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Plant bio

Plant development II

QuestionAnswer
Regulating developmental pathways in plants Transcription factors determine cell, tissue and organ identity • Developmental pathways are controlled by networks of interacting genes. These networks are regulated by transcription factors
Regulating developmental pathways in plants • Movement of transcription factors can contribute to the patterning process • Development is regulated by cell-to-cell signalling • Cell fate is determined by position, not clonal history (cell extrinsic information)
Radial patterning WT Arabidopsis roots have a distinct radial patterning of cell layers ▪ Defined plane of cell division in stem cells gives rise to specific cell files ▪ In roots, cell divisions are mostly anticlinal
Radial patterning - periclinal division is necessary to give rise to endoderm and cortex
Genetic control of root development in Arabidopsis - mutants are lacking distinct endodermis and cortex Mutations SHORTROOT (SHR) and SCARECROW (SCR) genes result in slow growing roots that lack distinct endodermal and cortical cell layers ▪ Both genes encode GRAS transcription factors
Periclinal cell division - lack of this (blocked by shr and scr genes) fails to produce two cell types
Agrobacterium transformation to analyse expression patterns Localising expression of the SHORTROOT (SHR) mRNA and protein in Arabidopsis roots
Transcriptional fusion between pSHR and GFP ▪ A recombinant gene was constructed ▪ The cellular transcription pattern can be determined following UV illumination of longitudinal sections
Translational fusion between SHR and GFP ▪ A recombinant gene was constructed ▪ Translation of the mRNA produces a chimeric protein ▪ The cellular location of the protein can be determined
Analysing SHR transcription and protein localization - Transcriptional fusion: SHR promoter: GFP fusion shows SHR is transcribed in cells of the vascular cylinder - Translational fusion: SHR protein: GFP fusions show the protein is also located in the adjacent endodermal cell layer and the QC.
Analysing SCR protein localization The SCR protein is expressed in a single cell layer in roots – the endodermis, but NOT the cortex. ▪ SCR expression is weak in the endodermal cell layer when SHR is absent. ▪ SHR expression stimulates SCR!
SHR role in endodermal cell fate determination SHR synthesised in stele cells moves in both directions across the nuclear pore complex (this accounts for the diffuse appearance of GFP in stele cells) ▪ SHR moves through plasmadesmata to neighbouring cell
SHR role in endodermal cell fate determination ▪ Modification of SHR when it enters the endodermal cell prevents further transport through PD ▪ SHR activation of SCR initiates endodermal cell development
SCR role in endodermal cell fate determination In scr mutants the single cell layer has features of both cell types. In shr mutants the single cell layer lacks endodermal features ▪ SCR and SHR are required in the same cell to specify endodermal characters
SCR role in endodermal cell fate determination ▪ Studies show the SCR and SHR proteins form a heterodimer complex that switches on expression of endodermal genes
Why are these important? Plane of cell division plays an important role in development • Transcription factors play an important role • Developmental pathways are controlled by networks of interacting genes
Why are these important? • Development is regulated by cell-to-cell signalling • Movement of transcription factors can contribute to the patterning process
Floral transition - the switch from vegetative to reproductive development - the SAM changes from a vegetative meristem to an inflorescence meristem
Factor that influence floral transition Age Photoperiod vernalization Ambient temperature Gibberelin Light quality Abiotic stress Sucrose
Timing of floral transition - several factors influence the timing All flowering cues are acting on the floral integrators: FT, SOC1, LFY
Floral transition mediation • Floral integrators activate the meristem identity genes on the position where the floral meristem will form • Floral meristem identity genes: LFY, AP1 (CAL)
Meristem identity genes are necessary to form a flower - CAULIFLOWER (CAL) is another floral meristem identity gene
Ap1/cal double mutant produces inflorescence meristems instead of floral meristems - Meristem identity genes activate downstream genes required for floral organ development
Flower body plan - four main floral organs: carpels, stamens, petals, sepals
Forward genetics helped understand flower development - Many mutants in flower development affect 2 consecutive whorls
Homeotic mutants in arabidopsis - ie. apetala2-2: 1 and 2 whorls affected
ABC-model of flower development • 3 classes of activity: A-, B- and C-function • Each of the ABC function encompasses 2 whorls • A and C function mutually repress each other
Floral organ identity genes - sepal and petal: APETALA1/2 - petal: APETALA3/PISTILLATA - stamen: PISTILLATA/AGAMOUS - carpel: AGAMOUS - phenotypes can be predicted using the ABC-model
Quadruple ABC-mutant changes floral organs into leaves • Quadruple mutant of Ap1, ap2, ap3/pi, Ag • Ectopic expression of these genes in leaves did not lead to homeotic transformations into floral organs - E class genes also needed for flower development
ABCE model is sufficient to specify floral organs Ectopic expression of A- , B- and E-genes gives rise to petals
SEPALLATA1-4 - additional gene needed for flower development
ABCE genes encode transcription factors Most ABCE genes encode MADSdomain transcription factors. • These transcription factors bind to DNA in dimers • These dimers can also form tetramers
Gene regulatory network alongside ABCE model ABCE genes regulate hundreds of genes • The regulatory networks downstream of the ABCE-proteins is complex
Created by: reub8n
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