Shoot apical meristems (SAMs), which maintain stem cells at the tips of stems, and axillary meristems (AMs), which arise at leaf axils for branch formation, play significant roles in the establishment of plant architecture. Previously, we showed that, in Arabidopsis thaliana, activation of NB-LRR (nucleotide-binding site-leucine-rich repeat)-type UNI proteins affects plant morphology through modulation of the regulation of meristems. However, information about genes involved in the processes was still lacking. Here, we report that ERECTA (ER) receptor kinase family members cooperatively mediate the morphological alterations that are stimulated by activation of UNI proteins. uni-1D is a gain-of-function mutation in the UNI gene and uni-1D mutants exhibit early termination of inflorescence stem growth and also formation of extra AMs at leaf axils. The former defect involves modulation of the SAM activity and is suppressed by er mutation. Though the AM phenotype is not affected by a single er mutation, it is suppressed by simultaneous mutations of ER-family members. It was previously shown that trans-zeatin (tZ)-type cytokinins were involved in the morphological phenotypes of uni-1D mutants and that expression of CYP735A2, which is essential for biosynthesis of tZ-type cytokinins, was modulated in uni-1D mutants. We show that this modulation of CYP735A2 expression requires activities of ER-family members. Moreover, the ER activity in UNI-expressing cells contributes to all morphological phenotypes of uni-1D mutants, suggesting that a cross-talk between ER-family-dependent and UNI-triggered signaling pathways plays a significant role in the morphological alterations observed in uni-1D mutants.