lec 6- fertilization in plants

Question 1

what is fertilization of plants called?

Answer 1

pollination

Question 2

how does the immobile pollen reach its target flower?

Answer 2

either by pollinators or the wind

Question 3

what happens when pollen lands on the stigma of the flower?

Answer 3

-when pollen lands on the stigma, the pollen germinates
-it grows into a pollen tube that goes down the style and becomes attracted to the ovules due to their scent

Question 4

what happens if a foreign pollen lands on a stigma?

Answer 4

-the pollen is unable to read any of the cues present on the stigma, in the style, or sent from the ovules
-this means the pollen grain cant germinate at all, if it does it will not be able to fertilize the ovules

Question 5

how does a pollen turn into a pollen tube?

Answer 5

-when the pollen lands on the stigma, the first recognition event occurs
-the coat of the pollen and the interaction with the stigma determines if pollen tube can form, these ques are species-specific and can differ species to species
-water from the stigma causes the pollen to grow

Question 6

what is self incompatibility?

Answer 6

when a plant that produces pollen is unable to fertilize its own ovules

Question 7

how is the pollen tube attracted to the ovules?

Answer 7

-the pollen tube goes down the style then invades the ovaries
-the ovules secrete signals that indicate where they are located in the ovary
-the pollen senses the signals and it grows towards the ovules, species-specific cues

Question 8

when does fertilization occur?

Answer 8

when the pollen tube reaches the ovules

Question 9

what signals do the ovules secrete to attract the pollen tubes?

Answer 9

chemoattractants, which cause the pollen tube to grow towards it via chemotaxis

Question 10

are chemoattractants also species specific?

Answer 10

yes, (e.g. a corn pollen tube will only be attracted to corn chemoattractants from an ovule, not those of other plants)

Question 11

when the pollen tube reaches the ovule, what event occurs?

Answer 11

double fertilization

Question 12

what is double fertilization?

Answer 12

when the two nuclei in the pollen tube fuse with the two nuclei in the egg sac of the ovule

Question 13

how does pollen (the male gamete) form?

Answer 13

-a diploid cell inside the anther undergoes meiosis and created 4 haploid microspores
-the 4 microspores’ nuclei then divide by mitosis which creates a generative cell (pollen) and the tube nucleus
-after germination, the generative cell forms two sperm cells in the pollen tube

Question 14

how does the female gametophyte develop?

Answer 14

• cell within the ovules undergoes meiosis, creating 4 meiotic products but only 1 lives, it is called the megaspore
  -several mitotic divisions occur, and the resulting haploid nuclei migrate to specific places in the developing egg sac
  -cellularization occurs, mature egg sac contains several haploid cells

Question 15

explain double fertilization with detailed steps:

Answer 15

-the pollen tube reaches the ovule and ruptures the synergid cell letting the sperm cells be released, the synergid nuclei degenerate
-the sperm cells than fuse with the egg to form a 2N zygote and the central cell nucleus to form a 3N cell that gives rise to the endosperm

Question 16

does a block activate when the pollen tube penetrates the ovule?

Answer 16

yes, called polytubey

Question 17

what are the two types of self incompatibility?

Answer 17

-sporophytic self incompatibility
-gametophytc self incompatibility

Question 18

what is sporophytic self incompatibility?

Answer 18

when pollen grains are recognized as self or not self when they land on the stigma

Question 19

what is gametophytic self incompatibility?

Answer 19

-when the pollen grain has germinated into a tube and enters the style, it is in the style where it is recognized as being either self or non self

Question 20

what is the set of genes on the chromosome that control self incompatibility called?

Answer 20

the S loci

Question 21

explain sporophytic self incompatibility in terms of S loci?

Answer 21

-if a plant is heterozygous at the S loci with the genes S1S2, the pollen grains will have either S1 or S2 genes and they will interact with receptors on the stigma
-if the receptors on the stigma are also S1 or S2 it will result in incompatibility due to the receptors binding to the ligands on the S1 or S2 pollen
-if the pollen is another gene like S3 or S4, the receptors will not be triggered, not causing a signal transduction pathway which lets pollen grains to be germinated

Question 22

explain gametophytic self incompatibility in terms of S loci?

Answer 22

-if the plant is heterozygous at the S loci with the S1S2 genes, pollen will be S1 or S2
-the pollen will land on the S1S2 stigma and will still germinate but pollen tube growth will be aborted at the style
-if S1 or S2 pollen lands on a S1S3 stigma, s1 pollen tubes will be stopped in the style while S2 get through and fertilize the ovules
-if the S1 or S2 pollen lands on a S3S4 stigma, both will germinate and turn into pollen tubes that enter the ovary and fertilize the ovules
-the style releases S1 and S2 proteins that interact with the receptors on the pollen tube and it binds, causing influx of Ca and death of pollen tube