Gibberellin Metabolism and Signaling
Introduction
Unlike mammals, plants have evolved to be very plastic in their development. Every plant cell is ostensibly a “stem cell” capable of giving rise to a wide array of developmental fates in response to signals from plant hormones, also referred to as phytohormones. Also, unlike mammals, plants do not have clearly defined source and target organs for hormone signals. This has complicated the study of plant hormones. Numerous advances have been made in understanding the regulation of plant hormone accumulation, transport, and signaling through genetic, biochemical, and physiological approaches. This review is focused on the plant hormone gibberellin.
Gibberellins are a large family of tetracyclic diterpene plant hormones characterized by the ent‐gibberellane ring system (Fig. 1). Gibberellins have been shown to promote many facets of plant growth and development including germination and stem elongation, and in most species transition to flowering, pollen tube elongation, and seed development (Olszewski 2002, Sun 2004). Every hormone signal transduction pathway is composed of two essential components, the control of hormone accumulation and reception of the hormone signal. This chapter will: (1) briefly review the history of GA research and the role of GA in regulating plant growth and development; (2) review the control of GA hormone accumulation through gene regulation; (3) review GA signal reception in the context of its role in plant growth and development; and (4) review the interaction of GA signaling with other hormone‐signaling pathways.
Gibberellins were the first plant hormone identified (Phinney 1983, Tamura 1991). Ironically, the discovery of gibberellin by the Japanese scientist Eiichi Kurosawa in 1926 was based on its synthesis by the fungus Gibberella fujikuroi, the causative agent of bakanae disease in rice. The “foolish seedlings” infected by bakanae disease grew excessively tall and spindly. The rare infected seedlings that survived produced poor seed set. Kurosawa demonstrated that the fungal pathogen infecting these plants synthesized a chemical that could stimulate shoot elongation in rice and other grasses (Kurosawa, 1926). The structure of this chemical, gibberellin A3 or GA3, was proposed in 1956 and revised in 1961. The occurrence of gibberellins in higher plant species was discovered in the mid‐1950s. This discovery marked the beginning of research on the role of GA in plant growth and development.
Since their discovery, over 136 GAs have been identified in plants and fungi; however, only a small fraction of these are biologically active in plants (Olszewski et al., 2002). Each unique GA has a number ranging from GA1 to GA136. Gibberellins are divided into two classes based on the number of carbon atoms, C20‐GAs and C19‐GAs, in which C20 has been replaced by a gamma‐lactone ring. The synthesis of bioactive GAs is essentially a three‐step process involving: (1) the formation of ent‐kaurene in the proplastid, (2) the formation of GA12/53 in the ER, and (3) the formation of active GA in the cytoplasm by successive oxidation steps. In most plant species, GA1 or GA4 are the bioactive GA. GA1 and GA4 are formed by similar pathways differing only in early 13‐hydroxylation in the case of GA1.
The role of GA in plant growth and development has been elucidated through the physiological characterization of GA biosynthesis and signaling mutants and the characterization of GA‐responsive genes. This section deals with the role of GA in seed development and germination, plant growth and elongation, flowering, and meristem cell identity.
Our understanding of GA in seed development and germination is based on mutants or tissues with reduced accumulation of GAs (Bentsink 2002, Ni 1993, Singh 2002). For example, the ga1, ga2, and ga3 mutants of Arabidopsis were isolated in an elegant screen for GA‐dependent germination by Koornneef and van der Veen (1980). These mutants cause marked reduction in endogenous GA and are unable to germinate unless GA is applied externally. While seeds are an excellent source of GA, the failure to synthesize GA in these mutants does not completely block seed development (Bentsink and Koornneef, 2002). Thus, it was originally thought that GA is not required for seed development. However, physiological characterization of Arabidopsis plants constitutively expressing the GA catalytic enzyme GAox2 revealed that reduced accumulation of GA in seed leads to increased probability of seed abortion (Singh et al., 2002). This suggests that GA is actually required in seed development. Moreover, reduced GA accumulation leads to reduced seed set by interfering with pollen tube elongation and silique expansion (Singh 2002, Swain 2004). How does GA stimulate germination? Germination and seedling growth require the production of hydrolytic enzymes to weaken the seed coat, mobilize seed nutrient storage reserves, stimulate plant embryo expansion and hypocotyl elongation, and activate the embryo meristem to produce new shoots and roots (Bewley and Black, 1994). Gibberellin has been implicated in all of these processes.
The germination process is considered complete when any part of the plant embryo emerges from the seed (Bewley and Black, 1994). Initial studies in tomato and muskmelon suggested that the decision to germinate results from the balance between the internal pressure of an expanding embryo and the external restraint of the endosperm cap or seed coat (Groot 1987, Ni 1993). Gibberellin‐induced hydrolytic enzymes such as endo‐[β]‐mannase are apparently needed to weaken the endosperm cap in these species (Still and Bradford, 1997).
Gibberellin stimulation of seed nutrient storage mobilization is best illustrated by the cereal aleurone system (Jacobsen et al., 1995). Gibberellin synthesized by the plant embryo stimulates secretion of the hydrolytic enzymes including α‐amylase by the aleurone layer. Aleurone‐derived hydrolases diffuse to the adjacent endosperm where they degrade starch for use by the embryo (Fig. 2). Because the aleurone layer itself secretes no GA, it can be isolated and used to assay α‐amylase secretion in response to hormone (Bush 1988, Varner 1965). α‐Amylase is arguably the best characterized GA‐responsive gene. Measurement of α‐amylase enzyme activity and mRNA accumulation has been used to identify GA‐responsive promoter elements and transcription factors (Sun and Gubler, 2004).
