Greens on the right track
Ms Johnstone, a former East of Scotland 800m and 1500m champion, is in the running to become a list MSP for Lothian.
Lothian has had a Green parliamentarian for longer than any other part of the UK following Mr Harper's election as MSP in 1999.
Ms Johnstone praised his hard work over the past 12 years.
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Hide AdShe said: "I am determined to earn the same backing from voters. Lothian is the Green heart of Scotland, if not the UK."
The Greens will campaign on freeing up cash for public services by repairing the Forth Road Bridge rather than building a new one, opposing the large-scale biomass plant in Leith and on a comprehensive home insulation scheme.