). No such differences were found in CO344, a susceptible genotype. High levels of diferulates impede the action of cell wall hydrolases (Grabber et al., J. Sc. Food Agric. 77:193-200, 1996). The high level of dimerization of the cell wall may impede cell wall degradation and cell disruption necessary for the progression of Fusarium which is a necrotroph phytopathogen. Cross-links of cell wall proteins in response to fungal elicitors have already been described in bean and soybean cell suspension (Bradley et al., Cell 70: 21-30, 1992) and is considered as a quick and early response of the plant to pathogens.

Figure 1. Diferulates/ferulate ratio in maize pericarp of infected (s) and control (l) pericarp and aleurone layers. Disease ratings (m) are also shown (lower values indicate less infection).
 
 

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