TERMINAL FLOWER1 is a mobile signal controlling Arabidopsis architecture

Abstract

Shoot meristems harbor stem cells that provide key growing points in plants, maintaining themselves and generating all above-ground tissues. Cell-to-cell signaling networks maintain this population, but how are meristem and organ identities controlled? TERMINAL FLOWER1 (TFL1) controls shoot meristem identity throughout the plant life cycle, affecting the number and identity of all above-ground organs generated; tfl1 mutant shoot meristems make fewer leaves, shoots, and flowers and change identity to flowers. We find that TFL1 mRNA is broadly distributed in young axillary shoot meristems but later becomes limited to central regions, yet affects cell fates at a distance. How is this achieved? We reveal that the TFL1 protein is a mobile signal that becomes evenly distributed across the meristem. TFL1 does not enter cells arising from the flanks of the meristem, thus allowing primordia to establish their identity. Surprisingly, TFL1 movement does not appear to occur in mature shoots of leafy (lfy) mutants, which eventually stop proliferating and convert to carpel/floral-like structures. We propose that signals from LFY in floral meristems may feed back to promote TFL1 protein movement in the shoot meristem. This novel feedback signaling mechanism would ensure that shoot meristem identity is maintained and the appropriate inflorescence architecture develops.

Figure 1.

TFL1 Is a Cytoplasmic 20-kD Protein of Inflorescences. (A) Recombinant (r) CEN and TFL1 proteins (100 ng) were size-fractioned by SDS-PAGE and visualized with Coomassie staining to reveal the predicted size of 20 kD. Equivalent gels were blotted and probed with anti-TFL1 sera (aTFL1), anti-CEN sera (aCEN), or commercial anti-TFL1 sera (aTFL1c). (B) Anti-TFL1 sera were used to probe blots of rTFL1 compared with plant extracts. Total protein extracts were analyzed from inflorescences (1-cm-long shoots) of 16-d-old wild-type, tfl1-1, and tfl1-18 plants. Extracts of 12-d-old vegetative 35S-TFL1 seedlings and 26-d-old ap1 cal shoot meristem tissues were also analyzed. The arrow indicates the 20-kD TFL1 protein. The arrowhead indicates a nonspecific protein of 17 kD detected by the sera. (C) Total (T) protein extracts derived from ap1 cal meristem tissues were fractionated into a high-speed supernatant (S) fraction and membrane pellet (P). Proteins were blotted and probed with anti-TFL1 sera. (D) Total (T) protein extracts derived from ap1 cal meristem tissues were fractionated into a cytosolic (C) and nuclear fraction (N). Equal amounts of proteins were analyzed as in (C) and probed with anti-TFL1 sera (aTFL1). An equivalent blot was probed with anti-histone3 (aH3) sera.

Figure 2.

TFL1 mRNA Patterns in Shoot Inflorescence Meristems. (A) TFL1 mRNA pattern (purple stain) in sections of wild-type plants harvested after 12 LD. Strong TFL1 mRNA was detected in the main shoot and young axillary meristems (arrow). The tissue was counterstained to show cells (white). (B) No TFL1 mRNA was seen in any tfl1-18 meristems (10 LD is shown). (C) and (D) TFL1 expression was seen in 12-d-old tfl1-1 and tfl1-13 mutant plants in axillary meristems (arrow). Bar = 100 μm.

Figure 3.

TFL1 Protein Moves beyond Its mRNA Domain. (A) and (B) TFL1 protein in wild-type (Col) plants harvested after 12 or 16 LD, respectively. TFL1 protein (purple stain) was detected at 12 LD in young axillary shoot meristems (arrow). At 16 LD, TFL1 protein was detected also in the main shoot meristem, its upper axillary meristems, and axillary shoot meristems of rosette leaves. The tissue was counterstained to show cells (white). Bar = 100 μm. (C) TFL1 protein was also detected in wild-type Ler plants harvested after 14 LD. Inset shows higher magnification (×3) of a meristem, highlighting TFL1 in epidermis/L1. (D) TFL1 protein was not detected in tfl1-18 mutant plants harvested after any time point (12 LD is shown).

Figure 4.

TFL1 mRNA in SD Vegetative Axillary Meristems. (A) and (B) TFL1 mRNA expression patterns in sections of wild-type plants grown for 30 SD (A) and then transferred to inductive LD and harvested after 4 LD (B). Bar = 100 μm. (C) and (D) tfl1-18 mRNA expression patterns in tfl1-18 mutant plants after 30 SD ([C]; inset shows that axillary meristems can have weak tfl1-18 mRNA at low frequency) and then after 2 LD induction (D). A section from 2 LD probed for AP1 expression (see Supplemental Figure 4 online) showed that many shoot meristems were present (arrows in [D]). (E) and (F) tfl1-1 mRNA expression pattern in tfl1-1 mutant plants after 30 SD (E) or after 4 LD induction (F).

Figure 5.

TFL1 Protein Moves in Vegetative Meristems. (A) to (D) TFL1 protein in wild-type plants grown for 30 SD (A) or after 3, 5, or 6 LD induction, respectively ([B] to [D]). Axillary meristems with strong TFL1 protein are marked in (C) with asterisks. (E) and (F) TFL1 protein pattern (E) was directly compared with its TFL1 mRNA pattern (F) in sections of the same material harvested after 30 SD + 5 LD. Again, TFL1 mRNA was restricted, while TFL1 protein was throughout the shoot meristems. (G) and (H) No mutant protein was detected in tfl1-18 mutant plants after 30 SD (G), after 1 LD induction (H), or any other times. Dark spots are dirt on tissue/slide. (I) Similarly, no mutant protein was detected in tfl1-1 mutant plants at any time point (30 SD + 2 LD is shown) despite abundant mRNA. Bar = 100 μm.

Figure 6.

TFL1 Protein Patterns in lfy and ap1 cal Mutants. (A) to (D) Analysis of TFL1 protein in lfy mutants ([A] to [C]) and a wild-type segregant (D). The lfy-6 allele was analyzed after 23 LD ([A], main shoot; inset shows secondary shoot with young axillary shoot meristem) or after 27 LD (B). The lfy-14 allele was analyzed after 27 LD (C) and compared with the wild type (D). (E) to (H) TFL1 protein was detected in ap1 cal mutant shoot meristems after 12, 14, 16, and 20 LD, respectively. Bar = 100 μm.

Figure 7.

Model for TFL1 Expression, Movement, Regulation, and Action. TFL1 mRNA is first found throughout all cells of young axillary meristems in the axils of leaves. As the meristem develops, TFL1 mRNA is excluded from the tip, but TFL1 protein moves into this region to coordinate cell identity. In the mature shoot meristem, TFL1 mRNA becomes limited to central cells, while TFL1 protein continues to move to the outer cells (black arrows). In the inflorescence, LFY is expressed in peripheral cells (anlagen) and floral meristems, and signals (gray arrows) from LFY promote TFL1 protein movement in the shoot meristem and thus restrict LFY and AP1 expression.