The single evolutionary origin of chlorinated auxin provides a phylogenetically informative trait in the Fabaceae

Abstract

<p>Chlorinated auxin (4-chloroindole-3-acetic acid, 4-Cl-IAA), a highly potent plant hormone, was once thought to be restricted to species of the tribe Fabeae within the Fabaceae, until we recently detected this hormone in the seeds of <i>Medicago, Melilotus</i> and <i>Trifolium</i> species. The absence of 4-Cl-IAA in the seeds of the cultivated species <i>Cicer aeritinum</i> from the Cicerae tribe, immediately basal to the Fabeae and Trifolieae tribes, suggested a single evolutionary origin of 4-Cl-IAA. Here, we provide a more robust phylogenetic placement of the ability to produce chlorinated auxin by screening key species spanning this evolutionary transition. We report no detectable level of 4-Cl-IAA in <i>Cicer echinospermum</i> (a wild relative of <i>Cicer aeritinum</i>) and 4 species (<i>Galega officinalis, Parochetus communis, Astragalus propinquus</i> and <i>A. sinicus</i>) from tribes or clades more basal or sister to the Cicerae tribe. We did detect 4-Cl-IAA in the dry seeds of four species from the genus <i>Ononis</i> that are either basal to the genera <i>Medicago, Melilotus</i> and <i>Trigonella</i> or basal to, but still within, the Fabeae and Trifolieae (ex. <i>Parochetus</i>) clades. We conclude that the single evolutionary origin of this hormone in seeds can be used as a phylogenetically informative trait within the Fabaceae.</p