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Sunday, February 22, 2009

Male Sterility in Plants


Male Sterility

Plant that do not produce viable, functional pollen grains.

Male Sterility: An inability to produce or to release functional pollen as a result of failure of formation or development of functional stamens, microspores or gametes






Types of Sterility


Three types of sterility:
  • “Pollen sterility” in which male sterile individuals differ from normal only in the absence or extreme scarcity of functional pollen grains (the most common and the only one that has played a major role in plant breeding).
  •  “Structural or staminal male sterility” in which male flowers or stamen are malformed and non functional or completely absent
  • “Functional male sterility” in which perfectly good and viable pollen is trapped in indehiscent anther and thus prevented from functioning

Classification of Male Sterility


Based on its inheritance or origin:

1. Cytoplasmic male sterility (CMS) = sterile cytoplasm (S)

Male sterility comes about as a result of the combined action of nuclear genes and genic or structural changes in the cytoplasmic organellar genome

maternally inherited

2. Nuclear male sterility (NMS) = Genic, genetic, mendelian

Male sterility is governed solely by one or more nuclear genes

Nuclear inherited

3. Non genetic, chemically induced male sterility
Application of specific chemical (gametocides or chemical hybridizing agents)

Stamen (anther and filament) and pollen grains are affected

It is divided into:

a. Autoplasmic

CMS has arisen within a species as a result of spontaneous

mutational changes in the cytoplasm, most likely in the

mitochondrial genome

b. Alloplasmic

CMS has arisen from intergeneric, interpecific or occasionally

intraspecific crosses and where the male sterility can be

interpreted as being due to incompatibility or poor co-operation

between nuclear genome of one species and the organellar

genome another

CMS can be a result of interspecific protoplast fusion

Cytoplasmic Male Sterility

Origins:

1. Intergeneric crosses


2. Interspecific crosses


3. Intraspecific crosses


4. Mutagens (EMS, EtBr)


5. Antibiotic (streptomycin and Mitomycin)


6. Spontaneus


CMS Mechanism of Action

1. Abnormal behavior of the tapetum in the anther


2. Genetic determinant of CMS reside in mitochondria


3. Nuclear gene control the expression of CMS

CMS Limitations 

1. Pleiotropic negative effect of the CMS on agronomic quality performance of plants in the CMS cytoplasm


2. Enhanced disease susceptibility


3. Complex and environmentally unstable maintenance of male sterility and/or male fertility restoration


4. Inability to produce commercial quantities of hybrid seed economically because of poor floral characteristic of cross pollination

Utilization of CMS

1. It provides a possible mechanism of pollination control in plants to permit the easy production of commercial quantities of hybrid seeds

2. It consists of a male sterile line (the A-line), an isogenic maintainer line (The B line), and if necessary also restore line (the R-line)

3. A lines are developed by back-crossing selected B-lines to a CMS A-line for 4 – 6 times to generate a new A-line

4. B and R-lines are developed by similar back cross procedures using a CMS R-line as female in the original cross and a new line as the recurrent parent in 4 – 6 backcrosses

Nuclear Male Sterility [NMS]

1. Originated through spontaneous mutation or mutation by ionizing radiation and chemical mutagens such as ethyl methane sulphonate (EMS) and ethyl imine (EI) or by genetic engineering, protoplast fusion, T-DNA transposon tagging and affecting the synthesis of flavonoids


2. can probably be found in all diploid species


3. Usually controlled by mutations in genes in the single recessive genes affect stamen and pollen development, but it can be regulated also by dominant genes


Methods of Inducing Male Sterility


1. Through Chemical Agents


2. Through Recombinant DNA (rDNA) Technology

Chemical Agents Used

1. Plant hormones/hormones antagonists

a. auxins and auxin antagonists (NAA, IBA, 2,4-D, TIBA, MH)

b. Gibberellins and antagonist (GA3, GA4+7, CCC: 2-chloroethyl-trimethyl ammonium chloride)

c. Abscisic acid

2. Other substances

a. LY195259

b. TD1123

Recombinant Methods

Targetting the expression of a gene encoding a cytotoxin by placing it under the control of an ather specific promoter (Promoter of TA29 gene)

Expression of gene encoding ribonuclease (chemical synthesized RNAse-T1 from Aspergillus oryzae and natural gene barnase from Bacillus amyloliquefaciens)

RNAse production leads to precocious degeneration of tapetum cells, the arrest of microspore development and male sterility. It is a dominant nuclear encoded or genetic male sterile (GMS), although the majority of endogenous GMS is recessive

Fertility Restoration

Restorer gene (RF) must be devised that can suppress the action of the male sterility gene (Barstar)
a specific inhibitor of barnase

Also derived from B. amyloliquefacien Served to protect the bacterium from its own RNAse activity by forming a diffusion-dependent, extreemely one to one complex which is devoid of residual RNase activity

The use of similar promoter to ensure that it would be activated in tapetal cells at the same time and to maximize the chance that barstar molecule would accumulate in amounts at least equal to barnase

Inhibiting the male sterility gene by antisense. But in the cases where the male sterility gene is itself antisense, designing a restorer counterpart is more problematic

Strategies to Propagate Male-Sterile Plant

1. Selection by herbicide application

2. Inducible sterility

3. Inducible fertility

4. Two-component system