Correlation between leaf growth variables suggest intrinsic and early controls of leaf size in Arabidopsis thaliana

Leaf development is affected by both internal (genetic) and external (environmental) regulatory factors. The aim of this work was to investigate how leaf growth variables are related to one another in a range of environments. The leaf growth variables of wild-type Arabidopsis thaliana and leaf development mutants (ang4, ron2-1, elo1, elo2 and elo4) were studied under different incident light treatments (light and shade). The leaves studied were altered in various leaf development variables, such as the duration of expansion, relative and absolute expansion rates, epidermal cell size, epidermal cell number and initiation rate. Final leaf area was correlated to maximal absolute leaf expansion rate and cell number, but not to duration of leaf expansion or cell size. These relationships were common to all studied genotypes and light conditions, suggesting that leaf size is determined early in development. In addition, the early variables involved in leaf development were correlated to one another, and initial relative expansion rate was negatively correlated to the duration of expansion. These relationships between the leaf development variables were used to construct a conceptual model of leaf size control.     

INDETERMINATE GROWTH OF LEAVES IN GUAREA (MELIACEAE): A TWIG ANALOGUE

Leaves of seed plants are generally characterized as organs of determinate growth. In this regard, Guarea and related genera seem unusual in that the pinnately compound leaves of these plants contain a bud at their tip from which new pinnae expand from time to time. Previous studies (based upon superficial examinations of leaf-tip buds) have produced contradictory conclusions regarding how long the leaf apex remains meristematic and produces new pinna primordia. In order to determine whether leaf development in Guarea is truly indeterminate, we microscopically examined leaf-tip buds of G. guidonia and G. glabra. In both species, the leaf apex remains meristematic and continues to produce new pinna primordia as the leaf ages. Unexpanded leaves of G. guidonia contained an average of 23 pinna primordia, while the oldest leaves we examined had initiated an average of 44 total pinnae. In G. glabra, unexpanded leaves contained 8 pinnae, whereas an average of 28 pinnae had been initiated on the oldest leaves. These results indicate that leaf development in Guarea is truly indeterminate. Periodic examination of individual intact leaves indicated that the leaves commonly continue their growth for 2 or more years (observed maximum = 51 months). As new leaflets are initiated at the shoot apex (and subsequently expand in rhythmic flushes), older (basal) leaflets may abscise. In addition, the petiole and rachis of the leaf thicken and become woody as a result of the activity of a vascular cambium. Guarea leaves therefore seem to function as the analogue of a typical twig (stem) in general habit as well as in their indeterminate apical growth and secondary thickening.     

Co-Ordination of Cell Division and Tissue Expansion in Sunflower, Tobacco, and Pea Leaves: Dependence or Independence of Both Processes?

Abstract Temporal analyses of cell division and tissue expansion in pea, tobacco, and sunflower leaves reveal that both processes follow similar patterns during leaf development. Relative cell division and relative tissue expansion rates are maximal and constant during early leaf development, but they decline later. In contrast, relative cell expansion rate follows a bell-shaped curve during leaf growth. Cell division and tissue expansion have common responses to temperature, intercepted radiation, and water deficit. As a consequence, final leaf area and cell number remain highly correlated throughout a large range of environmental conditions for these different plant species, indicating that cell division and tissue expansion are co-ordinated during leaf development. This co-ordination between processes has long been explained by dependence between both processes. Most studies on dicotyledonous leaf development indicate that leaf expansion rate depends on the number of cells in the leaf. We tested this hypothesis with a large range of environmental conditions and different plant species. Accordingly, we found a strong correlation between both absolute leaf expansion rate and leaf cell number. However, we showed that this relationship is not necessarily causal because it can be simulated by the hypothesis of independence between cell division and tissue expansion according to Green's theory of growth (1976). Received 23 February 2000; accepted 3 March 2000     

Heteroblasty in the moss,Aphanoregma patens (Physcomitrella patens), results from progressive modulation of a single fundamental leaf developmental programme

