Avocado (Persea americana) – a major fruit crop worldwide – is threatened by root rot caused by Phytophthora
cinnamomi. This pathogen is known to infect the plant via the feeder roots leading to branch dieback,
and eventually tree mortality. While it is known that different avocado rootstocks have varying degrees
of susceptibility to Phytophthora root rot, little research has been done on the avocado–Phytophthora
interaction. In this study, transcript abundance levels of defence-related genes coding for phenylalanine
ammonia-lyase, lipoxygenase, pathogenesis-related protein 5, endochitinase, gluthathionine S-transferase
and metallothionein were characterised and compared among five rootstocks with varying susceptibility to
root rot, after exposure to P. cinnamomi. Root samples were collected at 0 h, 3 h, 6 h, 12 h, 24 h, 48 h and
72 h post-infection and transcript abundance of the defence-related genes was determined using quantitative
real-time reverse transcription PCR. The results indicated the involvement of PR-5 and endochitinase in the
defence response of all avocado rootstocks to P. cinnamomi but these genes could not be directly linked to
the observed phenotypic resistance. PR-5 and endochitinase were highly upregulated at 72 h post-infection.
Differences in transcript abundance of phenylalanine ammonia-lyase and lipoxygenase genes were seen
when comparing tolerant and less tolerant rootstocks, which may suggest that transcripts of these genes
contribute to resistance. These data provide important insights into plant defence and into how different
avocado rootstocks may exhibit increased resistance to infection by P. cinnamomi.