1. miRNAs are small non-coding RNA molecules that regulate gene expression through base-pairing with mRNA. 2. The first miRNA, lin-4, was discovered in C. elegans in 1993. miRNAs are processed from hairpin pre-miRNAs by the enzymes Drosha and Dicer. 3. Mature miRNAs are incorporated into the RISC complex and silence genes by degrading or inhibiting the translation of mRNA.

1. UNIVERSITY OF AGRICULTURAL SCIENCES, DHARWAD
COLLEGE OF AGRICULTUE,VIJAYAPURA
Topic : miRNAs and their use in Plant Breeding
Presented By
Surya Charan
PGS22AGR9086
Jr Msc(agri) GPB
Submitted To
Dr. M D Patil
Scientist
ARS,Dharwad

2. Index
1. What is miRNA
2. History of miRNA
3. miRNA Biogenesis
4. Mechanism of action of miRNA
5. miRNA in Plants

3. What is miRNA?
A microRNA is a small non coding RNA molecule
found in plants, animals and some viruses , that
functions in RNA silencing and post transcriptional
regulation of gene expression.
miRNAs function via base-pairing with
complementary sequences within mRNA molecules.
miRNAs are hair pin-like structure found only in
Eukaryotes

4. Single stranded and small in size(19 to 25
nucleotides).It folds back on itself.
Play important regulatory roles in by targeting
mRNAs for cleavage or translation expression.
One miRNA can bind to more than one mRNA
(Vice versa)
Aberrant level of mRNA is controlled by miRNA.

5. History of miRNA
miRNA was first discovered in 1993 during a study
of the lin-4 gene in the study of C.elegans by Ambros
and coworkers.
 Also in 2000, let-7 RNA was regulated lin-41
mRNA(L4) to adult.
Researchers realized several small regulating RNA
were present in C.elegans, Drosophila and human
cells. Hence named miRNA.

6. miRNA Biogenesis
miRNA is transcribed as long pre-miRNA, which
undergoes cleavage and processing to form mature
miRNA.
miRNA is transcribed by RNA polymerase II and III.
Many miRNA genes lie in the introns of other genes.
The pre-miRNA is much longer and comprises the 5’ cap
and 3’ poly(A) tail.
The pre-miRNA undergoes nuclear and cytoplasmic
processing.

8.  In the nucleus, pre-miRNA is processed by the enzyme
Drosha.
It has an RNase III domain to form a microprocessor
complex.
In the cytoplasm, pre-miRNA is processed by another
enzyme called Dicer. It is also an RNase III enzyme.
It forms a miRNA duplex of 22 nucleotides.
Either of the strands has the capability to act as a
miRNA.
In the RISC complex, only one strand is incorporated
that targets the mRNA.

9. Mechanism of Action
miRNA like siRNA is involved in gene silencing
through RNA interference. It is involved in post-
transcriptional gene silencing (PTGS).
The mature miRNA is associated with other proteins to
form the RNA-Induced Silencing Complex (RISC). It is
also referred to as miRISC.
The processing of pre-miRNA by Dicer also causes
unwinding of the duplex and only one miRNA strand is
associated with the RISC complex. The other strand is
generally degraded.

10. miRNA silences a gene either by degrading mRNA or
by inhibiting the translation.
It also destabilises mRNA by shortening the poly(A)
tail.

11. miRNAs in Plants
miRNAs are considered to be key regulators of many
developmental, homeostatic, and immune processes in
plants.
Their roles in plant development include shoot apical
meristem development, leaf growth, flower formation,
seed production, or root expansion.
In addition, they play a complex role in responses to
various abiotic stresses comprising heat stress, low-
temperature stress, drought stress, light stress, or
gamma radiation exposure.

12. miR156 is highly expressed in the embryo and early
seedling stage and declines with increasing plant
stage.
High levels of miR156 early in plant development
suppress flowering and are necessary for expression
of the juvenile phase.
mi160 is a major regulator of root growth and
graviotropism.
miR172 regulate tassel formation.