Abstract

Leaves at the apex of a mature Aphanoregma patens (Hedw.) Lindb. (Physcomitrella patens (Hedw.) Bruch Schimp. in B.S.G.) gametophore differ markedly in size and form from those at its base. To determine how these differences are produced during development, we first examined qualitative and quantitative differences between successive leaves along the stem and among leaves at different developmental stages. Differences between successive leaves were slight and cumulative. Local changes in cell number and size combined to produce a regularly shaped and approximately bilaterally symmetrical leaf suggesting that cell division and cell expansion are regionally regulated and coordinated at the organ level. The midrib and marginal teeth are discrete characters, which were prefigured by changes in cell shape in leaves that lacked these characters. In leaf primordia, cell proliferation was responsible for most of the changes in leaf form and size early in development and may have continued as cell expansion took over as the primary contributor to leaf growth and morphogenesis. Thus, leaf heteroblasty in Physcomitrella probably results from modulation of a single developmental programme by external and/or internal forces, which alter progressively in intensity as a gametophore grows. We applied exogenous cytokinin and auxin separately to growing cultures to explore their effects on leaf growth. Cytokinin and auxin stimulated leaf cell division and leaf cell elongation, respectively. Also, young upper leaves of gametophores exposed to exogenous auxin closely resembled basal leaves of untreated plants. Therefore, endogenous cytokinins and auxins may be among the modulating internal forces involved in leaf morphogenesis and the establishment of leaf heteroblasty.     

Expansion Kinematics are an Intrinsic Property of Leaf Development and are Scaled from Cell to Leaf Level at Different Nutrient Availabilities

Abstract: Leaves of Nicotiana tabacum L. and Ricinus communis L. develop with a wide range of different final sizes due to leaf position and nutrient availability. The aim of this study was to investigate, on different organizational levels of leaf growth, which parameters are affected by external nutrient availability and which parameters are not affected and might thus be intrinsic, general patterns of growth that govern plant architecture. We found that leaf size and final cell size was larger with higher external nutrient availability, and that hexose concentrations in N. tabacum were lower with higher nutrient availability. Despite these differences, several dynamic parameters of leaf development were not affected by the nutrient treatment. Leaves of all sizes within a species exhibited the same relationship between relative leaf growth rate and relative leaf area (RLA), which is defined as the ratio of momentary and final leaf area. External nutrient availability did not affect chlorophyll concentration per parenchyma cell, which increased linearly with leaf development. Leaves of identical RLA exhibited identical cell density patterns within their interveinal tissue layers. This indicates a close connection between the kinematics of cell expansion and RLA and, hence, reveals that kinematics are an intrinsic property of growing leaves that can be scaled from the cell to the leaf level.     

Elevated CO2 and plant structure: a review

Consequences of increasing atmospheric CO₂ concentration on plant structure, an important determinant of physiological and competitive success, have not received sufficient attention in the literature. Understanding how increasing carbon input will influence plant developmental processes, and resultant form, will help bridge the gap between physiological response and ecosystem level phenomena. Growth in elevated CO₂ alters plant structure through its effects on both primary and secondary meristems of shoots and roots. Although not well established, a review of the literature suggests that cell division, cell expansion, and cell patterning may be affected, driven mainly by increased substrate (sucrose) availability and perhaps also by differential expression of genes involved in cell cycling (e.g. cyclins) or cell expansion (e.g. xyloglucan endotransglycosylase). Few studies, however, have attempted to elucidate the mechanistic basis for increased growth at the cellular level. Regardless of specific mechanisms involved, plant leaf size and anatomy are often altered by growth in elevated CO₂, but the magnitude of these changes, which often decreases as leaves mature, hinges upon plant genetic plasticity, nutrient availability, temperature, and phenology. Increased leaf growth results more often from increased cell expansion rather than increased division. Leaves of crop species exhibit greater increases in leaf thickness than do leaves of wild species. Increased mesophyll and vascular tissue cross-sectional areas, important determinates of photosynthetic rates and assimilate transport capacity, are often reported. Few studies, however, have quantified characteristics more reflective of leaf function such as spatial relationships among chlorenchyma cells (size, orientation, and surface area), intercellular spaces, and conductive tissue. Greater leaf size and/or more leaves per plant are often noted; plants grown in elevated CO₂ exhibited increased leaf area per plant in 66% of studies, compared to 28% of observations reporting no change, and 6% reported a decrease in whole plant leaf area. This resulted in an average net increase in leaf area per plant of 24%. Crop species showed the greatest average increase in whole plant leaf area (+ 37%) compared to tree species (+ 14%) and wild, nonwoody species (+ 15%). Conversely, tree species and wild, nontrees showed the greatest reduction in specific leaf area (– 14% and – 20%) compared to crop plants (– 6%). Alterations in developmental processes at the shoot apex and within the vascular cambium contributed to increased plant height, altered branching characteristics, and increased stem diameters. The ratio of internode length to node number often increased, but the length and sometimes the number of branches per node was greater, suggesting reduced apical dominance. Data concerning effects of elevated CO₂ on stem/branch anatomy, vital for understanding potential shifts in functional relationships of leaves with stems, roots with stems, and leaves with roots, are too few to make generalizations. Growth in elevated CO₂ typically leads to increased root length, diameter, and altered branching patterns. Altered branching characteristics in both shoots and roots may impact competitive relationships above and below the ground. Understanding how increased carbon assimilation affects growth processes (cell division, cell expansion, and cell patterning) will facilitate a better understanding of how plant form will change as atmospheric CO₂ increases. Knowing how basic growth processes respond to increased carbon inputs may also provide a mechanistic basis for the differential phenotypic plasticity exhibited by different plant species/functional types to elevated CO₂.     

