L2 Plant Model Organisms

Model Plants

• Extensively studied plant species used for investigating biological phenomena, or for biotechnology or agronomy value.
• Models are chosen for their close relationships with crop plants, providing relevant biological insights.
• Scientific and commercial value comes from model plants with shorter evolutionary distances from the crop plant.
• Arabidopsis has been used since the 1980s.
• Knowledge is not always transferable to complex species like cereals with starch metabolism.
• Scientific advances like gene editing, genome sequencing, and speed breeding have expanded the possibilities for studying more complex plants.
• Other models include cereals and legumes.

Arabidopsis and Other Models

• Arabidopsis is the primary plant model dating to the 1980s.
• Third-generation models include:
  • Marchantia polymorpha
  • Setaria viridis
  • Phragmites australis
  • Eutrema salsugineum
  • Cardamine hirsuta
  • Pisum sativum

Recent Developments Redefining Model Plants

• Insights from Arabidopsis have laid the foundation for molecular biology work.
• Availability of genome sequences, recent developments in gene editing and speed breeding are used to solve challenges with major crops.
• Fundamental understanding of cereal crops can provide genetic solutions more rapidly.
• Non-model plant models cover areas where Arabidopsis is lacking, like land plant evolution.
• Having a small genome size is no longer key for use as a reference genome

The Primary Model PLant: Arabidopsis Thaliana

• Arabidopsis is a small, annual plant from the Brassicaceae (mustard) family within the eudicotyledonous angiosperms.
• Also known as thale cress or mouse-ear cress.
• Native to Eurasia and Africa and naturalized in North America around the 17th century
• Has no commercial value.

Model Plant Advantages

• Insights are gained as a reference system, therefore it's possible to forward with research and improve economically important plants
• Genome size is ~133.7 Mb (haploid), with 5 chromosomes.
• Comprehensive knowledge of a complete plant species.
• Hypotheses can be tested quickly and easily.

More Advantages

• Each plant produces 10,000 to 40,000 seeds in 6 weeks
• Many variants are available - mutations can be generated by irradiating the seeds or treatment with mutagenic chemicals.
• Self-pollination allows recessive mutations to be expressed because of homozygosity

Additional Properties

• Requires light, air, water, and minerals to complete its life cycle.
  • Short cycle of approximately 6 weeks
  • Its small size (6-12 inches) suits limited spaces
  • Undergoes self-pollination
  • Grown in greenhouses or indoor growth chambers
• Small and genetically tractable genome facilitates easier and faster genetic engineering.
• Has 133,725,193 base pairs (1.33 x 108 bp) of DNA in 5 chromosomes (2n = 10).
• DNA has very little "junk" DNA.
• 27,583 genes are encoded by the DNA.
• Transgenic plants can be made with Agrobacterium tumefaciens to introduce foreign genes.
• Majorly involved in the study of plant genome organization, gene regulation, genetics of plant development and genetics of flowering

Mutants for different developmental stages:

• Embryogenesis - fertilisation to seed
• Vegetative development - germination to an adult plant
• Reproductive development - flowering

Mutants

• Provide a powerful tool to guide researchers in their search for genes with a degree of control over development.
• Provide insights to molecular, cellular, and developmental mechanisms underlying life that spawns new research.
• Genetic modification has been made for staple crops to increase yields and to confer pest and disease resistance, plus increased nutritional value.
• Plant geneticists use mutants to see how they behave in development
• Mutants help connect developmental processes with responsible genes
• Gene families have been found, gene functions are discovered and those same functions discovered are similar to others

Embryognesis

• Patterning of the Arabidopsis embryo can be altered by mutation
• Mutations alter the heart shape of the embryo have been found and suggest genes control the apical-basal axis.
• Apical mutants (gurke) are missing cotyledons and shoot meristem.
• Central mutants (fackel) have no hypocotyl, and the cotyledons are connected to a root.
• Basal mutants (monopterous) have no hypocotyl nor root.
• Fackel is required for cell division and expansion.
  • Hypocotyl severely shortened and cells failed to elongate at globular stage.
  • Fackel is associated with loss of sterol C-14 reductase activity (essential for cellulose synthesis in the building of the Cell wall).
• Normal aerial development
• Specialised zone (SZ), Elongation zone (EZ) Meristematic zone (MZ); A – wild type, C - mutant short-root type, ceased elongation

Mutant to Understand Root Development

• Root tissues are produced from root Apical Meristems (RAM)
• The root is setup early in the late heart shape with a set of initial calls
• Each column of root cells originates with a specific cell in the meristem via cell division

Examples of Mutants

• Short-root (shr)
  • Traced back to heart-stage embryo
  • A mutation causes a determinate root growth (causes the loss of internal root cell layers (has no endodermis)
• Cobra (cob)
  • Aerial parts are similar to the wild type
  • Roots are much larger
  • Epidermal cells are 15 times larger
  • Has abnormal roots that expand faster than others rather than longer

Flower Mutants

• Agamous (AG) is a C-function floral organ identity gene
• Required for reproductive organs that the 3rd and 4th whorls are located to the WT Flowers are indeterminate and produce the basic pattern of sepal, petal, petal.