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 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

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 FACKEL – hypocotyl/central region, auxin response factor, transcriptional factor that regulates auxin response determinants, regulates PIN7 expression (auxin distribution)

Radial patterning - specialized meristems develop in globular stage embryos and differentiate tissues of the embryo Ground tissue/cortex & endodermis - ground meristem Vascular tissue - procambium Epidermis Epidermis (tissue) - protoderm (meristem)

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 scr scr – 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 layers Central 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 initials Meristem - 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 Cortical and endodermal initials Vascular initials