Rewatering plants after a long water-deficit treatment reveals that leaf epidermal cells retain their ability to expand after the leaf has apparently reached its final size

Background and Aims: Leaves expand during a given period of time until they reachtheir final size and form, which is called determinate growth.Duration of leaf expansion is stable when expressed in thermal-timeand in the absence of stress, and consequently it is often proposedthat it is controlled by a robust programme at the plant scale.The usual hypothesis is that growth cessation occurs when cellexpansion becomes limited by an irreversible tightening of cellwall, and that leaf size is fixed once cell expansion ceases.The objective of this paper was to test whether leaf expansioncould be restored by rewatering plants after a long soil water-deficitperiod. Methods: Four experiments were performed on two different species (Arabidopsisthaliana and Helianthus annuus) in which the area of leavesthat had apparently reached their final size was measured uponreversal of water stresses of different intensities and durations. Key Results: Re-growth of leaves that had apparently reached their finalsize occurred in both species, and its magnitude depended onlyon the time elapsed from growth cessation to rewatering. Leafarea increased up to 186% in A. thaliana and up to 88% in H.annuus after rewatering, with respect to the leaves of plantsthat remained under water deficit. Re-growth was accounted forby cell expansion. Increase in leaf area represented actualgrowth and not only a reversible change due to increased turgor. Conclusions: After the leaf has ceased to grow, leaf cells retain their abilityto expand for several days before leaf size becomes fixed. Aresponse window was identified in both species, during whichthe extent of leaf area recovery decreased with time after the‘initial’ leaf growth cessation. These results suggestthat re-growth after rewatering of leaves having apparentlyattained their final size could be a generalized phenomenon,at least in dicotyledonous plants.     

Cytokinin-Regulated Mesophyll Cell Division and Expansion during Development of Cucurbita pepo Leaves

The role of cytokinins in the development of mesophyll structure was studied in developing pumpkin Cucurbita pepo L. leaves. Leaves were treated with cytokinins at different stages of growth: when they reached 25 or 50% of their final size (S _max), immediately after leaf growth ceased, and during senescence. At the early stages of leaf development, treatment with exogenous benzyladenine accelerated division of mesophyll cells. At the later stages of development, BA treatment activated expansion of growing cells and those, which have just accomplished their growth. The exogenous cytokinin did not affect the senescent leaf cells. The content of endogenous cytokinins changed during mesophyll development. The juvenile leaves (25% of S _max) were characterized by low level of these phytohormones. In the expanding leaves (50% of S _max), the content of phytohormones increased and decreased when leaf growth ceased. In the senescent leaves, the cytokinin content decreased markedly. It was concluded that the response of mesophyll cells to cytokinin depended on the cell growth phase at the moment of hormone action. Furthermore, in the young leaves, lower cytokinin concentrations were required for division of mesophyll cells in vivo than for cell expansion at the final stage of leaf development.     

