Intracellular shuttle that promotes movement of SHR from the stele into the endodermis (PubMed:21924907). Required for SHR association to endosomes and localization, and for intercellular movement of SHR (PubMed:25124761). Interacts with SHR, MGP, SCR, JKD, CPC, TMO7 and AGL21, but not with LFY or STM. Intact microtubules promote the localization to endosomes and the cell cortex. Up-regulated by SHR. Embryonic lethal, when homozygous and no visible phenotype, when heterozygous. Hypomorphs (siel-3 and siel-4) have reduced movement of SHR and cause defects in root patterning (PubMed:21924907). Inhibition of microtubules results in mis-localization of SIEL (PubMed:23294290).