Growth of plant organs relies on coordinated cell proliferation followed by cell growth, but the nature of the cell-cell signal that specifies organ size remains elusive. The Arabidopsis receptor-like kinase (RLK) ERECTA regulates inflorescence architecture. Our previous study using a dominant-negative fragment of ERECTA revealed the presence of redundancy in the ERECTA-mediated signal transduction pathway. Here, we report that Arabidopsis ERL1 and ERL2, two functional paralogs of ERECTA, play redundant but unique roles in a part of the ERECTA signaling pathway, and that synergistic interaction of three ERECTA-family RLKs define aerial organ size. Although erl1 and erl2 mutations conferred no detectable phenotype, they enhanced erecta defects in a unique manner. Overlapping but distinct roles of ERL1 and ERL2 can be ascribed largely to their intricate expression patterns rather than their functions as receptor kinases. Loss of the entire ERECTA family genes led to striking dwarfism, reduced lateral organ size and abnormal flower development,including defects in petal polar expansion, carpel elongation, and anther and ovule differentiation. These defects are due to severely reduced cell proliferation. Our findings place ERECTA-family RLKs as redundant receptors that link cell proliferation to organ growth and patterning.

The aboveground body of higher plants has a modular structure of repeating units, or phytomers. As such, the position, size, and shape of the individual phytomer dictate the plant architecture. The Arabidopsis (Arabidopsis thaliana) ERECTA (ER) gene regulates the inflorescence architecture by affecting elongation of the internode and pedicels, as well as the shape of lateral organs. A large-scale activation-tagging genetic screen was conducted in Arabidopsis to identify novel genes and pathways that interact with the ER locus. A dominant mutant, super1-D, was isolated as a nearly complete suppressor of a partial loss-offunction allele er-103. We found that SUPER1 encodes YUCCA5, a novel member of the YUCCA family of flavin monooxygenases. The activation tagging of YUCCA5 conferred increased levels of free indole acetic acid, increased auxin response, and mild phenotypic characteristics of auxin overproducers, such as elongated hypocotyls, epinastic cotyledons, and narrow leaves. Both genetic and cellular analyses indicate that auxin and the ER pathway regulate cell division and cell expansion in a largely independent but overlapping manner during elaboration of inflorescence architecture.The aboveground body of higher plants is a consequence of the continual activity of the shoot apical meristem (SAM), which generates repeating units called phytomers. Each phytomer is composed of a node, stem (internode), leaf (lateral organ), and axillary bud, the latter of which allows branching. Modification of the position, size, and shape of the individual phytomer provides immense variations in plant architecture. Such diversity in plant architecture has significance in the domestication and breeding of crop plants. For instance, the maize (Zea mays) teosinte branched1 gene regulates branching pattern, and alteration in the teosinte branched1 expression level played a pivotal role in the domestication of maize (spp. mays) from its wild ancestor, Teosinte (maize spp. Parviglumis; Doebley et al., 1997). Another example is semidwarf varieties of cereals, in which reduction in the internode length has directly contributed to dramatic yield increase (Chrispeels and Sadava, 1994;Peng et al., 1999).The model plant Arabidopsis (Arabidopsis thaliana) forms a typical rosette. During the vegetative stage, the Arabidopsis plant produces rosette leaves with no apparent internodal elongation. As the SAM acquires reproductive characteristics, the primary inflorescence stem elongates rapidly (bolting). First, the SAM gives rise to a shoot that has both vegetative and reproductive characteristics, including bracts (cauline leaves), axillary buds, and branches. Subsequently, the SAM generates multiple floral meristems, each of which differentiates a flower at the tip and a pedicel at the base, while the SAM itself maintains its indeterminate state (Schultz and Haughn, 1991).Molecular-genetic studies have highlighted the in vivo role of phytohormones and growth regulators for normal growth of the inflorescence. For instance, mutants defectiv...