Networks in leaf development

Shoots are characterized by indeterminate growth resulting from divisions of undifferentiated cells in the central region of the shoot apical meristem. These cells give rise to peripheral derivatives from which lateral organ initials are recruited. During initial stages of cell recruitment, the three-dimensional form of lateral organs is specified. Lateral organs such as leaves develop and differentiate along proximodistal (base-to-tip), dorsoventral (top-to bottom) and mediolateral (middle-to-margin) planes. Current findings are refining our knowledge of the genes and genetic interactions that regulate these early processes and are providing a picture of how these pathways may contribute to variation in leaf form.     

Gene expression during leaf development in Lolium temulentum: Patterns of protein synthesis in response to heat-shock and cold-shock

At an optimal growth temperature of 20°C, expending 4th leaves of Lolium temulentum L. synthesised a broad spectrum of polypeptides which altered with the maturity of the leaf tissue. Elevation of the temperature to 35°C, or above, induced synthesis of heat-shock proteins (hsp), and all parts of the 4th leaf were capable of this response. The threshold temperature for induction of hsp synthesis was little affected by the growth temperature (5 or 20°C). In contrast, a sudden 15–18°C decrease in temperature did not result in a marked alteration of protein synthesis patterns. It is concluded that in this species adaptation to rapid temperature reduction is not mediated by stress protein synthesis.     

Patterns of leaf morphology and leaf N content in relation to winter temperatures in three evergreen tree species

The competitive equilibrium between deciduous and perennial species in a new scenario of climate change may depend closely on the productivity of leaves along the different seasons of the year and on the morphological and chemical adaptations required for leaf survival during the different seasons. The aim of the present work was to analyze such adaptations in the leaves of three evergreen species (Quercus ilex, Q. suber and Pinus pinaster) and their responses to between-site differences in the intensity of winter harshness. We explore the hypothesis that the harshness of winter would contribute to enhancing the leaf traits that allow them to persist under conditions of stress. The results revealed that as winter harshness increases a decrease in leaf size occurs in all three species, together with an increase in the content of nitrogen per unit leaf area and a greater leaf mass per unit area, which seems to be achieved only through increased thickness, with no associated changes in density. P. pinaster was the species with the most intense response to the harshening of winter conditions, undergoing a more marked thickening of its needles than the two Quercus species. Our findings thus suggest that lower winter temperatures involve an increase in the cost of leaf production of evergreen species, which must be taken into account in the estimation of the final cost and benefit balance of evergreens. Such cost increases would be more pronounced for those species that, like P. pinaster, show a stronger response to the winter cold.     

Protein synthesis in Xanthium leaf development

Young, expanding Xanthium leaves had many soluble proteins;older leaves had progressively fewer. The leaves that grew themost rapidly incorporated the most ¹⁴CO₂ into their proteins.The relative intensity of ¹⁴CO₂ incorporation into the differentsoluble proteins changed with leaf development. (Received November 17, 1969; )     

Small RNAs in tomato fruit and leaf development

Tomato fruit and leaf development offers excellent systems to study the evolution of gene regulation underlying development of different organs. We have identified over 350 and 700 small RNAs from tomato fruit and leaf, respectively. Except for conserved microRNAs, more than 90% of the small RNAs are unique to tomato. We confirmed expression of some conserved as well as novel putative microRNAs by Northern hybridization. Our results help form a basis for comparative studies on how small RNA-mediated gene expression has contributed to the evolution of common and distinct developmental pathways of fruits and leaves. We have established a website (http://ted.bti.cornell.edu/digital/sRNA/) for public access to all of our small RNA sequences, their expression patterns in respective tissues, and their matching genes or predicted target genes in a searchable manner.     

