The receptor kinase CORYNE of Arabidopsis transmits the stem cell-limiting signal CLAVATA3 independently of CLAVATA1

Abstract

Stem cells in shoot and floral meristems of Arabidopsis thaliana secrete the signaling peptide CLAVATA3 (CLV3) that restricts stem cell proliferation and promotes differentiation. The CLV3 signaling pathway is proposed to comprise the receptor kinase CLV1 and the receptor-like protein CLV2. We show here that the novel receptor kinase CORYNE (CRN) and CLV2 act together, and in parallel with CLV1, to perceive the CLV3 signal. Mutations in CRN cause stem cell proliferation, similar to clv1, clv2, and clv3 mutants. CRN has additional functions during plant development, including floral organ development, that are shared with CLV2. The CRN protein lacks a distinct extracellular domain, and we propose that CRN and CLV2 interact via their transmembrane domains to establish a functional receptor.

Figure 1.

Phenotype of crn-1 Mutants. (A) Concept of the mutant screen. The arrowheads indicate the possible positions of suppressors in the CLV3 signaling pathway. (B) Cartoon illustrating that suppressor mutants form inflorescence shoots with flowers. (C) Wild-type (Ler) and crn-1 plants, 5 weeks after germination. (D) crn-1 siliques are club-shaped and carry more carpels than wild-type siliques. (E) and (F) An H4:CLV3 transgene arrests SAM development in wild-type (E) but not in crn-1 (F) seedlings. (G) to (I) Increased meristem size in crn-1. SAM diameter was measured across the surface (arrowheads in [H] and [I]) (n = 40; ±se). (J) crn-1 inflorescence at 29°C. The enlarged SAM arrests (arrowhead) with the formation of carpelloid organs. (K) to (M) Wild-type (K) and crn-1 ([L] and [M]) meristems at 22°C. The arrowhead in (L) indicates the extra sepal. In (M), the inflorescence meristem is strongly fasciated. (N) Wild-type flower, with four petals and six stamens. (O) to (S) crn-1 flowers grown under continuous light. (O) and (P) The gynoecium is enlarged, and some flowers carry five petals or additional sepals (asterisk in [P]). (Q) Lateral view of the same flower shown in (O). Two stamens are shorter and lack anthers (arrowhead). (R) Third whorl organs; three stamens lack anthers. The arrow indicates the flower basis. (S) crn-1 stamens may remain immature (arrowhead). In (P) to (S), organs or complete whorls were removed. (T) Organ number in crn-1 mutant flowers (gray bars; n = 20) compared with wild-type flowers (white bars; n = 10) grown under continuous light. The light gray bars indicate the number of stamens lacking anthers. se is indicated. Bars = 10 mm in (C), (E), and (F), 1 mm in (D), (J), and (N) to (S), and 50 μm in (H), (I), and (K) to (M).

Figure 2.

Similarities and Differences between crn and clv Mutants. (A) and (B) Pedicel lengths of crn and clv mutants (n = 120; ±se). Pedicel lengths are reduced in Ler due to a mutation in the ER gene. crn-1, clv2-1, and clv3-2, but not clv1-11, partially suppress the er phenotype. (C) to (F) Overexpression of CLV3 restricts root growth by reducing root apical meristem (RAM) size (n = 12; ±se). crn-1 and clv2-6, but not clv1-14, restore normal root growth. (G) Carpel number of crn-1, clv1-11, and clv2-1 mutants increases from two to four with increased daylengths (n = 200; ±se). clv3-2 mutants respond similarly, but carpel number is higher under all conditions. (H) Carpel number of crn-1 increases when the plant is shifted from short days to continuous light during flowering. The asterisk indicates that young flowers at the shoot tip were removed. Bars = 5 mm in (B) and (H) and 500 μm in (E) and (F).

Figure 3.

CLV3 and WUS Expression in crn-1 Mutants. RNA in situ hybridization was performed on wild-type and crn-1 meristem sections. Bars = 100 μm. (A) to (F) CLV3 expression. (A) and (B) CLV3 is expressed in an expanded domain in crn-1 mutant SAMs. (C) to (F) In wild-type flowers at stage 6, CLV3 expression in the stem cell domain is lost when carpels are formed (asterisk in [D]: stage 3 flower; black arrowheads, carpel primordia; white arrowheads, stamen primordia). Expression is maintained in crn-1 flowers, and a fifth whorl (5W) is formed ([E] and [F]). (G) and (H) WUS expression. Like CLV3 expression, WUS expression expands in a crn-1 SAM.

Figure 4.

