Vegetative Development of Plants HORT 301 – Plant Physiology September 29, 2008 Taiz and Zeiger – Chapter 16 Web Topics 16.1, 16.2 & 16.5 Web Essay 16.2 [email protected]Angiosperms (flowering/seed plants) – three developmental stages; embryogenesis, vegetative development and reproductive development (flowering) Embryogenesis – first part of vegetative development, formation of an embryo from a zygote Post-germinal vegetative development – cell, tissue and organ differentiation and growth that determine size, structure and form of plants
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Vegetative Development of Plants HORT 301 – Plant Physiology September 29, 2008 Taiz and Zeiger – Chapter 16 Web Topics 16.1, 16.2 & 16.5 Web Essay 16.2.
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Vegetative Development of PlantsHORT 301 – Plant Physiology
September 29, 2008Taiz and Zeiger – Chapter 16Web Topics 16.1, 16.2 & 16.5
Annual and perennial plants - mechanisms and processes of embryogenesis and vegetative development are similar
Determinate growth – developmental genetic program transitions shoot meristem from vegetative to reproductive growth and then plant senescence, e.g. annuals, biennials
Indeterminate growth – genetic programs regulate vegetative and reproductive growth during a growth season, usually sequentially, e.g. perennials
No genetic program for senescence, although individual cells (tracheary element), tissues (xylem wood) and organs (leaves and fruits) may senesce
Plants differ from animals by retaining stem cells, undifferentiated and indeterminate allowing plants to deviate from the genetic developmental program
Capacity to adjust growth and development in response to chemical or environmental stimuli, e.g. stress episode, respond to injury
Arabidopsis has a determinate growth pattern
Double fertilization – one generative nucleus fuses with the egg forming the zygote and other fuses with the polar nuclei producing the endosperm
Embryogenesis
Genetic programs control embryogenesis, which is the differentiation and development of a zygote into a rudimentary plant
Developmental genetic program – transcriptional and post-transcriptional regulatory pathways that control cell patterning
Regulating gene expression in specific cell types and at appropriate times
Hormones, other endogenous cues, and the environment affect plant developmental genetic programs
Cells exhibit developmental patterning, zygote exhibits polarity illustrated by the vacuolar and cytoplasmic sides
Consequently, first zygotic division is asymmetric, resulting in the progenitor of the embryo and suspensor
Suspensor anchors the embryo to the ovular wall to allow movement of nutrients and chemical signals from maternal cells
Embryogenesis is illustrative of cell, tissue and organ developmental patterning; polarity is critical
Polarity in the embryo becomes more evident with the development of shoot and root structures, apical meristems
Embryogenesis in dicots and monocots is different but within each group patterning is very similar
Dicot embryogenesis stages:
A. Zygotic – single cell from gametic fusion, which divides asymmetrically to form the apical and basal cells
B-D. Globular shape – symmetric divisions proceed first and than asymmetric divisions to initiate the protoderm (progenitors of the epidermis)
E-F. Heart shape – rapid cell division of cells on the sides of embryo initiate the progenitors of cotyledons
G. Torpedo shape – primarily cell expansion in axial and radial polar growth patterns of the embryonic axis, differentiation and development of shoot and root apical meristems, internal tissues
H. Mature – cell growth ceases, dehydration occurs, reserves are stored and dormancy
Pattern of divisions in the developing embryo indicate that coordination is required to form tissues and organs
Coordination is based on regulatory signals and genes that specify cell type and location, and polarity
root meristem
Auxin is a chemical morphogen of embryogenesis
Auxin induces formation of embryos from somatic cells
PIN auxin efflux transporters are responsible for asymmetric distribution of auxin throughout embryos and plants
Mutations to PIN correlate with developmental lesions in embryogenesis, additional inference that auxin regulates embryogenesis
DR5 (promoter)-reporter systems (GUS or GFP) monitor auxin movement and levels
Auxin movement and concentration establishes polarity and regulates patterning in the zygote and globular and heart stage embryos
Proembyro
Prosuspensor
Genes involved in embryogenesis of Arabidopsis
Axial patterning
GURKE – shoot, encodes acetyl CoA carboxylase
MONOPTERIS – root, sterol C-14 reductase
GNOM – root and shoot, guanine nucleotide exchanger, facilitates vesicular targeting that controls PIN localization
Epidermal, cortical & endodermis, pericycle and vascular cells in late heart shaped stage embryos, typical root patterning
Protoderm (epidermis) development - ARABIDOPSIS THALIANA MERISTEM LAYER 1 (ATML1) and PROTODERMAL FACTOR 2 (PDF2), homeodomain transcription factors, regulate development of the epidermis
Vascular tissue differentiation – cytokinin is implicated based on pharmacological and genetic evidence
WOODEN LEG (WOL) – encodes a cytokinin receptor and regulates phloem development, wol – no protophloem
Ground tissue (cortex and endodermis) development
