McClung C. Robertson (Orcid ID: 0000-0002-7875-3614) A fiber-optic pipeline lets the root circadian clock see the light C. Robertson McClung Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire, 03755 USA 1. INTRODUCTION Circadian clocks are endogenous timekeepers that allow organisms to synchronize their biological processes with the daily environmental cycles driven by the rotation of the earth. Accumulating evidence supports a role for plant circadian clocks in local adaptation (Yerushalmi & Green, 2009). For example, in a number of species circadian period increases with latitude (Greenham et al., 2017, Michael et al., 2003). Tomato domestication and the concomitant range expansion from the tropics into higher latitudes has selected for altered clock function, including both lagging phase and lengthened period (Figure 1) (Müller et al., 2016). This article is protected by copyright. All rights reserved. This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/pce.13343