35S Promoter question

[Jelly]

Hello,

I have a problem with a review concerning Arabidopsis flower induction.
TEM and CO are two antagonist genes : CO activates flowering, TEM represses flowering.

The authors use 35S::TEM1/+ 35S::CO plants and say they flower at an intermediate time between 35S::CO plants and 35S::CO 35S::TEM1 plants.

What does the /+ mean ?

Thank you.