Compound leaf development and evolution in the legumes

Across vascular plants, Class 1 KNOTTED1-like (KNOX1) genes appear to play a critical role in the development of compound leaves. An exception to this trend is found in the Fabaceae, where pea (Pisum sativum) uses UNIFOLIATA, an ortholog of the floral regulators FLORICAULA (FLO) and LEAFY (LFY), in place of KNOX1 genes to regulate compound leaf development. To assess the phylogenetic distribution of KNOX1-independent compound leaf development, a survey of KNOX1 protein expression across the Fabaceae was undertaken. The majority of compound-leafed Fabaceae have expression of KNOX1 proteins associated with developing compound leaves. However, in a large subclade of the Fabaceae, the inverted repeat-lacking clade (IRLC), of which pea is a member, KNOX1 expression is not associated with compound leaves. These data suggest that the FLO/LFY gene may function in place of KNOX1 genes in generating compound leaves throughout the IRLC. The contribution of FLO/LFY to leaf complexity in a member of the Fabaceae outside of the IRLC was examined by reducing expression of FLO/LFY orthologs in transgenic soybean (Glycine max). Transgenic plants with reduced FLO/LFY expression showed only slight reductions in leaflet number. Overexpression of a KNOX1 gene in alfalfa (Medicago sativa), a member of the IRLC, resulted in an increase in leaflet number. This implies that KNOX1 targets, which promote compound leaf development, are present in alfalfa and are still sensitive to KNOX1 regulation. These data suggest that KNOX1 genes and the FLO/LFY gene may have played partially overlapping roles in compound leaf development in ancestral Fabaceae but that the FLO/LFY gene took over this role in the IRLC.     

Changes in leaf trichomes and epicuticular flavonoids during leaf development in three birch taxa

BACKGROUND AND AIMS: Changes in number of trichomes and in composition and concentrations of their exudates throughout leaf development may have important consequences for plant adaptation to abiotic and biotic factors. In the present study, seasonal changes in leaf trichomes and epicuticular flavonoid aglycones in three Finnish birch taxa (Betula pendula, B. pubescens ssp. pubescens, and B. pubescens ssp. czerepanovii) were followed. METHODS: Trichome number and ultrastructure were studied by means of light, scanning and transmission electron microscopy, while flavonoid aglycones in ethanolic leaf surface extracts were analysed by high-pressure liquid chromatography. KEY RESULTS: Density of both glandular and non-glandular trichomes decreased drastically with leaf expansion while the total number of trichomes per leaf remained constant, indicating that the final number of trichomes is established early in leaf development. Cells of glandular trichomes differentiate before those of the epidermis and produce secreted material only during the relatively short period (around 1-2 weeks) of leaf unfolding and expansion. In fully expanded leaves, glandular trichomes appeared to be at the post-secretory phase and function mainly as storage organs; they contained lipid droplets and osmiophilic material (probably phenolics). Concentrations (mg g(-1) d. wt) of surface flavonoids decreased with leaf age in all taxa. However, the changes in total amount ( microg per leaf) of flavonoids during leaf development were taxon-specific: no changes in B. pubescens ssp. czerepanovii, increase in B. pendula and in B. pubescens ssp. pubescens followed by the decline in the latter taxon. Concentrations of most of the individual leaf surface flavonoids correlated positively with the density of glandular trichomes within species, suggesting the participation of glandular trichomes in production of surface flavonoids. CONCLUSIONS: Rapid decline in the density of leaf trichomes and in the concentrations of flavonoid aglycones with leaf age suggests that the functional role of trichomes is likely to be most important at the early stages of birch leaf development.     

Patterns of leaf surface wetness for montane and subalpine plants

The frequency and duration of water on leaf surfaces have important consequences for plant growth and photosynthetic gas exchange. The objective of the present study was to compare the frequency and duration of leaf wetness under natural field conditions among species and to identify variation in structural features of leaves that may reduce surface wetness. During June–September 1992 in the central Rocky Mountains (USA), natural leaf wetting due to rain and dewfall was observed on 79 of 89 nights in open meadow habitats compared to only 29 of 89 nights in the understorey. Dew formation occurred at relative humidities that were often well below 100% because of radiational heat exchange with cold night skies and low wind speeds (< 0.5 m s^?1). A survey of 50 subalpine/montane species showed that structural characteristics associated with the occurrence and duration of leaf surface wetness differed among species and habitats. Both adaxial and abaxial surfaces accumulated moisture during rain and dewfall events. Leaf surfaces of open-meadow species were less wettable (P= 0.008), and had lower droplet retention (P= 0.015) and more stomata P= 0.017) than adjacent understorey species. Also, leaf trichomes reduced the area of leaf surface covered by moisture. Ecophysiological importance is suggested by the high frequency of leaf wetting events in open microsites, influences on growth and gas exchange, and correspondence between leaf surface wettability and habitat.     

