VL 7: RNAi II

Question 1

What are microRNAs (miRNAs), and what is their role in gene regulation?

Answer 1

miRNAs are small non-coding RNAs encoded by MIR genes that regulate gene expression, but are trans-acing regulatory factors.

• by slicing target mRNAs or
• interfering with their translation.

They play crucial roles in plant developmental timing, stress responses, and other physiological processes.
Some MIR gene fmilies are highly conserved and present in all plants or all angiosperms.

Question 2

How are miRNAs processed and how do they function?

Answer 2

1. miRNAs are encoded by MIR genes and are transcribed as primary miRNAs (pri-miRNAs)
2. pri-miRNAs fold into double-stranded structures and are processed by Dicer-like enzymes (DCL1) into mature miRNAs.
3. These miRNAs associate with Argonaute proteins (often AGO1) to regulate target mRNAs. (always cleaves at the same position)

They are highly conserved and are important gene regulators –> neraly half of there targets are transcription factors

Question 3

How do miRNAs regulate the vegetative phase change in plants?

Answer 3

• miRNAs like miR156 regulate the transition from juvenile to adult growth. (change in morphology and reproductive competence)
• miR156 targets SPL genes, which are promoters of phase change.
• As miR156 levels decrease with plant age, SPL gene expression increases, leading to vegetative phase change. (Pflanze wächst zu adulter flanse heran, bekommt manchmal andere blätternnn)
• (HASTY is a protein needed for miRNA export from nucelus to cytoplam, mutant shows a shortened juvenile phase)

MIR156 family highly conserved, always has the same sequence because it is crucial to recognize the target

Question 4

What happens in miR156 overexpression or loss-of-function mutants in plants?

Answer 4

• Overexpression of miR156 prolongs the juvenile phase,
• while loss-of-function mutants exhibit precocious (früreif) phase change, demonstrating miR156’s role in controlling the timing of developmental transitions.

Question 5

microRNAs Summary Slide

Answer 5

• miRNAs are thought to have evolved from siRNAs, and are produced and processed somewhat similarly
• Plants have a small number of highly conserved miRNAs, and a large number of non-conserved miRNAs
• miRNAs are encoded by specific MIR genes but act on other genes – they are trans-acting regulatory factors
• miRNAs in plants regulate developmental and physiological events

Question 6

Conclusion slide about Small RNAs

Answer 6

• Small RNAs contribute to the regulation and defense of the genome, and confer silencing specificity through base-pairing
• siRNA targets include repetitive-rich heterochromatin, transposons, viruses or other pathogens
• miRNAs and siRNAs targets include regulatory genes affecting developmental timing or patterning, nutrient homeostasis and stress responses

Question 7

In what organism is miRNA also needed for developmental expression?

Answer 7

• In the nematode C. elegans, lin-4 silencing of lin-14 is required for developmental progression
• lin-14 promotes the juvenile state.

Question 8

How does miR165/6 regulate leaf polarity?

Answer 8

• miR165/6 is expressed on the abaxial (lower) side of the leaf.
• suppresses PHABULOSA (PHB) and PHAVOLUTA (PHV) transcription factors in the lower leaf domain
• In phb-1d mutants, miR165/6 binding is prevented, causing ectopic PHB activity and radialized, adaxialized leaves.

Question 9

How are miRNAs involved in root patterning?

Answer 9

• miR165/6 moves from the endodermis into the vascular cylinder, regulating HD-ZIPIII genes.
• Establishes radial patterning in the root.

Question 10

What role do miRNAs play in nutrient signaling?

Answer 10

Moves from shoot to root, signaling nutrient demand.
e.g.: * miR399 genes are specifically induced by phosphate starvation. If there is an overexpression, plants accumulate to much phosphate

Question 11

Phosphat Homeostasis

Answer 11

• The rate of nutrient uptake in the roots is regulated by signals from the shoot. miR399 is upregulated and trasported down to root
• PHO2 encodes a ubiquitin-conjugating E2 that presumably contributes to directed proteolysis
• PHO2 levels are regulated by miR399 which accumulates upon phosphate starvation
• Starvation-induced miR399 moves from shoot to root, regulating phosphate uptake through PHO2
• PHO2 expression is also controlled by IPS1, a target mimic of miR399

Wenn PHO2 downreguliert meht Phosphat Aufnahme.