CLAVATA3 Signaling Buffers Arabidopsis Shoot Apical Meristem Activity in Response to Photoperiod

Abstract

Land plants grow throughout their life cycle via the continuous activity of stem cell reservoirs contained within their apical meristems. The shoot apical meristem (SAM) of Arabidopsis and other land plants responds to a variety of environmental cues, yet little is known about the response of meristems to seasonal changes in day length, or photoperiod. Here, the vegetative and reproductive growth of Arabidopsis wild-type and clavata3 (clv3) plants in different photoperiod conditions was analyzed. It was found that SAM size in wild-type Arabidopsis plants grown in long-day (LD) conditions gradually increased from embryonic to reproductive development. clv3 plants produced significantly more leaves as well as larger inflorescence meristems and more floral buds than wild-type plants in LD and short-day (SD) conditions, demonstrating that CLV3 signaling limits vegetative and inflorescence meristem activity in both photoperiods. The clv3 phenotypes were more severe in SDs, indicating a greater requirement for CLV3 restriction of SAM function when the days are short. In contrast, clv3 floral meristem size and carpel number were unchanged between LD and SD conditions, which shows that the photoperiod does not affect the regulation of floral meristem activity through the CLV3 pathway. This study reveals that CLV3 signaling specifically restricts vegetative and inflorescence meristem activity in both LD and SD photoperiods but plays a more prominent role during short days.

Keywords: Arabidopsis; CLV; carpel; meristem; peptide; photoperiod; rosette; signaling; stem cells.

Figure 1

Shoot apical meristem size increases over time in wild-type plants grown in long-day conditions. (A) Col-0 embryonic meristem (EM), (B) Col-0 vegetative meristem at 7 days after germination (VM), and (C) Col-0 inflorescence meristem (IFM) at bolting. (D) Mean meristem diameter and (E) mean meristem height. Lower case letters indicate statistically significant differences (p < 0.001). n = 12–14. Scale bar, 25 µm.

Figure 2

The leaf initiation rate and rosette diameter of wild-type and clv3 plants grown in long-day conditions. A (A) Col plant, (B) clv3-10 plant, and (C) clv3-15 plant at 10 days after germination (DAG). A (D) Col plant, (E) clv3-10 plant, and (F) clv3-15 plant at 15 DAG. The (G) Col plant, (H) clv3-10 plant, and (I) clv3-15 plant at the transition to flowering (TtF). The (J) mean total leaf number and (K) mean rosette diameter +/− standard deviation (S.D.). Asterisks denote statistically significant differences at * p < 0.05 and ** p < 0.0001. n = 22. Scale bar, 1 cm.

Figure 3

The leaf initiation rate and rosette diameter of wild-type and clv3 plants grown in short-day conditions. A (A) Col plant, (B) clv3-10 plant, and (C) clv3-15 plant at 28 days after germination (DAG). A (D) Col plant, (E) clv3-10 plant, and (F) clv3-15 plant at 42 DAG. A (G) Col plant, (H) clv3-10 plant, and (I) clv3-15 plant at the transition to flowering (TtF). The (J) mean total leaf number and (K) mean rosette diameter +/− standard deviation (S.D.). Asterisks denote statistically significant differences at * p < 0.05 and ** p < 0.0001. n = 23–28. Scale bar, 1 cm.

Figure 4

The inflorescence meristem activity of wild-type and clv3 plants grown in long-day or short-day conditions. (A) Col, (B) clv3-10, and (C) clv3-15 inflorescences in LD conditions and (D) Col, (E) clv3-10, (F) clv3-15 inflorescences in SD conditions. The (G) mean inflorescence meristem (IFM) diameter in LD and SD conditions. The (H) mean stage 1-5 floral bud number per IFM in LD and SD conditions. Lower case letters indicate statistically significant differences (p < 0.001). n = 9–14. Scale bar, 20 µm (A,D), 100 µm (B,C,E,F).

Figure 5

The carpel number of wild-type and clv3 plants grown in long-day or short-day conditions. (A) Col, (B) clv3-10, and (C) clv3-15 carpels in LD conditions and (D) Col, (E) clv3-10, and (F) clv3-15 carpels in SD conditions. (G,H) The mean carpel number in (G) long-day and (H) SD conditions. (I,J) The percentage distribution of carpels per flower in (I) LD and (J) SD conditions. White asterisks denote carpels. Black bars indicate the mean. Lower case letters indicate statistically significant differences (p < 0.001). n = 100 gynoecia per genotype. Scale bar, 1 mm.