Phase identity of the maize leaf is determined after leaf initiation

Abstract

The vegetative development of the maize shoot can be divided into juvenile and adult phases based on the types of leaves produced at different times in shoot development. Models for the regulation of phase change make explicit predictions about when the identity of these types of leaves is determined. To test these models, we examined the timing of leaf type determination in maize. Clones induced in transition leaf primordia demonstrated that the juvenile and adult regions of these leaves do not become clonally distinct until after the primordium is 700 m in length, implying that these cell fates were undetermined at this stage of leaf development. Adult shoot apices were cultured in vitro to induce rejuvenation. We found that leaf primordia as large as 3 mm in length can be at least partially rejuvenated by this treatment, and the location of rejuvenated tissue is correlated with the maturation pattern of the leaf. The amount and distribution of juvenile tissue in rejuvenated leaves suggests that rejuvenation occurs nearly simultaneously in all leaf primordia. In vitro culture rejuvenated existing leaf primordia and the P0 primordium, but did not change the identity of subsequent primordia or the total number of leaves produced by the shoot. This result suggests that leaf identity can be regulated independently of the identity of the shoot apical meristem, and it implies that vegetative phase change is not initiated by a change in the identity of the shoot apical meristem. P lants produce different types of leaves or leaf-like organs at different stages in their development (a phenomenon called heteroblasty or phase change) (1-4). Among these are cotyledons, the variety of leaf types produced during the vegetative growth of the shoot, inflorescence bracts, and the highly modified leaves found in flowers and other reproductive structures. Genetic and molecular analyses of floral morphogenesis have provided a detailed model (the ABC model) for the regulation of floral organ identity (5, 6). In contrast, the specification of vegetative organ identity remains very poorly understood (7).One of the questions that is still unresolved is when the identity of a leaf is determined. Many investigators have reported that heteroblastic variation in leaf anatomy and morphology is discernable very early in leaf development (8-13), and have therefore concluded that leaf identity is specified during the earliest stages of leaf initiation. However, this conclusion has rarely been tested experimentally. The effect of exogenous factors on leaf morphology has been studied extensively (1, 4), but there is surprisingly little information about when phase-specific aspects of leaf anatomy and morphology are determined during leaf development. Sussex and Clutter (14) found that adult leaf primordia of the fern Osmunda cinnamomea develop as juvenile leaves when cultured on a medium containing low levels of sucrose, demonstrating that leaf identity is not specified until after leaf initiation in this species. In addition, experimental an...

Dual Effects of miR156-TargetedSPLGenes andCYP78A5/KLUHon Plastochron Length and Organ Size inArabidopsis thaliana

“…An even stronger phenotype is seen in ProSPL9:rSPL9 plants, in which the juvenile phase is lost (this work). Clonal analyses with maize have indicated that vegetative phase change is not conferred by the shoot apical meristem, but rather that phase identity is determined autonomously in each leaf primordium (Orkwiszewski and Poethig, 2000). Our results are consistent with this observation, as SPL9 is normally expressed in leaf primordia, and increasing SPL9 levels in leaf primordia by expression from the ANT promoter is sufficient to accelerate phase change.…”

Leaf architecture, lignification, and tensile strength during vegetative phase change in Zea mays

“…The metachromatic stain Toluidine blue O stains non-lignified cell walls of maize purple and lignified cell walls blue/green (Lawson and Poethig 1995;Orkwiszewski and Poethig 2000). Juvenile leaves did not exhibit lignification in any tissues except for the secondary walls of the xylem in the vascular bundles (Figs 8, 9, 12, 13) and some of the parenchyma cells in the midrib at the base of leaf 5 (Figs 14 and 15).…”

“…Samples were then dehydrated through an ethanol series and transferred into Citrisolv (Fisher Scientific, Pittsburgh, Pennsylvania USA) and slowly infiltrated with liquid paraffin over 3 d. Subsequently, samples were poured into plastic holders and solidified over crushed ice. Sections (4-6 µm) were cut on a rotary microtome (American Optical, Buffalo, New York USA), de-paraffinized in Citrisolv, rehydrated in an ethanol series, and stained with Toluidine Blue O, a metachromatic stain for cellulose and lignin (Lawson and Poethig 1995;Orkwiszewski and Poethig 2000). Photographs were obtained using an Olympus BX60 light microscope with an Olympus SC 35 mm camera (Tokyo, Japan).…”

“…Each stage of development has unique phytomers that exhibit different anatomical features. In Zea mays there are also one to three transitional leaves between the juvenile and adult vegetative stage (Bongard-Pierce et al 1996;Orkwiszewski and Poethig 2000). Juvenile leaves are characterized by the presence of epicuticular wax, the absence of trichomes, lack of lignins in epidermal cell walls and spherical epidermal cells, while adult leaves have no epicuticular wax, have trichomes, stain positive for lignins in the cell walls of epidermal cells, and have rectangular-shaped epidermal cells (Poethig 1988;Lawson and Poethig 1995;Sylvester et al 2001).…”

“…Juvenile leaves are characterized by the presence of epicuticular wax, the absence of trichomes, lack of lignins in epidermal cell walls and spherical epidermal cells, while adult leaves have no epicuticular wax, have trichomes, stain positive for lignins in the cell walls of epidermal cells, and have rectangular-shaped epidermal cells (Poethig 1988;Lawson and Poethig 1995;Sylvester et al 2001). Transitional leaves have distinct regions where their anatomy and morphology exhibit either juvenile or adult vegetative anatomical characters (Orkwiszewski and Poethig 2000).…”