Abstract:
Avocado is a globally significant fruit tree crop. However, Phytophthora root rot, caused by Phytophthora cinnamomi (Pc), is the primary limitation to production in most countries. This soil borne oomycete pathogen infects and damages avocado tree root systems, leading to canopy decline and reduced yields. The disease can be managed successfully in an integrated manner, but not totally eliminated. An important aspect of integrated control that could potentially provide a lasting solution is the use of Pc-resistant rootstocks. Although selection of these rootstocks is time-consuming, better understanding of the host-pathogen interaction may aid in defence marker identification which could enhance selection efficiency.
This study used Pc-susceptible and resistant rootstocks to examine the histological and/ or quantitative biochemical differences in callose, lignin, reactive oxygen species (ROS), antioxidants, phenolics, tyloses and gums in response to infection. This was the first quantitative study of callose and lignin deposition in R0.38, R0.06 and R0.09, and the first time Pc-resistance mechanisms were examined in R0.38.
Notably, the early pervasive growth of Pc into the stele of R0.38 concurred with its susceptibility to Pc. Although Pc-susceptible R0.38 produced more callose than Pc-resistant R0.06 at the earliest time point of 6 hours post inoculation (hpi), Pc-resistant R0.09 produced the most callose in the entire trial at the proposed biotrophic to necrotrophic switch of 12 hpi in Pc. This response in R0.09 is a potential biochemical resistance marker to Pc. There were no significant increases in lignin deposition in response to infection, but resistant rootstocks showed a significant decrease in lignin at 24 hpi, which may have been due to degradation by Pc. Hence the study of lignin is unlikely to aid defence marker identification. There were no significant changes in ROS production, possibly due to assay insensitivity, but all rootstocks did show significant changes in antioxidant production. This suggests that there may have been small changes in ROS, which could have been linked to defence signalling. Phenolics and gums were produced in greater abundance and earlier in R0.38, as compared to the resistant rootstocks, and hence they were not associated with resistance to Pc but possibly susceptibility. Therefore they are not useful in the search for resistance markers. The role of tyloses in defence against Pc was inconclusive, as tyloses were observed in both R0.38 and R0.09 at the same time point of 12 hpi.