植物叶发育的分子机理

叶是植物进行光合作用和蒸腾作用的主要场所, 对植物的生长发育具有重要的作用。叶的发育包括叶原基的形成和极性的建立, 大量研究表明, 叶发育建成受到众多转录因子、小分子RNA以及生长素等因子的调控。文章综述了近年来叶发育和形态建成的分子机制研究进展, 以期了解叶发育的调控网络。     

The control of leaf development

The formation of a leaf is a basic aspect of plant development. This review provides an overview of our present understanding of the process from initiation to the final form of the leaf. Molecular genetic and cell biology approaches have yielded significant advances in this area, adding not only to our knowledge of leaf development but also to fundamental principles in plant biology. These principles will be highlighted, as well as areas where our understanding is still incomplete, in particular the problem of coordinating the multifaceted steps involved in the generation of the leaf structure.     

Clonal analysis of leaf development in cotton

Clonal analysis has been used to describe the cellular parameters of leaf development in American Pima cotton (Gossypium barbadense). Sectors (clones) induced before leaf initiation indicate that the leaf primordium arises from ~100 cells on the flank of the shoot meristem. An analysis of sector frequency during the period of leaf expansion suggests that the rate of cell division is fairly uniform throughout the length of the leaf, but is lower at the margin of the lamina than in intercalary regions. The shapes of marginal sectors indicate that the orientation of cell division (as defined by the orientation of the new cell wall) in this region is more often parallel to the margin than perpendicular to it, although the degree of polarization varies along the length of the margin. There is a slight gradient in the duration of cell division along the length of of the lamina late in development, with cell division ceasing progressively from the lamina tip to the base over two cell cycles. The parameters of cell division in cotton are therefore similar to those described for tobacco with the notable exception of the behavior of cells at the leaf margin.     

Enzymatic and non-enzymatic lipid peroxidation in leaf development.

Enzymatic and non-enzymatic lipid peroxidation has been implicated in programmed cell death, which is a major process of leaf senescence. To test this hypothesis we developed a high-performance liquid chromatography (HPLC) method for a simultaneous analysis of the major hydro(pero)xy polyenoic fatty acids. Quantities of lipid peroxidation products in leaves of different stages of development including natural senescence indicated a strong increase in the level of oxygenated polyenoic fatty acids (PUFAs) during the late stages of leaf senescence. Comprehensive structural elucidation of the oxygenation products by means of HPLC, gas chromatography/mass spectrometry and (1)H nuclear magnetic resonance suggested a non-enzymatic origin. However, in some cases a small share of specifically oxidized PUFAs was identified suggesting involvement of lipid peroxidizing enzymes. To inspect the possible role of enzymatic lipid peroxidation in leaf senescence, we analyzed the abundance of lipoxygenases (LOXs) in rosette leaves of Arabidopsis. LOXs and their product (9Z,11E,13S,15Z)-13-hydroperoxy-9,11,15-octadecatrienoic acid were exclusively detected in young green leaves. In contrast, in senescing leaves the specific LOX products were overlaid by large amounts of stereo-random lipid peroxidation products originating from non-enzymatic oxidation. These data indicate a limited contribution of LOXs to total lipid peroxidation, and a dominant role of non-enzymatic lipid peroxidation in late stages of leaf development.     

Patterns in specific leaf area and the structure of a temperate heath community

Relationships between the distribution and specific leaf area (SLA: leaf area per unit dry mass) of six heath (Ericaceae) species were investigated along an environmental gradient between peat bogs and conifer forest in British Columbia, Canada. I asked whether patterns in SLA could help to identify the processes shaping plant distributional patterns. Specifically, I assessed whether (i) species’ distributions across the environmental gradient are correlated with SLA (ii) relationships between plant distributional patterns and SLA are similar among bogs with different shrub species (iii) intraspecific patterns in SLA parallel interspecific relationships between distributions and SLA, and (iv) intraspecific patterns are environmentally determined. Results showed that distributional patterns were often correlated with SLA; species with lower SLA were more abundant towards the centre of bogs, while species with higher SLA were more abundant in forest. Intraspecific patterns in SLA paralleled distributional patterns across the gradient; individuals located towards the centre of bogs had lower SLA than those growing in forest. A transplantation experiment showed that plants typically altered their SLA according to local environmental conditions. However, one bog showed no relationship between species’ distributions and SLA. This bog lacked the two species with lowest SLA, which typically occurred at the centre of other bogs. In their absence, species with higher SLA that typically occurred in forest increased in abundance towards the centre of the bog, where they obtained lower values of SLA. Therefore, while distributional patterns were often closely associated with SLA, plasticity in SLA was associated with increased breadth of species’ distributions across the gradient. Overall results indicate SLA may serve as a useful proxy for a range of life history traits to help elucidate the processes structuring plant communities.