Plant Cell Organelles and Functions

Introduction

• Plant cells need both chloroplasts and mitochondria for vital functions
• This is due to the endosymbiotic theory

Structure of Plastids & Peroxisomes

• Peroxisomes are membrane-bound organelles involved in oxidative metabolism, lipid metabolism (production, scavenging, degrading hydrogen peroxide)
• Peroxisomes are closely associated with chloroplasts, mitochondria, and the ER
• Does not contain DNA
• Plastids play a crucial role in energy production and metabolism within plant cells
• Different plastid types exist and can interconvert
• Plastids have their own DNA and replication mechanisms

Plastid Introduction

• Chloroplasts and mitochondria are similar in that they are both powerhouses of plant cells, having their own DNA and replication processes.
• Chloroplasts are found in photosynthetic eukaryotes, and are used for photosynthesis
• Mitochondria are found in most eukaryotes, and are used for cellular respiration
• The endosymbiotic theory explains the origin of these organelles.

Plastid Types & Interconversion

• Plastids can specialize into many roles
• All plastid types can interconvert into another given the correct conditions
• The plastid type within a cell reflects the cell's identity and function
• Proplastid: a precursor to other plastid types, found in the least differentiated tissues (e.g., meristems, gametes)
• Etioplast: a precursor to chloroplasts, found in tissues grown in the dark
• Chloroplast: the "solar powerhouse," critical for photosynthesis found in green tissues. Most complex plastid type.
• Chromoplast: stores non-polar pigments (red, orange, yellow) found in fruits, tubers, and flowers
• Leucoplast: colorless, non-pigmented plastids, lacking thylakoids
  • Amyloplast: starch synthesis and storage within storage tissues
    • Found in storage tissues like tubers and roots
    • Can have gravity-sensing function
  • Elaioplast: synthesis and storage of fatty acids, terpenes, and other lipids
  • Proteinoplast: synthesis and storage of large proteins found in seeds
• Chloroplast & Chromoplast interconversion is possible, as is Amyloplast <-> Chloroplast via appropriate conditions

Plastid Replication and Genome

• Plastids cannot be made by the plant cell. They replicate through the division of existing plastids and are inherited from parent to offspring
• All plastid types divide to keep pace with cell division, metabolic needs, and environmental cues
• Division by binary fission, similar to bacteria
• DNA replication, transcription, and translation processes are also similar to prokaryotes
• Plastid genome is small, circular, and structured like bacterial genomes, but much smaller (<2000 kbp).
• Most genome has transferred to the nucleus
• Codes for some proteins and RNA for essential processes (regulation and signaling, ribosomes, and parts of photosynthetic machinery).

Plastid Genome-Nuclear Interactions

• Necessary for proper regulation of plastid-related gene expression and development.
• Multiple different signal feedback loops and transfer of proteins coordinate the nuclear and plastid activities
• Intermediates in chlorophyll biosynthesis can be used as developmental signals
• All proteins found in plastids are coded in the nucleus and must be imported from the cytosol
• Redox signals are important for indicating stress levels and coordinating repair or stress-avoidance responses.
• Plastid import/export and signaling is important for proper cell development
• Import protein mutations can impact development and growth

Summary Questions (T/F)

• Plastids can't be made by the cell and have their own replication processes (True)
• Only chloroplasts can exchange signals with the nucleus (False)
• Plastids require communication with the nucleus for proper function.