Anchor Root Development: A World within Worlds

Many plant species develop shoot-borne adventitious roots(ARs)with specialized functions that contribute to plant adaptability by enhancing resource capture and stress avoidance.In Arabidopsis,as in many dicots,the postembryonic root system is primarily composed of lateral roots(LRs),which originate from the xylem pole pericycle cells through a process that involves periodic oscillation in gene expression mediated by a β-carotenoidderived signal(Dickinson et al.,2019).Our understanding of the key molecular and signaling events regulating AR initiation has recently been obtained from studies in Arabidopsis,where ARs are formed on etiolated hypocotyls upon transfer to light(Lakehal et al.,2019)or from excised leaf explants(Zhang et al.,2019).Anchor roots(ANRs)are a type of AR that originate immediately below the root-hypocotyl junction in an auxindependent manner and that require a diapocarotenoid-derived signal(anchorene)to emerge(Jia et al.,2019).ANR primordia activation may represent a generic mechanism to compensate for the loss of the growth potential of the primary root(PR),as it is enhanced by mutations affecting PR growth,by whole-root excision or by nitrogen deficiency(Jia et al.,2019).A recent paper by Bai et al.(2020)provides direct evidence that a regulatory module involving the AP2/ERF transcription factor ABSCISIC ACID INSENSITIVE 4(ABI4),the mitogen-activated protein kinases(MAPKs)MPK3 and MPK6,and the phosphatase PP2C12 controls ANR emergence in Arabidopsis.