Maintenance of stem cell populations in plants

Abstract

Flowering plants have the unique ability to produce new organs continuously, for hundreds of years in some species, from stem cell populations maintained at their actively growing tips. The shoot tip is called the shoot apical meristem, and it acts as a self-renewing source of undifferentiated, pluripotent stem cells whose descendents become incorporated into organ and tissue primordia and acquire different fates. Stem cell maintenance is an active process, requiring constant communication between different regions of the shoot apical meristem to coordinate loss of stem cells from the meristem through differentiation with their replacement through cell division. Stem cell research in model plant systems is facilitated by the fact that mutants with altered meristem cell identity or accumulation are viable, allowing dissection of stem cell behavior by using genetic, molecular, and biochemical methods. Such studies have determined that in the model plant Arabidopsis thaliana stem cell maintenance information flows via a signal transduction pathway that is established during embryogenesis and maintained throughout the life cycle. Signaling through this pathway results in the generation of a spatial feedback loop, involving both positive and negative interactions, that maintains stem cell homeostasis. Stem cell activity during reproductive development is terminated by a temporal feedback loop involving both stem cell maintenance genes and a phase-specific flower patterning gene. Our current investigations provide additional insights into the molecular mechanisms that regulate stem cell activity in higher plants.

Fig. 1.

Organization of the Arabidopsis SAM. Confocal laser scanning microscope optical section through a WT flowering SAM and developing floral meristem. The section shows the cell layers (L1, L2, and L3) and the histologically defined domains. The central zone (CZ) lies at the apex of the SAM and harbors the stem cell reservoir. The surrounding peripheral zone (PZ) consists of progenitor cells for lateral organs, and the underlying rib zone (RZ) consists of progenitor cells for the core of the stem. The SAM has been stained with propidium iodide to visualize the cell nuclei. [Modified with permission from ref. (Copyright 2000, Elsevier).]

Fig. 2.

Schematic of the CLV signaling complex. The CLV1 LRR-RLK forms a heteromeric complex with the CLV2 receptor-like protein at the plasma membrane of interior SAM cells. Binding of the CLV3 polypeptide, possibly in association with another protein (X), is proposed to stimulate assembly of an active signaling complex that also contains a phosphatase (KAPP) and a Rho-like GTPase (Rop). The signal is relayed from the cytosol to the nucleus, potentially via a MAPK cascade, to limit WUS expression. P, phosphorylation site; SS, disulphide bond. [Reprinted with permission from ref. (Copyright 2002, Annual Reviews, www.annualreviews.org).]

Fig. 3.

Targeting to the vacuole blocks the activity of CLV3. (A) Scheme of the fusion constructs. The three Vac constructs contain fusions of CLV3 to the C-terminal vacuolar sorting signal from barley lectin (ctVSSBL) or tobacco chitinase A (ctVSSCH). The ctVSSBL with two additional Gly residues (GG), which no longer functions as a vacuolar-sorting signal, was attached at the carboxyl terminus of CLV3 (Sec1) or CLV3-T7 (Sec2). Sec3 contains the full-length CLV3 protein with no additional tag. (B) clv3-2 plants transformed with the fusion constructs. Primary transformants were grouped into six phenotypic classes according to the severity of the meristem phenotype. Class 1 plants showed no transgene activity and resembled untransformed clv3-2 plants. Class 6 plants showed the strongest transgene activity and exhibited a gain-of-function phenotype in which the SAM of each plant terminated prematurely before flowering. Plants in the other classes fell between these two extremes. The graph at the bottom shows for each construct the percentage of transformed plants recovered in each phenotypic class. (C) WT plants transformed with the fusion constructs. Primary transformants were grouped into classes 3, 4, 5, and 6. The graph at the bottom shows for each construct the percentage of transformed plants recovered in each phenotypic class. [Reprinted with permission from ref. (Copyright 2002, American Society of Plant Biologists).]

Fig. 4.

Temporal feedback loop regulating stem cell termination in determinate floral meristems. (Upper) Schematic of an indeterminate SAM, showing the interaction between CLV3 and WUS in their respective domains (gray circles). In the SAM, LFY is absent and AG expression is not induced. (Lower) Schematic of a determinate floral meristem over time. LFY is present throughout the young floral meristem. Both LFY and WUS bind to enhancer sequences and cooperate to induce AG transcription in the center of the developing flower. At the time of carpel (ca) initiation, AG and an additional factor (X) repress WUS expression to terminate stem cell activity. [Reprinted with permission from ref (Copyright 2002, Annual Reviews, www.annualreviews.org).]