Hi, I'm working on the DE analysis of my RNA-seq data from the green algae Chlamydomonas, and I'm able to generate a normal DE result by DESeq2 like this:

I've also downloaded a text file of gene symbol and transcript ID from JGI (https://phytozome.jgi.doe.gov/pz/portal.html):

I'm wondering if there is a direct way to add a column of gene symbols to my DE result by mapping the transcripts ID to the text file above? I've done some research and I'm not sure if the org.Hs.eg.db package can help me to do it Thanks!

There are numerous ways to do this in R, and Rina, has provided one above. 

I think the easiest approach is:

res <- results(dds, tidy=TRUE)

This will put the row names as a column called "row".

Then once your table of additional gene symbols has the first column called "row", you can just do:

names(new.table) <- c("row", "symbol")
m <- merge(res, new.table, all.x=TRUE)

The all.x=TRUE argument says that it should include rows of res even if there is no matching row of new.table.