Gibberellin indirectly promotes chloroplast biogenesis as a means to maintain the chloroplast population of expanded cells

Chloroplast biogenesis needs to be well coordinated with cell division and cell expansion during plant growth and development to achieve optimal photosynthesis rates. Previous studies showed that gibberellins (GAs) regulate many important plant developmental processes, including cell division and cell expansion. However, the relationship between chloroplast biogenesis with cell division and cell expansion, and how GA coordinately regulates these processes, remains poorly understood. In this study, we showed that chloroplast division was significantly reduced in the GA‐deficient mutants of Arabidopsis (ga1‐3) and Oryza sativa (d18‐AD), accompanied by the reduced expression of several chloroplast division‐related genes. However, the chloroplasts of both mutants exhibited increased grana stacking compared with their respective wild‐type plants, suggesting that there might be a compensation mechanism linking chloroplast division and grana stacking. A time‐course analysis showed that cell expansion‐related genes tended to be upregulated earlier and more significantly than the genes related to chloroplast division and cell division in GA‐treated ga1‐3 leaves, suggesting the possibility that GA may promote chloroplast division indirectly through impacting leaf mesophyll cell expansion. Furthermore, our cellular and molecular analysis of the GA‐response signaling mutants suggest that RGA and GAI are the major repressors regulating GA‐induced chloroplast division, but other DELLA proteins (RGL1, RGL2 and RGL3) also play a role in repressing chloroplast division in Arabidopsis. Taken together, our data show that GA plays a critical role in controlling and coordinating cell division, cell expansion and chloroplast biogenesis through influencing the DELLA protein family in both dicot and monocot plant species.     

RESPONSE OF LEAF ANATOMY AND PHOTOSYNTHETIC CAPACITY IN ALOCASIA MACRORRHIZA (ARACEAE) TO A TRANSFER FROM LOW TO HIGH LIGHT

Alocasia macrorrhiza plants were grown in 1% and 20% full sunlight, and their leaf anatomical and physiological parameters were measured. Total leaf thickness was 41% greater and mesophyll thickness was 52% greater in high-light leaves than in low-light leaves. This increase in thickness resulted from both increased cell size and number. Maximum leaf photosynthetic capacity was also 66% greater in high- than in low-light leaves. When low-light plants were transferred to high light, the thickness of mature leaves did not increase but the thickness of the first leaf to expand after the transfer was significantly greater than that of the low-light leaves. Thus, only leaves that were still expanding at the time of transfer developed leaf thickness greater than plants remaining in low light. Fully mature leaves showed no change in photosynthetic capacity in response to transfer. Leaves that had just completed expansion at the time of low- to high-light transfer were able to develop slightly higher maximum photosynthetic capacities than older leaves. However, full photosynthetic acclimation to the new light environment did not occur until the second new leaf expanded after transfer. These results are discussed in relation to the timing and mechanisms of whole plant acclimation to increased light.     

Polygalacturonase45 cleaves pectin and links cell proliferation and morphogenesis to leaf curvature in Arabidopsis thaliana

Regulating plant architecture is a major goal in current breeding programs. Previous studies have increased our understanding of the genetic regulation of plant architecture, but it is also essential to understand how organ morphology is controlled at the cellular level. In the cell wall, pectin modification and degradation are required for organ morphogenesis, and these processes involve a series of pectin-modifying enzymes. Polygalacturonases (PGs) are a major group of pectin-hydrolyzing enzymes that cleave pectin backbones and release oligogalacturonides (OGs). PG genes function in cell expansion and separation, and contribute to organ expansion, separation and dehiscence in plants. However, whether and how they influence other cellular processes and organ morphogenesis are poorly understood. Here, we characterized the functions of Arabidopsis PG45 (PG45) in organ morphogenesis using genetic, developmental, cell biological and biochemical analyses. A heterologously expressed portion of PG45 cleaves pectic homogalacturonan in vitro, indicating that PG45 is a bona fide PG. PG45 functions in leaf and flower structure, branch formation and organ growth. Undulation in pg45 knockout and PG45 overexpression leaves is accompanied by impaired adaxial–abaxial polarity, and loss of PG45 shortens the duration of cell proliferation in the adaxial epidermis of developing leaves. Abnormal leaf curvature is coupled with altered pectin metabolism and autogenous OG profiles in pg45 knockout and PG45 overexpression leaves. Together, these results highlight a previously underappreciated function for PGs in determining tissue polarity and regulating cell proliferation, and imply the existence of OG-based signaling pathways that modulate plant development.     

