IBIP seminar – “Development and architecture of root systems”

Thursday 9 April 2015

A small local defect with big systemic effects: protophloem differentiation, phosphoinositides & root branching

Christian Hardtke
Department of Plant Molecular Biology, University of Lausanne

In Arabidopsis mutants with impaired primary root protophloem differentiation, brevis radix (brx) and octopus (ops), meristematic activity and overall root growth are strongly reduced. 2nd site mutation in the protophloem-specific 5-phosphatase COTYLEDON VASCULAR PATTERN 2 (CVP2) partially rescues brx defects. Consistently, both CVP2 hyperactivity and mutation of protophloem-specific 5-kinases result in brx/ops root phenotypes. Paradoxically however double mutants of cvp2 and its homolog cvl1 with strongly reduced 5-phosphatase activity also display brx/ops root defects. Thus, tightly balanced phosphatidylinositol-4,5-biphosphate (PIP2) levels are essential for protophloem differentiation. Genetically, OPS is an effector of PIP2 balance, since cvp2 mutation cannot rescue ops defects, whereas increased OPS dosage rescues cvp2 cvl1 defects. Finally, all protophloem mutants display systemic shifts in auxin response, where auxin activity is reduced in the meristematic zone, but increased in the differentiation zone. This phenotype is associated with a higher frequency of lateral root initiation and can also be created through peptides that specifically suppress protophloem differentiation. Discontinuous protophloem strands thus create an “auxin traffic jam” as a consequence of suboptimal auxin delivery into the meristem, thereby systemically shaping root system architecture through a range of secondary effects that encompass formative cell divisions as well as hormone activities.

Contact : Alexandre Martinière

Contacts IBIP :
Sabine Zimmermann
Alexandre Martiniere
Christine Granier
Chantal Baracco