The syllabus for Paper I of UPSC Botany Optional subject consists of the following topics:

1. Microbiology and Plant Pathology:
- Structure and reproduction of viruses, bacteria, and fungi.
- Principles of plant pathology, classification of plant diseases, and their control measures.
- Physiology of parasitism and disease resistance in plants.

2. Cryptogams:
- Classification, structure, reproduction, and life cycles of algae, bryophytes, pteridophytes, and gymnosperms.
- Evolutionary trends in the reproductive structures of these groups.
- Economic importance of algae and fungi.

3. Phanerogams:
- Morphology, anatomy, embryology, and reproduction of angiosperms.
- Comparative account of the families of angiosperms.
- Economic importance of angiosperms.

4. Plant Physiology:
- Water relations of plants, mineral nutrition, and ion transport.
- Photosynthesis, respiration, and nitrogen metabolism.
- Growth and development, plant movements, and plant hormones.
- Photomorphogenesis and phytochrome.

5. Biochemistry and Plant Metabolism:
- Structure and function of carbohydrates, proteins, lipids, and nucleic acids.
- Enzymes and their kinetics, mechanisms of enzyme action, and factors affecting enzyme activity.
- Metabolism of carbohydrates, proteins, lipids, and nucleic acids.
- Secondary metabolites and their roles in plant defense and other physiological processes.

6. Genetics and Plant Breeding:
- Mendelian principles, gene interaction, and genetic mapping.
- Mutation, chromosomal aberrations, and their significance in plant breeding.
- Quantitative genetics, heritability, and variability.
- Plant breeding methods and objectives, biotechnology in plant breeding.

7. Ecology and Plant Geography:
- Concepts of ecology, ecosystem, and community ecology.
- Plant succession, population ecology, and conservation biology.
- Biogeochemical cycles, pollution, and environmental issues.
- Plant geography and phytogeography.

This syllabus provides a comprehensive understanding of various aspects of botany, including microbiology, plant pathology, plant physiology, genetics, biochemistry, and ecology. It also covers the classification, structure, reproduction, and economic importance of different groups of plants. The syllabus aims to develop a deep understanding of the fundamental principles and applications of botany.