Q.49 Cytoplasmic male sterility via the chloroplast genome can be induced by the expression of Pha A
gene encoding
(A) β-Ketothiolase
(B) Acetoacetyl CoA carboxylase
(C) Acetoacetyl CoA reductase
(D) PHB synthase
Cytoplasmic Male Sterility PhaA Gene
Cytoplasmic male sterility (CMS) is a valuable trait in hybrid seed production, engineered via chloroplast transformation with the PhaA gene from the polyhydroxybutyrate (PHB) pathway. Question 49 states: “Cytoplasmic male sterility via the chloroplast genome can be induced by the expression of PhaA gene encoding (A) β-Ketothiolase (B) Acetoacetyl CoA carboxylase (C) Acetoacetyl CoA reductase (D) PHB synthase.”
This MCQ targets biotech applications in plant genetic engineering, focusing on plastid-expressed genes disrupting pollen development.
Correct Answer: (A) β-Ketothiolase
The PhaA gene encodes β-ketothiolase, the first enzyme in bacterial PHB biosynthesis, catalyzing acetyl-CoA to acetoacetyl-CoA. When overexpressed in chloroplasts (using psbA promoter), it accumulates toxic intermediates, impairing anther tapetum function and causing pollen abortion—inducing CMS without affecting female fertility.
Studies confirm PhaA expression leads to homoplasmic integration, high transcription in anthers, and sterile pollen, ideal for three-line hybrid systems in crops like rice or maize.
Explanation of All Options
β-Ketothiolase (Correct Choice)
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PhaA enzyme initiates PHB pathway: 2 acetyl-CoA → acetoacetyl-CoA.
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Chloroplast expression disrupts pollen metabolism via metabolic burden.
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Light-regulated (psbA promoter); confirmed sterile anthers via microscopy.
Acetoacetyl CoA Carboxylase (Option B)
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Encoded by phaC or similar; carboxylates acetoacetyl-CoA to 3-hydroxybutyrate-CoA.
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Not PhaA; later PHB step, less studied for CMS induction.
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Involved in PHA diversity, not primary male sterility trigger.
Acetoacetyl CoA Reductase (Option C)
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Typically phaB gene; reduces acetoacetyl-CoA to R-3-hydroxybutyryl-CoA.
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Mid-pathway enzyme; overexpression alone insufficient for CMS.
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Used in full PHB operons but not the specific PhaA target.
PHB Synthase (Option D)
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phaC gene product; polymerizes 3-hydroxybutyryl-CoA into PHB granules.
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Final PHB enzyme; causes polymer accumulation but pleiotropic effects.
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Nuclear expression problematic; chloroplast PhaC studied separately from PhaA.
| Option | Gene | PHB Pathway Step | CMS Role? |
|---|---|---|---|
| β-Ketothiolase | PhaA | Acetyl-CoA → Acetoacetyl-CoA | Yes (anther disruption) |
| Acetoacetyl CoA Carboxylase | phaC-like | Carboxylation step | No |
| Acetoacetyl CoA Reductase | phaB | Reduction step | No |
| PHB Synthase | phaC | Polymerization | Partial |
Relevance for Biotechnology and Exams
Understanding cytoplasmic male sterility PhaA gene is key for plant biotech, enabling transgene containment and hybrid vigor without chemical hybridizing agents. It appears in advanced GATE/CSIR questions on chloroplast engineering and metabolic pathways.
This approach leverages maternal inheritance of chloroplasts, preventing transgene escape via pollen—crucial for GM crop biosafety.
