Conditional genetic screen in <i>Physcomitrella patens</i> reveals a novel microtubule depolymerizing-end-tracking protein

<div><p>Our ability to identify genes that participate in cell growth and division is limited because their loss often leads to lethality. A solution to this is to isolate conditional mutants where the phenotype is visible under restrictive conditions. Here, we capitalize on the haploid growth-phase of the moss <i>Physcomitrella patens</i> to identify conditional loss-of-growth (CLoG) mutants with impaired growth at high temperature. We used whole-genome sequencing of pooled segregants to pinpoint the lesion of one of these mutants (<i>clog1</i>) and validated the identified mutation by rescuing the conditional phenotype by homologous recombination. We found that <i>CLoG1</i> is a novel and ancient gene conserved in plants. At the restrictive temperature, <i>clog1</i> plants have smaller cells but can complete cell division, indicating an important role of <i>CLoG1</i> in cell growth, but not an essential role in cell division. Fluorescent protein fusions of CLoG1 indicate it is localized to microtubules with a bias towards depolymerizing microtubule ends. Silencing <i>CLoG1</i> decreases microtubule dynamics, suggesting that CLoG1 plays a critical role in regulating microtubule dynamics. By discovering a novel gene critical for plant growth, our work demonstrates that <i>P</i>. <i>patens</i> is an excellent genetic system to study genes with a fundamental role in plant cell growth.</p>