genes G C A T T A C G G C A T Review Circadian Clock Components Offer Targets for Crop Domestication and Improvement C. Robertson McClung   Citation: McClung, C.R. Circadian Clock Components Offer Targets for Crop Domestication and Improvement. Genes 2021, 12, 374. https://doi.org/10.3390/genes1203 0374 Academic Editor: Paloma Mas Received: 30 January 2021 Accepted: 4 March 2021 Published: 6 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA; c.robertson.mcclung@dartmouth.edu; Tel.: +1-603-646-3940 Abstract: During plant domestication and improvement, farmers select for alleles present in wild species that improve performance in new selective environments associated with cultivation and use. The selected alleles become enriched and other alleles depleted in elite cultivars. One important aspect of crop improvement is expansion of the geographic area suitable for cultivation; this frequently includes growth at higher or lower latitudes, requiring the plant to adapt to novel photoperiodic environments. Many crops exhibit photoperiodic control of flowering and altered photoperiodic sensitivity is commonly required for optimal performance at novel latitudes. Alleles of a number of circadian clock genes have been selected for their effects on photoperiodic flowering in multiple crops. The circadian clock coordinates many additional aspects of plant growth, metabolism and physiology, including responses to abiotic and biotic stresses. Many of these clock-regulated processes contribute to plant performance. Examples of selection for altered clock function in tomato demonstrate that with domestication, the phasing of the clock is delayed with respect to the light–dark cycle and the period is lengthened; this modified clock is associated with increased chlorophyll content in long days. These and other data suggest the circadian clock is an attractive target during breeding for crop improvement. Keywords: domestication; crop improvement; circadian rhythm; circadian clock; molecular breeding; photoperiodic flowering 1. Introduction 1.1. Plant Domestication About 10,000 years ago, shortly after the end of the most recent ice age, humans began the transition from foraging to farming with extraordinary consequences for both the human domesticators and their domesticated plants and animals, as well as for their non-domesticated counterparts [1]. Plants were domesticated in multiple regions around the globe. For example, emmer and einkorn wheat, barley, peas, lentils, chickpeas, and flax were domesticated in the so-called Fertile Crescent of southwest Asia [1]. Rice was domesticated in China [2] and teosinte was domesticated to maize in Mesoamerica [3]. Plant domestication entails the exposure of wild species to new selective environments associated with human cultivation and use [1,4,5]. Although the ways in which plants are cultivated and used (for example, plants can be grown for fruits or seeds versus vegetative organs) influences the traits selected for during domestication, it is generally recognized that a common suite of traits are associated with the domestication of seed and fruit crops [1,6]. Typically, domestication is associated with increased fruit or grain size, although often the number of fruits or seeds is reduced, an increase in overall plant robustness, more determinate growth or increased apical dominance (reduced growth of side stems in comparison to the central stem), and a retention of the seeds on the plant for easy harvest. Other commonly encountered domestication traits include a loss of seed dormancy, a decrease in bitter substances in edible structures, changes in photoperiod sensitivity, and synchronized flowering [1]. Genes 2021, 12, 374. https://doi.org/10.3390/genes12030374 https://www.mdpi.com/journal/genes