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First published online December 30, 2003
doi: 10.1242/10.1242/dev.00998

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LRR-containing receptors regulating plant development and defense

Department of Molecular, Cellular and Developmental Biology, University of Michigan, Ann Arbor, MI 48109-1048, USA

View larger version (22K): [in a new window]   Fig. 1. Life cycle of Arabidopsis thaliana. The mature plant possesses primary and secondary roots, rosette and cauline leaves, and inflorescences. Flowers are composed of sepals, petals, stamens (male reproductive organs) and carpels (female reproductive organs). After pollination, the fertilized egg develops into an embryo inside the silique. The embryo possesses two meristems, a shoot meristem (SAM) and a root meristem (RAM), where new organs and tissues are initiated during post-embryonic growth. Seeds germinate and give rise to seedlings composed of the embryonically-formed hypocotyl and cotyledons. Further organogenesis depends on the action of shoot and root meristems.

View larger version (41K): [in a new window]   Fig. 2. The shoot meristem. Scanning electron microscopy of a wild-type shoot meristem (A) reveals little morphological differentiation, except for the distinct organ primorida (op), whereas molecular and detailed histological studies have revealed important subdomains (PZ, peripheral zone; CZ, central zone; RZ, rib zone) of the shoot meristem (D). The flower meristem gives rise to a regular number of floral organs and organ types (B), eventually forming the fruit (also called the silique; C). In plants carrying a mutation in one of the CLV genes, the shoot meristem can be massively enlarged (E), as a result of expansion of the population of stem cells (H). clv flower meristems also accumulate stem cells, resulting in the formation of additional organs of each type (F), and leading to distorted, club-shaped fruits (G). (I) A model for CLV signaling. The CLV3 protein (blue) is secreted from the stem cells and diffuses to the underlying cell layers, where CLV1 (red) is expressed. There, CLV1 binds to CLV3 and activates signaling to repress WUS expression, as well as to sequester CLV3 to prevent it from diffusing to the WUS-expressing region. CLV2 (green) is a putative co-receptor with CLV1. Scale bar in A and E: 50 µm.

View larger version (17K): [in a new window]   Fig. 3. bri1 mutant plants and signaling. (A) Representation of wild type and the severe dwarf phenotype observed in bri1 mutant plants. Rosette leaves of bri1 plants are `cabbage-like', because of defects in stem elongation, and cell expansion of hypocotyls and petioles. (B) Model for the BRI1 signaling pathway. The type II serine carboxypeptide BRS1 activates a putative steroid binding protein (proSBP), which then associates with the brassinosteriod BR. SBP-BR interacts with the LRR-RLK BRI1 (red) and the receptor BAK1 (dark blue). BAK1/BRI1 then inactivate the GSK3-like kinase BIN2, which is an upstream regulator of BES1 and BRZ1. In the absence of BR, BIN2 is constitutively active, and phosphorylates BES1 and BRZ1, leading to their degradation. When BES1 and BRZ1 are not phosphorylated, they are localized to the nucleus where they activate transcription of brassinosteroid responsive genes.