Control of shoot development after germination

The shoots of higher plants develop by the addition of structures such as leaves, internodes and vegetative or floral buds at the shoot apex (Poethig, 2003) . Many features of these structures, such as the shape and size of a leaf or the plane of elongation of the internode change continuously throughout plant development and respond to environmental conditions. Some of these changes can be gradual and progressive, such as the transition from producing juvenile leaves to adult leaves. In contrast other changes, such as the initiation of flowering or changes in phyllotaxy, are much more abrupt, occurring rapidly when a predetermined condition has been met (Allsopp, 1967; Poethig, 1990, 2003).

There are three major phases of post-embryonic shoot development.

These are defined as the juvenile vegetative, adult vegetative, and the reproductive phases. The relationship between juvenile and adult development, and competence to flower is not always clear (Vega et al., 2002; Poethig, 2003) . In Arabidopsis, a plant will not flower during the juvenile phase, whereas after the transition to the adult vegetative phase plants can respond to floral evocation signals, and initiate reproductive development (Poethig, 1990; Lawson and Poethig, 1995; Poethig, 2003) . The shift from juvenile to adult vegetative phase can be commercially important, for example many fruit trees must go through a prolonged period of juvenility before being able to bear fruit (Pena et al., 2001).

The juvenile and adult vegetative phases of Arabidopsis can be distinguished based on leaf characters. Juvenile leaves are smaller and rounder in shape than adult leaves, and carry trichomes on their adaxial surfaces. Adult leaves are elongated and carry trichomes on both surfaces (Chien and Sussex, 1996; Telfer et al., 1997; Bowman and Eshed, 2000) . Furthermore cauline leaves, which are formed during the reproductive phase of Arabidopsis development, have a marked reduction of trichomes on their adaxial surface (Telfer et al., 1997) .

The transition to flowering

The transition from the adult vegetative phase to the reproductive phase (i.e. the transition to flowering) involves major changes in cell differentiation and morphogenesis at the shoot apical meristem (SAM). The SAM consists of a population of pluripotent stem cells that are formed during embryogenesis, and give rise to all shoot structures (Bowman and Eshed, 2000) . The SAM, which is located at the tip of the shoot apex, is responsible for the production of lateral organs, and stem tissues.

SAMs are dome shaped and can be subdivided into two main regions. The tunica consists of the epidermis (L1) and the underlying cell layer (L2), both of which undergo cell divisions in an anticlinal plane. In contrast cells below the L2 represent a mass of cells called the corpus (L3), which divide in all directions. An alternative and much used classification of meristemic areas is to divide them into cytological zonations, a peripheral zone, a central zone and a rib zone. Lateral organs derive from the peripheral zone, stem cells derive from the rib zone and the central zone consists of cells that replenish both the peripheral and the rib zones (Steeves and Sussex, 1989; Sussex, 1989; Meyerowitz, 1997; Barton, 1998).

After the transition to flowering the SAM becomes an inflorescence meristem, which can be distinguished from vegetative SAMs early in their development by their larger size and different shape. During the floral transition there is a marked increase in cell division in the central zone. The first primordia produced by the primary inflorescence meristem of Arabidopsis form cauline leaves and the associated axillary meristems produce secondary inflorescences. Subsequently floral primordia are produced in the peripheral zone of the inflorescence meristem (Weigel and Jurgens, 2002).

Competence and determination of floral evocation

Prior to flowering, the cells in the SAM must first become competent to respond to a signal(s) that evokes floral initiation (McDaniel et al., 1992). In photoperiodic flowering, this signal is produced in the leaves, which can be demonstrated by grafting of plants exposed to different day lengths.

If an apical bud is able to respond to these developmental signals then it is said to have acquired competence (McDaniel et al., 1992).

The next stage is called determination. Once a shoot apex is determined to flower, then it will do so even after removal of leaves forming the floral stimulus. For example if a determined apex is grafted onto a vegetative shoot, which is not producing a stimulus, it will still flower (McDaniel and Hartnett, 1996).

In some plants, such as Lolium temulentum, flowering of a determined apex is delayed in the absence of transmissible signals, although flowering still occurs. For example, even when the meristem has become determined by a long-day photoperiod signal, flowering is delayed unless gibberellic acid is supplied (Taiz and Zeiger, 2002).

Overview of the control of flowering time

Some plants initiate flowering regardless of their environment, showing an autonomous flowering response, whereas others will only flower when environmental requirements are met, showing an obligate/qualitative response to their environment. Many plants are strongly influenced by environmental factors, but will also make the transition to flowering if these stimuli are not received after a pre-determined period of growth, and are said to show a facultative/quantitative response to their environment.

Many environmental factors, such as stress and nutrient availability, influence flowering time, but perhaps the most important are light and temperature. With regards to light the quantity, quality and duration (photoperiod) are all important variables. During the control of flowering photoperiod and temperature are perceived in different parts of the plant. Photoperiod is perceived in the leaves, whereas vernalisation (a prolonged exposure to low temperatures; see section is perceived in the shoot apex (Lang, 1965; Zeevaart and Boyer, 1987).

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