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Tuesday, February 3, 2009

Cytoplasmic Male Sterility in Plants

Male Sterility

Plants with male sterility don’t produce functional / viable pollen grains.
Male Sterility is of two types:
Cytoplasmic Male sterility
Genic Male sterility

Advantages of Male Sterility & Its Importance in Plant Biotechnology

In palnt breeding to obtain hybrids cross pollination need to be done among selected parental lines High level of self pollination any occur especially in bisexual plants. This hampers hybrid seed production to overcome this problem and ensure crosses between the selected female and selected male, several types of pollination control need to be used.

Mannual emasculation
CMS
GMS
Self Incompatibility
male Gametocides
Use of genetically Engineered Systems for Male Sterility

Male sterility in plants may be nuclear or Cytoplasmic. Cytoplasmic male sterilityis invariably due to defective functioning of to the tapetum of the anthers. The main function of tapetum is to supply nutrients to developing pollen grains. So far 40 plant species are reported to have Cytoplasmic male sterility.
In maize there are specific nuclear genes which are called as fertility restoring genes (Rf). Rf genes restore polar fertility after crossing. In such case when male sterile plants with CMS is crossed with maize with Rf genes produce male fertile plants. Hence the defect from Mitochondrial function can be corrected by product encoded by nuclear genome of restorer maize.
Cytoplasmic male sterility is due to presence of additional DNA sequence in mitochondria designated as T-urf13, that codes for 13000 molecular weight polypeptide.
Restorer nuclear genome suppresses the synthesis of this polypeptide.CMS has become a major focus in Agriculture Biotechnology
In plant breeding two parental types are crossed to produce high yielding varities. Hybrid vigour or heterosis expressed only in F1 generation. Yield reduces rapidly with subsequent generations. Hence to cross two parental types the anthers of Female plant i.e., Seed producing plant is either removed or emasculated. This process is a labour intensive process and it is impractical. to perform on large scale.
Hence CMS lines play a role as they are genetically emasculated. These CMS lines are not capable of producing viable pollen, emasculation is not necessary to produce hybrid seeds. During hybrid seed production 3 lines of plants are maintained.

A – Line (Hybrid Seed Producing Parent) with CMS (defective mitochondrial genes) and recessive restorer genes in the nucleus.
B – Line: These are called as maintainers. This maintainers have normal(N) cytoplasmic genes but recessive restorer genes.
C- Line: Restorer Plant. They have normal cytoplasm and dominant nuclear restorer genes.
When A line is Crossed with B line


A
Both Cytoplasmic and nuclear Normal Cytoplasm but Nuclear
Male sterility.No viable pollen male sterility because of normal cytoplasm viable pollen
(Female) are produced
(Male)



Hybrid Plant
(Maintainer Plants)
No viable Pollen

Cytoplasmic Male Sterility [CMS]

Cytoplasmic Male Sterility is noticed in more than 150 species. Cytoplasmic Male Sterility is invariably due to defects in mitochondrial genome and it is exclusively maternally inherited. Female fertility is not affected by Cytoplasmic male sterility. Palnts with Cytoplasmic male sterility do produce seeds if viable pollen is provided.
To increase crop productivity, in recent years there has been the development of hybrid cultivar in a wide range of crops crosses between genetically distinct parental lines or populations give rise to progeny that exhibit heterosis or hybrid vigor. Hybrids in general are more resistant to disease and insects, less suceptible to environmental stress and yield more seed.
Inorder to do cross pollination CMS is most beneficial factor as it is not needed to emasculate the flower which is practically very difficult to perform in minute flowers.
The CMS is mainly due to defective functioning of tapetum of the anther. The main function of pollen is to supply nutrients to developing pollen grains. In nature CMS orginates because of intergenc crosses, Interspecific crosses,

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