Plant Physiology Cell differentiation and Senescence

Cell differentiation

Differentiation: both ways

Totipotency: the ability of a single cell to divide and produce all the differentiated cells in an organism Tracheary Elements: Vessel elements+Tracheids • facilitate water and solute movement • Construct secondary wall composed of cellulose and lignin

Cells of adult plants remain totipotent: cloning a carrot Regenerated adult plant… 1 mm 3 fragments (“explants”) from adult root… Induce with hormones to initiate shoot and root formation e.g. Auxin, Cytokinin, Ethylene Culture “embroid” in liquid culture, then agar… Culture explants in liquid culture medium… Cells “dedifferentiate” and begin to divide, forming “callus” tissue… Move to soil…

Culturing Plant Tissue - the steps

• Establishment of the explant in a culture medium. The medium sustains the plant cells and encourages cell division. It can be solid or liquid • Each plant species has particular medium requirements that must be established by trial and error

Culture Medium constituents

• Inorganic salt formulations • Source of carbohydrate • Vitamins • Water • Plant hormones - auxins, cytokinins, GA’s • Solidifying agents

Media Composition Macronutrients

(required content in the plant - 0.1% or % per dry weight) - C, H, O, P, K, N, S, Ca, Mg

Micronutrients

(requirement - ppm/dry weight) - Fe, Mn, Zn, Cu, B, Cl, Mo, Na, Se and Si are essential for some plants • Two Hormones Affect Plant Differentiation: – Auxin: Stimulates Root Development-cell enlargement – Cytokinin: Stimulates Shoot Development-cell division • Generally, the ratio of these two hormones can determine plant development: – ↑ Auxin ↓Cytokinin = Root Development – ↑ Cytokinin ↓Auxin = Shoot Development – Auxin = Cytokinin = Callus Development

Plant Tissue Culture Applications

Micropropagation

A single explant can be multiplied into several thousand plants in less than a year - this allows fast commercial propagation of new cultivars e.g. Strawberry, Orchids

Developmental Pathways Transcription Factors

• 1500 in Arabidopsis

MADS box TFs

• 30 in Arabidopsis • Developmental events in roots, leaves, flower, ovule and fruit

Homeobox TFs

• Contain Homeodomain • KNOTTED1 (KN1) involved in shoot apical meristem maintenance

Cell Signaling

Different proteins, hormones interact to control plant development

Protein Kinases Mitogen Activated Protein Kinases

• Protein Phosphorylation • Arabidopsis MPK4

Arabidopsis 60 MKKKs 10 MKKs 20 MAPKs Receptor Kinases

• Contain receptor domain CLAVATA-mutations increased size of vegetative shoot apical and floral meristems •

Receptor Ligand

• Small proteins or peptides that activates the receptor kinases

Leaves in fall tell a story about life and death

Plant Senescence and Organ Abscission

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Concept

Senescence: A program in which the function of

organ or whole plant naturally declines to death.

This is a genetically programmed essential phase of the growth and development in plant.

Abscission: Specific cells in petiole form an

abscission layer, thus facilitating the senescent organ to separate from the plant

Types of plant senescence

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(1) Overall Senescence

Senescence occurs in whole plant body, such as annuals which senesces to death after flower and setting.

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(2)Top Senescence

The part aboveground dies with the end of growth season but the part underground is alive for several years. Perennial weeds , corm and bulb— —lily.

In summer In winter

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(3) Deciduous senescence

The leaf falls in specious season, in summer or winter.

Deciduous trees

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(4)Progressive senescence

Senescence only occurs in older organ or tissue. New organ or tissue develops while old ones are senescing.

Green trees 。

(A) Development of Arabidopsis thaliana plants is shown at various times after germination. Photographs show plants at 14, 21, 37, and 53 days after germination ( left to right). Note the yellowing of shoots of the 53 day-old plant. (B) Age-related changes in rosette leaves of Arabidopsis 7, 9, and 11 days after leaf expansion had ceased. Note the progressive yellowing of leaves, beginning farthest from the main veins

Senescence in Arabidopsis thaliana.

Arabidopsis mutants affected in senescence

Arabidopsis leaf senescence Rice leaf senescence

leaf Abscission zone at base of leaf at the where it joints the stem stem

Auxin prevents abscission

Senescence Regulation

SDG Senescence down regulated genes

include chlorophyll a/b-binding protein gene (CAB), Rubisco small subunit gene (SSU). SAG (up-regulated during leaf senescence)

Class I SAG Senescence associated genes

expressed only during senescence (senescence-specific).

Class II SAG Senescence associated genes

have basal level of expression during early leaf development, but expression increases during senescence

Basal expression (SAG II) Leaf develops to full expansion Onset (SAG I) Senescence progression

Leaf Senescence in Arabidopsis

Class I – genes that control developmental aging Class II – genes that control both senescence and other growth processes Class III- genes that control senescence in response to environmental factors Class IV- regulatory genes that up-regulate senescence activity AND down regulate cellular maintenance activities Class V- genes that control degradation of senescence regulatory factors

Programmed cell death is a specialized type of senescence

Process whereby individual cells activate an intrinsic senescence program = Programmed Cell Death (PCD) In animals, PCD may be initiated by specific signals (errors in DNA replication during division) - involves expression of a characteristic set of genes, resulting in cell death - accompanied by morphological and biochemical changes (apoptosis, Greek: “falling off”) - during apoptosis, cell nucleus condenses and DNA fragments in a specific pattern

Programmed cell death is a specialized type of senescence

PCD in plants, less well characterized - but similar histological changes as in animals observed - PCD occurs during differentiation of xylem tracheary elements, during which nuclei and chromatin degrade and cytoplasm disappears → activation of genes encoding nucleases and proteases - protection against pathogenic organisms - infection by pathogen causes plant cells to quickly accumulate high concentrations of toxic phenolic compounds and die (it’s not quite as simple) → dead cells form small circular island of cell death (necrotic lesion) - necrotic lesions isolate and prevent infection from spreading to surrounding healthy tissues by surrounding the pathogen with a toxic and nutritionally depleted environment (hypersensitive response)

Further Readings

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Growth and Development, Plant Physiology by Taiz and Zeiger