Mapping of the Classical Mutation rosette Highlights a Role for Calcium in Wound-induced Rooting

Abstract

Removal of the root system induces the formation of new roots from the remaining shoot. This process is primarily controlled by the phytohormone auxin which interacts with other signals in a yet unresolved manner. Here, we study the classical tomato mutation rosette (ro) which lacks shoot-borne roots. ro plants were severely inhibited in the formation of wound-induced roots and have reduced rates of auxin transport. We mapped ro to the tomato ortholog of the Arabidopsis thaliana BIG and the mammalians UBR4/p600. RO/BIG is a large protein of unknown biochemical function. In A. thaliana, BIG was implicated in the regulation of auxin transport and calcium homeostasis. We show that exogenous calcium inhibits wound-induced root formation in both tomato and A. thaliana ro/big mutants. Exogenous calcium antagonized the root-promoting effects of the auxin IAA, but not of 2,4-D, an auxin analog that is not recognized by the polar transport machinery, and accumulation of the auxin transporter PIN1 was sensitive to calcium levels in the ro/big mutants. Consistent with a role for calcium in mediating auxin transport, both ro/big mutants and calcium-treated wild-type plants were hypersensitive to treatment with polar auxin transport inhibitors. Subcellular localization of BIG suggests that like its mammalian ortholog, it is associated with the endoplasmic reticulum (ER). Analysis of subcellular morphology revealed that ro/big mutants exhibited disruption in cytoplasmic streaming. We suggest that RO/BIG maintain auxin flow by stabilizing PIN membrane localization, possibly by attenuating the inhibitory effect of Ca2+ on cytoplasmic streaming.