Genetic Interactions of CRN. (A) to (C) wus-1 mutants are epistatic to crn-1. wus-1 (A), crn-1 wus-1 (B), and crn-1 (C) plants at 30 d after germination. Arrowheads mark the arrested SAMs. (D) Siliques of crn-1 and pol-1 crn-1. (E) to (H) Top view of inflorescences at 4 weeks after germination. Shoot meristems (arrowheads) of crn clv1-11 double mutants are larger than those of crn-1 and clv1-11 single mutants and comparable to the SAM of clv3-2. In the insets, the SAM is outlined. (I) Stem diameter of crn-1 clv1-11 double mutants is increased compared with that in the single mutants. (J) The wild-type phenotype is restored in crn-1 mutants carrying a transgenic copy of the At5g13290 region. (K) and (L) Overexpression of a CRN cDNA does not affect wild-type development (K) but rescues the crn-1 mutant (L). Arrows in (J) to (L) indicate siliques with two carpels. Bars = 10 mm in (A) to (D), 1 mm in (E) to (I), and 5 mm in (J) to (L).

Figure 5.

Gene Structure and Expression Pattern of CRN. (A) Genomic region of CRN (At5g13290). The 3.3-kb genomic DNA fragment was used for the complementation test. The open reading frames of CRN (boxes, divided into three regions: SP, signal peptide; TMD; and kinase domain) and of the neighboring genes (open boxes) are shown. The mutation in crn-1 causes an exchange of the amino acid at position 70 from G to E. The crn-2 mutant results in an exchange of the amino acid at position 273 from D to N. The T-DNA insertion in the SALK line 056806, the SSLP marker CA72, and the 5′ and 3′ untranslated regions (thick black lines) are indicated. (B) Amino acid sequence of CRN (Col). Amino acids predicted from the Ler sequence and exchanges (boldface) found in the mutant alleles are in brackets. The signal sequence is underlined. The dark-shaded area indicates the TMD, and the light-shaded area indicates the kinase domain. The dotted line indicates amino acids deleted by facultative splicing of the first intron. The triangle shows the position of the second intron. (C) Alignment between the TMDs of CRN and the putative rice ortholog Os CRN. Regions of identity (dark shaded), similarity (light shaded), and difference (unshaded) are indicated. The asterisks mark the four (sm)xxx(sm) motifs. (D) to (H) CRN expression in wild-type embryos. The asterisks indicate weak staining of the embryo-surrounding region. (D) Sixteen-cell stage. The embryo but not the suspensor (black arrowhead) is stained. (E) Late globular stage. (F) to (H) From heart to torpedo stage, CRN expression focuses to the vascular initials of the hypocotyl (arrowhead), the inner cells of the cotyledons (arrow in [G]), and the primary root (arrowhead in [H]). Only weak expression is detected at the position of the developing SAM (arrow in [H]). (I) CRN is expressed in ovules (arrowhead). (J) In the inflorescence, CRN is expressed in the inflorescence meristem (asterisk), young flower primordia (white arrowhead), vasculature of sepals, pedicels, and stem (white arrows), stamen (black arrowheads), and carpel primordia (black arrow). (K) Flower primordium, stage 3. CRN is expressed in the whole flower meristem (white arrowhead) but not in the sepal primordia (arrow). (L) to (O) CRN:GUS reporter line. (L) Expression in the root meristem comprises the quiescent center and surrounding initials and extends apically into the root meristem. (M) Expression in the pericycle at sites of lateral root initiation. (N) Lateral root meristem. (O) CRN is expressed in the cotyledon vasculature. Bars = 25 μm in (D) to (H), 100 μm in (I) to (K), 50 μm in (L) to (N), and 1 mm in (O).

Figure 6.

Models of CLV3 Signaling via Two Independent Receptor Systems. (A) Integration of CRN into the CLV/WUS feedback loop. (B) Speculative model of CLV3 receptor interactions. P, Phosphorylation. CLV3 is depicted as a hydroxylated peptide. Both receptor systems act to restrict stem cell fate in shoot and floral meristems, and their joint activities permit the formation of only two carpels (2.0 C) in the wild type. (C) Model of CLV3 receptor complexes in crn and clv mutants. Recessive mutations in CRN (crn-1; [C1]), CLV1 (clv1-11; [C2]), or CLV2 (data not shown) cause stem cell accumulation to a similar degree, as indicated by the increased carpel number. (C3) crn and clv2 mutants are epistatic to each other, indicating that they act together and independently of CLV1. (C4) CLV1 acts separately from CRN and CLV2; therefore, the double mutant phenotypes are additive. (C5) Double mutants of crn-1 with the dominant allele clv1-1 are phenotypically weaker than the combination of null alleles, suggesting that the clv1-1 receptor has residual signaling activity. (C6) The CRN/CLV2 pathway is subject to inhibition by the clv1-1 protein and therefore is only partially active.