A comparison of changes in phenylalanine ammonia-lyase activity, lignin and phenolic synthesis in the roots of Eucalyptus calophylla (field resistant) and E. marginata (susceptible) when infected with Phytophthora cinnamomi

Abstract

Inoculation of primary roots of Eucalyptus marginata (jarrah, susceptible) and E. calophylla (marri, field resistant) with Phytophthora cinnamomi zoospores caused a lesion to develop within 12–16 h. Lesions in roots of E. calophylla were restricted within 3–4 days and ceased extending whereas those in roots of E. marginata continued to extend. The temporal and spatial changes in phenylalanine ammonia lyase activity, lignin concentration and the concentration of soluble phenolics in roots were followed for 96 h after inoculation. PAL activity increased in inoculated roots of E. calophylla within 24 h of inoculation, beginning at the root tip and then declining to below control levels over the succeeding 24–48 h. A similar pattern of increase in PAL activity, followed by a decline was recorded for four of the five successive 1 cm root segments distal to the root tip. There were no increases in PAL activity in inoculated roots of E. marginata. After inoculation, the amount of lignin m roots of E. calophylla was increased above control levels by as much as 53%. Lignin concentrations in inoculated roots of E. marginata were unchanged. The concentration of suberin remained at pre-inoculation levels throughout the experiment in roots of both species. There were rapid increases in total soluble phenolics in roots of E. calophylla after inoculation, but only minor increases in roots of E. marginata. In the third root segment from the root tip in E. calophylla levels of total phenolics were at their maximum of 97% above controls. Thin layer chromatography of an aqueous methanol extract of infected roots of E. calophylla revealed the presence of several new compounds which were not present in control roots of E. marginata. Treatment of roots of E. calophylla with the PAL inhibitor, aminooxyacetate, altered the resistance response through changes in the concentration of lignin and phenolics and produced what phenotypically resembled susceptibility. These results implicate lignin and phenolic synthesis in the resistance of cucalypts to P. cinnamomi.