胡杨枝芽生长特征及其展叶物候特征

以5个不同发育阶段的胡杨(Populus euphratica Oliv.)个体为研究对象,观测记录了枝芽展叶物候、枝芽生长特征和叶形变化的空间分布规律。结果表明:不同发育阶段的胡杨个体以及同一个体树冠的不同层次,其枝芽生长及其展叶物候期表现出不同的时空特征。随着树龄的增加和树冠层次的增高(由基向顶),当年新生枝条长度、枝条叶片数和叶形指数逐渐减小,但叶面积和叶片干重逐渐增大。5个不同发育阶段胡杨个体均表现出展叶物候始于树冠顶层,依次向下结束于树冠基部;展叶物候期共性表现在枝芽萌动期均在4月上旬,起始展叶期集中在4月中旬,展叶终期则在5月上旬到下旬;树龄较大的个体其枝芽萌动期、起始展叶期、展叶终期较树龄较小的个体早;其枝芽萌动期到展叶终期的时间进程较树龄较小的个体短;不同发育阶段的个体枝芽萌动期出现的时间较为离散,起始展叶期和展叶终期出现的时间较为集中。相关分析表明,出叶周期与枝条长度、枝条叶片数量和叶形指数呈极显著正相关,与叶面积和叶片干重呈显著负相关。     

Do small leaves expand faster than large leaves,and do shorter expansion times reduce herbivore damage?

Leaves are most vulnerable to herbivory during expansion. We hypothesised that one factor favouring small leaves could be that smaller-leaved species have shorter expansion times and are therefore exposed to high levels of herbivory for a shorter period than large leaves. In order to test this hypothesis, leaf expansion time and leaf area loss were measured for 51 species from Sydney, Australia. Strong positive correlations were found between leaf length and area and leaf expansion time, confirming that small leaves do expand in a shorter time than large leaves. The amount of leaf area lost was highly variable (from 0.5 to 90% of total leaf area), but was significantly related to both leaf expansion time and log leaf area. The amount of leaf area lost was not significantly correlated with specific leaf area nor with the presence of distasteful substances in the leaves, but was lower on species with hairy expanding leaves.     

Response of cassava leaf area expansion to water deficit: cell proliferation, cell expansion and delayed development

BACKGROUND AND AIMS: Cassava (Manihot esculenta) is an important food crop in the tropics that has a high growth rate in optimal conditions, but also performs well in drought-prone climates. The objectives of this work were to determine the effects of water deficit and rewatering on the rate of expansion of leaves at different developmental stages and to evaluate the extent to which decreases in cell proliferation, expansion, and delay in development are responsible for reduced growth. METHODS: Glasshouse-grown cassava plants were subjected to 8 d of water deficit followed by rewatering. Leaves at 15 developmental stages from nearly full size to meristematic were sampled, and epidermal cell size and number were measured on leaves at four developmental stages. KEY RESULTS: Leaf expansion and development were nearly halted during stress but resumed vigorously after rewatering. In advanced-stage leaves (Group 1) in which development was solely by cell expansion, expansion resumed after rewatering, but not sufficiently for cell size to equal that of controls at maturity. In Group 2 (cell proliferation), relative expansion rate and cell proliferation were delayed until rewatering, but then recovered partially, so that loss of leaf area was due to decreased cell numbers per leaf. In Group 3 (early meristematic development) final leaf area was not affected by stress, but development was delayed by 4-6 d. On a plant basis, the proportion of loss of leaf area over 26 d attributed to leaves at each developmental stage was 29, 50 and 21 % in Group 1, 2 and 3, respectively. CONCLUSIONS: Although cell growth processes were sensitive to mild water deficit, they recovered to a large extent, and much of the reduction in leaf area was caused by developmental delay and a reduction in cell division in the youngest, meristematic leaves.     