Gibberellin stimulation of plant stem elongation was the basis for the hormone's discovery and remains a reliable assay for GA response. Research suggests that GA stimulates stem elongation through stimulation of cell elongation and cell division (Huttly and Phillips, 1995). Gibberellin treatment causes microtubules to reorientate so as to encourage axial elongation (Shibaoka, 1994). It is thought that GA promotes cell elongation by induction of enzymes that promote cell wall loosening and expansion such as xyloglucan endotransglycosylase/hydrolase (XET or XTH), expansins, and pectin methylesterase (PME). Xyloglucan endotransglycosylases split cell wall xyloglucan polymers endolytically and then rejoin the free ends with another xyloglucan chain (Campbell and Braam, 1999). Xyloglucan endotransglycosylase activity has been associated with expanding regions and shown to be GA‐induced in Arabidopsis, lettuce, and pea (Kauschmann 1996, Ogawa 2003, Potter 1993). Expansins disrupt hydrogen bonding in the cell wall and appear to be GA‐induced in Arabidopsis and rice (Cosgrove 2000, Lee 2001, Ogawa 2003). Pectin methylesterase is thought to induce stem elongation by loosening the cell wall via pectin modification and is GA‐induced in Arabidopsis (Ogawa et al., 2003). Gibberellin was first shown to stimulate growth through induction of the cell cycle in rapidly growing deepwater rice (Sauter et al., 1995). In rice, GA induces expression of the cyclin cycA1;1 and the cyclin‐dependent kinase cdc2Os‐3 in the G2/M phase transition (Fabian et al., 2000). Microarray analysis in Arabidopsis has demonstrated GA induction of genes involved in the G1/S transition including cyclinD, MCM, and replication protein A (Ogawa et al., 2003). Further research on the mechanism of GA induction of these genes and their exact mode of action is needed.
In most species, the transition to floral development is stimulated by gibberellins (Sun and Gubler, 2004). However, gibberellins are not the sole factor in determining transition to flowering. In Arabidopsis, a facilitative long‐day (LD) plant, transition to flowering is controlled by the integration of signals from the GA pathway, the autonomous pathway, the vernalization pathway, and the light‐dependent pathway (Komeda, 2004). It is clear that gibberellins are required for transition to flowering in short days (SD, 8‐h light) because the strong GA biosynthesis mutant ga1‐3 cannot transition to flowering without application of GA under these conditions (Wilson et al., 1992). The failure of ga1‐3 to flower under SD appears to be due to reduced expression of the LEAFY (LFY) gene (Blazquez et al., 1998). The fact that the ga1‐3 mutant causes poor development of floral organs including petals and stamen shows that GA is also involved in the stimulation of floral development. Gibberellin has also been shown to induce expression of floral homeotic genes APETELA3, PISTILLATA, and AGAMOUS (Yu et al., 2004).
Studies on Lolium temulentum have suggested that GA is an inducer of flowering or “florigen” in LD‐responsive grasses (King and Evans, 2003). In Lolium, GA1, GA3, and GA4 are more active for stem elongation, whereas GA5 and GA6 are more active in triggering transition to flowering. It has been proposed that GA5 and GA6 are more active in the floral meristem because they have greater resistance to the expression of the GA catabolic enzyme GA2ox early in floral induction.
Studies in Arabidopsis have indicated an emerging role for GA in shoot apical meristem (SAM) cell identity (Hay 2002, Hay 2004). The SAM is a reservoir of undifferentiated cells that gives rise to the aerial leaves and stems of higher plants. Knotted‐like homeobox (KNOX) transcription factors appear to control meristem versus leaf cell identity. The KNOX gene SHOOTMERISTEMLESS (STM) has been shown to prevent expression of the GA biosynthesis gene GA20ox1 in the SAM (see Section II). The fact that ectopic GA signaling is detrimental to meristem maintenance suggests that GA signaling is antagonistic to meristem cell identity and may be involved in the transition from meristem to leaf cell fate.
Section snippets
Gibberellin Signal Transduction
Much has been learned about GA‐signal transduction using a combination of genetic, physiological, and biochemical analyses. Regulatory elements of the GA‐signal transduction pathway have been identified using: (1) screens for mutants with altered GA sensitivity, (2) identification of transcriptional regulators of the GA‐responsive genes, and (3) methods for identifying differentially expressed genes. Such approaches have recovered both positive and negative regulators of GA response that have
Cross‐talk with Other Hormone‐Signaling Pathways
The regulation of specific developmental processes is controlled by multiple plant hormones. It is therefore not surprising to find the existence of multiple levels of cross‐talk between these phytohormone‐signaling pathways. Cross‐talk between hormone‐signaling pathways is seen both in the control of hormone accumulation and in control of hormone sensitivity.
Perspectives
We have seen that mutations affecting GA biosynthesis and response have been essential for improving yields in many agronomically important crops. Although the molecular basis of several of the mutations has been revealed, in most cases, we still have little understanding of how they confer these beneficial traits. In contrast to GA metabolism, our knowledge of GA signaling and the downstream processes that promote GA‐responsive growth is rather limited. To further our understanding, it is
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