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

1 mm3 fragments (“explants”) from adult root…

Culture explants in liquid culture medium…Cells “dedifferentiate” and begin to divide, forming “callus” tissue…

Induce with hormones to initiate shoot and root formation e.g. Auxin, Cytokinin, Ethylene

Culture “embroid” in liquid culture, then agar…

Move to soil…

Regenerated adult plant…

Culturing Plant Tissue - the steps

• Establishment of theexplant in a culturemedium. The mediumsustains the plant cells andencourages cell division. Itcan be solid or liquid

• Each plant species hasparticular mediumrequirements that must beestablished by trial anderrorCulture Medium constituents• Inorganic salt formulations• Source of carbohydrate• Vitamins• Water• Plant hormones - auxins, cytokinins, GA’s• Solidifying agents

• 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

Media CompositionMacronutrients (required content in the plant - 0.1% or % per dry weight) - C, H, O, P, K, N, S, Ca, MgMicronutrients (requirement - ppm/dry weight) - Fe, Mn, Zn, Cu, B, Cl, Mo, Na, Se and Si are essential for some plants

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

Plant Tissue Culture Applications

Developmental Pathways

•1500 in ArabidopsisMADS box TFs•30 in Arabidopsis•Developmental events in roots, leaves, flower, ovule and fruitHomeobox TFs•Contain Homeodomain•KNOTTED1 (KN1) involved in shoot apical meristem maintenance

Transcription Factors

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 MAPKsReceptor 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

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 • (1) Overall Senescence • Senescence occurs in

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

• (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

• (3) Deciduous senescence • The leaf falls in specious

season, in summer or winter.

• Deciduous• trees

• (4)Progressive senescence• Senescence only occurs in older organ or

tissue. New organ or tissue develops while old ones are senescing.

• Green trees。

Senescence in Arabidopsis thaliana.

(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

Arabidopsis mutants affected in senescence

Arabidopsis leaf senescence

Rice leaf senescence

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

leaf

stem

Auxin prevents abscission

SDGSenescence 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 genesexpressed only during senescence (senescence-specific).

Class II SAGSenescence associated geneshave basal level of expression

during early leaf development, butexpression increases during senescence

Basal expression (SAG II)

Onset (SAG I)

Leaf develops to full expansion Senescence progression

Senescence Regulation

Leaf Senescence in ArabidopsisClass I – genes that control developmental agingClass II – genes that control both senescence and other growth processesClass III- genes that control senescence in response to environmental factorsClass IV- regulatory genes that up-regulate senescence activity AND down regulate cellular maintenance activitiesClass V- genes that control degradation of senescence regulatory factors

Programmed cell death is a specialized type of senescenceProcess 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 senescencePCD 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

• Growth and Development, Plant Physiology by Taiz and Zeiger