Assessing surface wax chemical diversity as a tool to defend against abiotic and biotic stress in canola

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This study identified the chemical composition of surface wax in B. napus and various other Brassica species. It also identified genes responsible for wax production. It advances the possibility of eventually selecting for canola lines better able to produce more or different wax in response to drought and pest threats.

Project Summary

The outer surface of a canola plant (Brassica napus) is covered by a complex mixture of water-repelling material referred to as cuticular wax. This layer plays a role in in prevention of water loss from the plant and in defence against attack by insect pests and fungal pathogens. For canola, little is known about the chemical composition of cuticular wax, its synthesis, biological function, and if there is chemical diversity within Canadian varieties.

A two-year study was conducted by Agriculture and Agri-Food Canada researchers to determine the chemical composition of wax in B. napus and to investigate the distribution of these chemicals on different plant parts and between different canola varieties. To aid in potential breeding efforts for new wax traits, researchers also identified genes encoding enzymes involved in wax biosynthesis and genes that regulate wax production.

Results showed that B. napus wax is a complex mixture of aliphatic (chain like) hydrocarbons, with five main components and many minor ones. The chemical composition of wax in B. napus appears relatively uniform over the plant, with significant differences in composition only seen in petals.

Low chemical diversity of wax was observed between B. napus varieties. The study surveyed canola varieties ranging in registration date from 1966 to 2019. Wax composition was similar for all varieties, but with some variation in total wax load and relative percentage of individual components. The results indicate that the development of Canadian spring canola varieties likely did not result in major changes to the wax profile. Expanding the study to include founder lines from a B. napus nested association mapping population (NAM) indicated that chemical diversity of epicuticular wax in B. napus is limited. Wax composition of other Brassica species B. rapa, B. oleracea, B. juncea and B. carinata was determined from multiple tissues of greenhouse grown plants.

Preliminary studies conducted on the effect of the environment on wax production indicates that the amount of wax on the plant increases under drought stress, but with little change in overall composition. Small but significant differences were seen when greenhouse grown plants were compared with the same varieties grown in the field, providing further evidence for the role of the environment in wax biosynthesis. No significant response in wax load or composition was seen under high or low nitrogen in field grown plants.

An epidermal transcriptome was developed and used as a resource to identify genes encoding proteins involved in wax biosynthesis and its regulation. The most highly expressed genes encode small proteins that are thought to play an important, but at this time unknown, function in wax export into the cell wall and may also be involved in host recognition or plant defence against fungal pathogens.

This work has considerably enhanced our understanding of wax chemistry and biosynthesis in B. napus and identified gaps where further knowledge is required. For example, knowing the chemistry of wax will help in determining how different components function in plant defence.