SCARECROW (SCR) and SHORTROOT (SHR) – encode GRAS family transcription factors that regulate genetic determinants of cortical and endodermal development
scrscr – cortex and endodermis are disturbed (merged)
shr – no endodermis
wol – no phloem
Meristems - progenitor cells retained throughout the plant life cycle, cells in which developmental programs are initiated, respond to hormones and environmental cues
The most evident meristems are the shoot apical meristem (SAM) and the root apical meristem (RAM) that are responsible for the shoot and root development, respectively
Other specialized meristems give rise to unique cell and tissue types, e.g. stomata, vascular tissue, intercalary tissue
Shoot apical meristem – source of undifferentiated cells from which specific cell types, tissues and organs develop
Shoot meristem development may not be directly affected by auxin
Direction of auxin movement (arrows) during the initiation of the shoot apical meristem (SAM) infers that auxin is not directly involved in SAM formation
Shoot Apical
Numerous genes contribute to the development of the shoot apical meristem
WUSCHEL (WUS) and CLAVATA 3 (CLV3) are critical determinants during the initial events of SAM cell patterning
WUS expression in early globular stage, and CLV3 expression in late heart stage
SAM zonation and cell layersCentral zone (CZ) - meristem initials (stem cells) that are progenitors of other cells in the shoot, slow but unlimited division for maintenance of meristem identity
Peripheral zone (PZ) – high division rate, produces leaf primordia
Rib zone (RZ) – subjacent to the central zone, produces internal tissues
Layers are L1, L2 and L3 – meristem initials that give rise to the epidermis (L1), subepidermal (L2) and cortical tissues (L3)
Control of the meristem organization is dependent on an auto feed-back loop of transcriptional regulation
Shoot development is genetically programmed, coordinated by transcriptional regulatory signal cascades that modulate gene expression in specific cell types and in a particular timeframe
Genetic program is regulated by chemical cues, such as hormones, small/micro RNAs, mRNAs and proteins but is also responsive to the environment
Root apical meristem
Root apical meristem (RAM) is the initiator of root development
RAM meristem identity and patterning are similar to that of the SAM
A basic difference between shoot and root development is the development of lateral branches of the system
Branching in roots occurs substantially away from the RAM, which means that lateral root primordia are not subjected to shear force cause by penetration of the primary root tip through soil
Auxin is involved in RAM cell identity
Auxin down-regulates expression of genes that encode negative regulators (auxin response factors, ARF), which lead to activation of PLETHORA (PLT) genes
PLT expression activates SCARECROW (SCR) and SHORTROOT (SHR), which specify quiescent center and stem cell identity
Four developmental zones in the root tip: root cap, meristematic zone, elongation zone and the maturation zone
Root cap – layer of cells that protects the stem cells, senses gravity (perhaps water and nutrients), produces a mucopolysaccharide that facilitates root penetration through soil
Meristmatic zone – meristematic cells including the quiescent center that contains meristem initialsMeristem - differentiation and development of the epidermis, cortex, and stele (vascular tissue)
Elongation zone – cells rapidly expand both radially and longitudinally (vertically), minimal cell division
Maturation zone – cell elongation ceases and development of root hairs and lateral roots
Differentiation may begin earlier but is completed in this region, lateral roots are differentiated from the pericycle, a specialized meristem
Shoot and root development – genetic programs for which numerous important determinants have been identified
Auxin regulates phyllotaxic organization
Auxin accumulates where primordia initiate, application of auxin changes leaf primordium development, modulation of PIN auxin transporters also alters primordia development
Shoot - pattern of leaf development (phyllotaxy) is regulated in the SAM, which can be altered by environmental responses
Leaf meristem has three developmental axes: tip to base, radial, and upper (adaxial) and lower (abaxial) surfaces
Shoot branching occurs from meristems in the axils of leaves an is a direct consequence of SAM activity
Root branching is due primarily to divisions of a specialized meristem, pericycle, in the maturation zone of the root
Senescence and programmed cell death – genetic program that is influenced by the environment, specific genes have been identified
Senescence occurs at the organismal, organ and tissue levels, leaves, fruits, etc.
Regulatory genes (e.g. ethylene biosynthesis) activate genes that encode hydrolytic enzymes like proteases, ribonucleases and lipases
Soybean plants are similar age but flowers were removed from the plant on the right
Some plants undergo senescence after one reproductive cycle (monocarpic senescence)
Programmed cell death is a cellular process – tracheary element formation and hypersensitive defensive responses are most classic
RAM lineage
Cells adjacent to the quiescent center are the progenitors of specific cell types, constitute the RAM
Columella initials – subjacent to the quiescent center, progenitors of the central part of the root cap
Epidermal and root cap initials – flank the quiescent center and produce the root epidermis