The study of a determinate growth orchid highlights the role of new leaf production in photosynthetic light acclimation

Plants usually respond to the changes of growth irradiance by a combination of the physiological modifications in their preexisting leaves and the production of new leaves. However, those with a determinate growth habit produce certain number of leaves in a growing season and cannot produce new leaves when light condition changes. We used an epiphytic orchid with only one leaf produced every growing season to examine whether and how determinate growth species adapt to changing environments after their preexisting leaves mature. Leaf photosynthesis and anatomy of Pleione aurita were investigated at full expansion and at 40 days after the fully expanded leaves were transferred from high to low light or from low to high light. Leaves show large physiological and morphological plasticity to light gradients at full expansion and the transferred leaves exhibited multiple physiological modifications, including reallocation of nitrogen between light harvesting and carbon fixation, and enhancement of thermal dissipation in their new environments, to optimize carbon assimilation and avoid photoinhibition. Irrespective of the various changes either to shade or sun, the sole preexisting leaf could not fully acclimate to new light environments due to the mesophyll thickness constraint. This leads to the consequence that only plants exposed to high light throughout the experiment had a positive annual biomass gain. Our results highlighted the importance of new leaf production in the carbon accumulation during photosynthetic light acclimation, and contribute new insights of epiphytes physiological responses to their highly dynamic arboreal habitat.     

Making leaves

Leaves are determinate organs that develop from the flanks of the shoot apical meristem through founder cell recruitment, establishment of proximodistal, dorsoventral and mediolateral axes, and subsequent growth, expansion and differentiation along these axes. Maintenance of the shoot apical meristem and production of leaves requires balanced partitioning of cells between pluripotent and differentiation fates. Hormones have a significant role in this balance but it is becoming apparent that additional intrinsic and extrinsic inputs influence hormone signalling to control meristem function and leaf initiation. As leaves develop, temporal and spatial regulation of growth and maturation determines leaf shape and complexity. Remarkably genes involved in leaf development in the context of the shoot apical meristem are also involved in elaboration of the leaf shape to generate subtle marginal serrations, more prominent lobes or a dissected compound leaf. Potentially these common regulatory modules represent a fundamental means of setting up boundaries separating discrete zones of growth. Defining gene networks involved in leaf shape variation and exploring interspecies differences between such networks is enabling exciting insight into changes that contribute to natural variation of leaf form.     

A dynamic analysis of the shade-induced plasticity in Arabidopsis thaliana rosette leaf development reveals new components of the shade-adaptative response

BACKGROUND AND AIMS: It is well known that plant aerial development is affected by light intensity in terms of the date of flowering, the length of stems and petioles, and the final individual leaf area. The aim of the work presented here was to analyse how shade-induced changes in leaf development occur on a dynamic basis from the whole rosette level to that of the cells. METHODS: Care was taken to ensure that light intensity was the only source of micro-meteorological variation in the study. The dynamics of leaf production, rosette expansion, individual leaf area expansion and epidermal cell expansion were analysed in Arabidopsis thaliana plants grown under two light intensities in three independent experiments. KEY RESULTS: The total area of rosette leaves was reduced by the shading treatment. Both the number of leaves produced and their individual leaf areas were reduced. The reduction in leaf number was associated with a reduction in leaf initiation rate and the duration of the phase of leaf production. The reduction in individual leaf area was associated with a reduction in leaf expansion rate and an increase in the duration of leaf expansion. The changes in leaf expansion dynamics were accompanied by a decrease in epidermal cell number which was partly compensated for by an increase in epidermal cell area. Overall, the whole rosette leaf expansion rate was reduced by shading, whereas the total duration of rosette leaf expansion was unaffected. This was mainly due to the accumulation of the increases in the durations of expansion of each individual leaf which was associated with an increase in cell expansion. CONCLUSIONS: The dynamic analysis presented here reveals a new shade-adaptative response mediated via the control of area expansion at the cell, organ and whole plant levels.     

Separation of cell enlargement and division in bean leaves

A method is presented for inducing cell enlargement in intact leaves and leaf strips of Phaseolus vulgaris L. without the complication of cell division. Primary bean leaves complete cell division and stop growing after 10 d in dim red light. Transfer to white light induces expansion (50% in 24 h) which is entirely the consequence of cell enlargement. Leaf strips from red-light-grown seedlings placed in white light and provided external solutes (10 mM KCl+10 mM sucrose) expand at the same rate as intact leaves in the light. This system makes possible future investigation of the mechanism of leaf cell enlargement.