Mutant Series: KOJAK (KJK)
Plant development consists of cell division, cell elongation, and differentiation. In the root of model plant Arabidopsis thaliana, we can follow all these three events easily. Cell starts to divide at the meristem, moves out from the meristematic region and then starts to elongate. After entering the maturation zone, it forms root hair and lateral root. Root hair comes first. The place from where we see the root hair, we consider it as maturation zone. Root hairs help to uptake water and ion, anchor and interact with symbiotic microorganisms.
If we look at the cross-section of root, the outer most layer is known as the epidermis. This layer has two types of cells: atrichoblast (non-root hair cell) and trichoblast (root hair cell). Only trichoblast produces root hairs. The formation of root hair follows few steps. Firstly, it creates a polar growth zone, popularly knows as "bulge". This "bulge" forms due to the thinning of the cell wall. In the next step, there is a polar cytoplasmic region formation close to the "protrusion". Once it forms a tip, it creates a cell wall around the tip. Then it keeps growing as a tubular shape. Without proper cell wall formation, the root hair fails to grow.
It had been known that the cellulose synthesis inhibitor DCB (2,6-dichlorobenzo nitrile) ruptures the root hair tips. As a quick reminder, cellulose is an absolute requirement for cell wall formation. Liam Dolan and his team were interested to identify mutants for ruptured root hair phenotypes. They had identified three mutants with ruptured root hair phenotypes. One from Ds transposon-mutagenized Landsberg erecta and another two from EMS-mutagenized Columbia. These three mutants show no root hairs! And, all these three mutants represent one gene. They named that gene KOJAK.
Cross-section of Arabidopsis thaliana root (left), root hair phenotype of wild-type and kjk (middle), picture of TV character Theo Kojak (right)
KOJAK is a TV series, where the main character is a bald police officer. His name is also Theo Kojak. This root hair mutant has bald phenotype and named after that bald character "KOJAK"!
KOJAK encodes a cellulose synthase like protein, AtCSLD3 (CELLULOSE SYNTHASE LIKE D3). It is required for the formation of the cell wall during root hair morphogenesis. Due to the mutation of KOJAK, root hairs rupture at their tip soon after initiation.