L7- Crop breeding Flashcards
what are the stages of plant development
flowering- determines potential of the yield
gamete development- male gamete is the pollen
pollen germination
fertilisation- where gametes come together
embryogenesis- fertilised zygote producing embryonic plant
seed filing- source of energy
what is gametogenesis
production of gametes, this precedes fertilisation
female-
start with single cell that undergoes meiosis, in most plants there is programmed cell death which makes it single haploid cell, single haploid cells undergoes three rounds of mitosis to produce single cells containing 8 nuclei, this then forms cell membranes that make multiple cells, these then specialise some go through cell death, polar nuclei fuse and form central cell (egg cell) on either side have synergy cells, these arent fertilised but are very important as they pump out lots of singals that communicate and attract pollen tube
male-
single cell undergoes meiosis, forms 4 haploid cells with a macrospore, all cells persist process of mitosis that splits nucleus, forming a vegetative nucleus and generative nucleus. plasma membrane seperates two nuclei, undergoes mitosis two nuclei within generative cell that is encapsulated within vegetative cells. get pollen grain that has 3 nuclei- 1 1 vegetative and 2 generative.
how do plants fertilise
in the carpel (ovary), you have the stigma where pollen grains land, if signalling is correct water is released from the stigma allowing pollen grains to germinate, this form pollen tube by expansion, they grow down through stigma, ovary and enter ovules
pollen enters ovule near synergism cells, one will degenerate other will fuse with central cell. firstly egg cell is fertilised and also central cell is fertilised- fertilised egg cell forms embryo plant, central cell goes from diploid to triploid which forms endosperm
what is embryogenesis
rounds of cell division that forms embryo - goes through octant, globular, heart and torpedo stage
endosperm is very important as it is major food source, rice is the starchy endosperm
why is plant breeding difficult
each step in fertilisation is affected by various conditions, such as chilling stress.
how were yields increased in the past
borlaug and the green revolution
- changes in plant architecture- short varieties
- use of nitrogen fertiliser
- use of herbicide and pesticides
- irrigation
how did seed yield increase
- wang case study
- changes in seed size
- changes in flower architecture
-changes in plant architecture ( shorter varieties, more tillers)
ahong wang and colleagues studied cultivated rice origins
created near isogenic lines (introgression of small sections of DNA into a different species, this should be homozygous, should both be two alleles from same parent so you know effect) between GLA4 (cultivated), and W1943 (wild), this was done via meiosis recombinant, and then many backcrosses
- it was found there was a 10-20% difference in grain length between GLA4 (cultivated) and the NIL of GLA4 and W1942, this wild rice is heterozygous for a premature stop codon
GL6 is a transcription factor regulating cell number in grains- found that the NIL lines had increased grain size, however- caused decrease in yield per plant
pleiotropic effect of the gene mean multiple traits affected
what is genetic abortion of floret development / production of sterile florets
abortion of flowers or sterile flowers are common in :
maize, sorgum, barley, wheat, oats and rice
in rice, florets are protected by structure called lemma, with two sterile lemma, mutations in gene G1, cause the sterile lemma to grow to normal size equivalent to none-sterile ones
in barley- VRS1 is a transcription factor that when lost there is complete reversion of fertility which increases the rows of grain
in wheat- GN1= VRS1 orthologue, a mutation here= partial reversion of fertility slight increase in rows of grain
what is the overview of understnading crop yield, and challneges here
helps to understand the mechanism behind each development stage, identify lines with positive increases in yield, find causal changes, target key proteins / metabolites for crop improvement
challenges
- complex genetics including ploidy - wheat is hexaploid means every copy of the genes need to be chnaged can be like 10
-most trait are quantitative, many genes of small effect
- pleiotropic effect of genes, many genes often effect mutliple parts of plant development - looks of different effects
are crops sensitive to climate extremes
um yes!
effect development, reproduction and grain development
what is effect of heat stress on pollen grain development
plants at flowering stage, were removed and put into climates 4 degrees warmer, resulted in decreased pollen grains, decreased by about 50%
disproportional effect, geminated and viable grains decreased significantly, non-viable grains increased
- pollen grain number, germination and viability are reduced at higher growth temps
possible link to reduced starch conc in anthers at 3 days prior to anthesis and lower soluble sugar conc in mature pollen grains
also! pollen tube growth was also effected by heat stress, the pollen tubes were stunted, male and female gamete signalling interrupted, those that did germinate did not grow correctly, found the ovules were also not viable
reproductive development are sensitive to heat stress (but also cold and drought stress), effects gamete development, pollen grain viability, embryogenesis and seed filing, cell division and source-sink relations
what is effect of stress on pollen tube reception / fertilisation
example of CrRLK1L of CrRLK1L
pollen tube reception is reliant on lots of signalling
CrRLK1L, is a receptor like kinase in arabidopsis, these are proteins which sit on surface of cell and communicate outside conditions into internal responses which are appropriate for environment of the cell
they have an kinase domain inside cell and malectin-like domain (MLD) known to bind two different signals, one is peptide and carbohyrates, detect status of cell wall and singals from themselves and other cells and integrate response
the regulate- fertilisation, cell wall integrity, tip growth, cell expansion and plant-pathogen interactions
HERK1 and ANJEA (ANJ) are involved in reproduction. functional redundancy between two proteins mean reproduction is affected when both proteins are lost, need one or other. in the double mutants, pollen tubes dont stop growing when they enter the embryo sac. HERK1 and ANJEA accumulate at the filiform apparatus, they are expressed in the ovule and expressed in synergid cells, these are ovular determinants of pollen tube reception
many groups investigating the interaction of CrRLK1L receptor kinases, peptide singals and the cell wall in reproduction
what is challenge to crop breeding
requires crossing germplasm, leads to heterozygous genome and subsequent segregation of advantage and disadvantage traits, how can we speed this up to get homozygous stable lines
what are NILs v RILs
near isogenic lines (NILs)
use meiosis and recombination, and then backcross to parent, to dilute to see certain traits
RILS
use meiosis and recombination, but then selfed multiple times, 50% homozygous after 1 gen, 75% homozygous after 2 gens, 87.5% homozygous after 3 gens, until you have kinda more even mixed traits
how can you speed breed
if yield is not an issue, then can go for a faster life cycle, in field 1-2 gens per yr
-under glasshouse cond 2-3 gens per yr
- increase light levels and day length, 6 gens per yr
-increase plant density, decrease generation time further with 19 seeds per wheat plant
