Arabidopsis thaliana Background
Arabidopsis thaliana is an angiosperm originating from the Eurasian continent. Its relatively short genome with a length of 135 Megabase pairs (Mbp) and short life cycle make it an ideal model organism for studying traits of flowering plants, such as light sensing as well as the development of structures such as flowers, pollen tubes, and trichomes. (TAIR)
Trichomes exist as a defense mechanism of the plant. They function primarily to block harmful UV radiation from reaching the leaves or main body of the plant and in some species, trichomes contain chemicals that discourage herbivores from eating the plant. Among trichome possessing species, trichome structure can vary from thin hairs to the star shaped trichomes of Arabidopsis thaliana, to glandular herbs (such as mint, basil, lavender, or oregano) that secrete oils or chemicals through their trichome tips. (Darling, D.)
Trichomes are a specifically interesting structure in the plant as they are typically made of only a single cell, yet have complex structures and patterning as well as important jobs. (Darling, D.) Single cell systems such as this allow researchers to decipher complex mechanisms non-invasively and conveniently. Three single-cell systems are studied in A. thaliana: Root hairs, pollen tubes, and trichomes, with the latter being the most complex. A. thaliana develops two types of trichomes, branched ones the leaves and unbranched on the stems and sepals. (Mathur et al. 1999)
If we better understand the mechanisms behind trichome development, we may be able to engineer plants with trichomes that make them more resistant to pests as well as UV radiation. We must first, however, understand how the trichomes develop and what genes and proteins are thought to affect them.
In the A. thaliana model organism, there are 6 stages of trichome cell development and many involved genes. These genes have either positive or negative protein product regulators of trichome fate and development. (Grebe 2012)
The first stage of trichome development is the cell fate determination of a trichome precursor cell. In this stage many positive trichome regulators are expressed in the base of the leaf and inside one cell, form a complex that activates trichome cell fate inhibitors in neighboring cells. In this fashion the cells surrounding a trichome precursor cell do not form trichomes that would interfere with the growth of the original trichome. The mechanism for trichome cell fate is described below in Figure 1. (Grebe 2012)
Figure 1. (Grebe 2012) A trichome-destined cell forms a complex (fate activator complex) inside its cytoplasm consisting of GL1, GL3, TTG1 and likely MYC1 (all positive regulators of trichome cell fate). This complex activates the transcription of trichome cell fate inhibitors TRY, CPC, ETC2 and other R3 MYB transcription factors that form a complex that moves to neighboring cells. GL1 is replaced in complexes ensuring that the cell remains a non-trichome cell (visualized on the right image). The GL1-TTG1-GL3-MYC1 complex activates transcription of GL2 (trichome cell differentiation gene) as well as SIM, a mitosis inhibitor. Expression of SIM causes endoreplication, or DNA replication without cell division. This lets the trichome express significantly higher levels of important genes as well as helping maintain trichome cell fate. Loss of SIM or overexpression of cell division activators can cause the fate activator complex to fail to reach the threshold level and the plant to fail to grow trichomes.
The second stage of trichome development is tubular growth. As shown in Figure 2A and B, the trichome-precursor cell begins growing perpendicular to the leaf. The cell nucleus, surrounded in an actin cage (perinuclear actin cage), as well as F-actin molecules organize towards the tip in preparation for stage three. (Mathur et al. 1999)
In the third stage, the F-actin filaments form foci at the points near the tip of the trichome that will be the branching point. These foci are connected to the perinuclear actin cage and the branches radiate outwards from this point, no longer growing perpendicular to the leaf plane. (Mathur et al. 1999)
In stages four and five, the braches extend rapidly and then orient themselves along the proximodistal axis before continuing to stage 6 where they mature.
Figure 2. (Mathur et al. 1999) A) shows each stage of trichome development under an electron microscope. B-G) show GFP tagged f-actin filaments, which form the structure of the trichome. B) shows stage 1 and 2 of trichome development. In C) stages 3 and 4 are pictured, showing the organization of actin filaments where the branching begins to take place, as well as the extension of these branches . D) shows the longitudinally organized actin filaments in a stage 5 trichome. E) and F) show the actin organization at branch tips and the base, respectively, and F shows that the nucleus of a trichome cell can be localized to the base or higher up the stalk as in G. G) depicts stage 6 matured trichomes that vary in size and branching structure.
The addiction of actin inhibitors (Lat-B, CD, Pha, and Jas) all caused distorted trichome shape, with Lat-B being the most potent(only needs 1-2 μM) and Pha and Jas the least potent(5-10 μM needed). (Mathur et al. 1999)
Loss of G13(g13) mutants lack one endoreplication cycle, while try mutants undergo an additional endoreplication cycle. Less endoreplication causes fewer branches in trichomes as well as a general decrease in the number of trichomes per leaf. Additional endoreplication causes an increase in trichome branching as well as overall trichome number, suggesting that endoreplication plays a strong central role in trichome patterning and structure. (Mathur et al. 1999)
Weaknesses of the Mathur et al. paper were that it did not discuss the differentiation of cells into trichome progenitors, however this was covered in the Grebe paper.
The following video depicts the development of trichomes on the leaves of a growing A. thaliana.
Darling, D. (n.d.). Trichome. Retrieved April 19, 2015, from http://www.daviddarling.info/encyclopedia/T/trichome.html
Grebe, M. (2012, February 1). The patterning of epidermal hairs in Arabidopsis — updated. Retrieved March 9, 2015, from http://www.sciencedirect.com/science/article/pii/S1369526611001713
Mathur, J., Spielhofer, P., Kost, B., & Chua, N. (1999, December 15). The actin cytoskeleton is required to elaborate and maintain spatial patterning during trichome cell morphogenesis in Arabidopsis thaliana. Retrieved March 9, 2015, from http://dev.biologists.org/content/126/24/5559.short
TAIR – About Arabidopsis. (n.d.). Retrieved March 10, 2015, from https://www.arabidopsis.org/portals/education/aboutarabidopsis.jsp