A carotenoid-derived signal controls root length and anchor root formation in Arabidopsis

Anwar S1, Nayak P1, Alagoz Y1, Watkins J2, Hou X2, Pogson B2 and Cazzonelli C1

  1. Hawkesbury Institute for the Environment, University of Western Sydney, Hawkesbury Campus, Bourke Street, Richmond, NSW AUSTRALIA 2753.
  2. Australian Research Council Centre of Excellence in Plant Energy Biology, College of Medicine, Biology and Environment, Research School of Biology, The Australian National University, Canberra, ACT 2601.

Carotenoids are organic pigments that are essential for animal and human health. They provide a precursor for Vitamin A and required for the prevention of eye diseases and certain types of cancer. In plants, they facilitate photosynthesis and photoprotection in chloroplasts. Carotenoids serve as substrates for the production of phytohormones (e.g. strigolactone and abscisic acid) and apocarotenoids (e.g. β-cyclocitral and β-ionone) that facilitate cellular acclimation to environmental stress, promote root-mycorrhiza interactions, maintain shoot dormancy and control nuclear gene expression. A function for a carotenoid-derived signal in controlling root architecture remains to be elucidated. We report the characterisation of a carotenoid mutant displaying a shorter primary root phenotype with enhanced anchor root formation. Carotenoid mutant root tissues accumulate upstream cis-carotenes, yet lack the accumulation of downstream xanthophyll carotenoids. Transcriptomic analysis of carotenoid mutant root tissues revealed a significant enrichment of genes involved in; 1) glucosinolate and glycoside metabolic process, 2) tetrapyrrole and heme binding related functions, and 3) controlling root meristem, nodule and cell division in response to nitrogen stress. A forward genetics approach identified revertants of the carotenoid mutant displaying a wild type-like root architecture. Roots from these revertants displayed an altered sensitivity to chemical inhibitors that impair carotenoid (e.g. norflurazon) and apocarotenoid (e.g. D15) biosynthesis, as well as phytohormone treatments (e.g. strigolactone, abscisic acid, auxin, brassinosteroid and ethylene). A simultaneous mapping and mutation identification by next generation sequencing approach is underway to identify genetic loci that perturb apocarotenoid signal processes that control root architecture. We discuss how a novel apocarotenoid signal pathway controls root architecture, and in particular the formation of anchor roots that provide support and enhance nutrient uptake.