Live Cell Microscopy-Based RNAi Screening in the Moss Physcomitrella patens

Using Spinning Disk Microscopy to Observe the Mitotic and Cytokinetic Apparatus in Physcomitrium patens

Protonemata of the moss Physcomitrium patens are ideal structures in which to observe cytoskeletal organization and dynamics. Special care is needed to prepare P. patens cultures for high-resolution microscopy. Here, we describe methods for spinning disk microscopy of dividing P. patens cells expressing sGFP-tubulin and H2B-mCherry, including detailed methods for culturing P. patens.

Exogenous 6-benzylaminopurine inhibits tip growth and cytokinesis via regulating actin dynamics in the moss Physcomitrium patens

Exogenous BAP but not 2iP disrupts actin structures and induces tip-growth retardation and cytokinesis failure in the moss Physcomitrium patens. Synthetic cytokinins have been widely used to address hormonal responses during plant development. However, exogenous cytokinins can cause a variety of cellular effects. A detailed characterization of such effects has not been well studied. Here, using Physcomitrium patens as a model, we show that the aromatic cytokinin 6-benzylaminopurine (BAP) inhibits tip growth at concentrations above 0.2 µM. At higher concentrations (0.6-1 µM), BAP can additionally block mitotic entry and induce cytokinesis defects and cell death. These effects are associated with altered actin dynamics and structures. By contrast, 2-isopentenyladenine (2iP) does not cause marked defects at various concentrations up to 10 µM, while t-zeatin (tZ) can moderately inhibit moss growth. Our results provide mechanistic insight into the inhibitory effects of BAP on cell growth and cell division and call for attention to the use of synthetic cytokinins for bioassays.

Spindle motility skews division site determination during asymmetric cell division in Physcomitrella

Asymmetric cell division (ACD) underlies the development of multicellular organisms. In animal ACD, the cell division site is determined by active spindle-positioning mechanisms. In contrast, it is considered that the division site in plants is determined prior to mitosis by the microtubule-actin belt known as the preprophase band (PPB) and that the localization of the mitotic spindle is typically static and does not govern the division plane. However, in some plant species, ACD occurs in the absence of PPB. Here, we isolate a hypomorphic mutant of the conserved microtubule-associated protein TPX2 in the moss Physcomitrium patens (Physcomitrella) and observe spindle motility during PPB-independent cell division. This defect compromises the position of the division site and produces inverted daughter cell sizes in the first ACD of gametophore (leafy shoot) development. The phenotype is rescued by restoring endogenous TPX2 function and, unexpectedly, by depolymerizing actin filaments. Thus, we identify an active spindle-positioning mechanism that, reminiscent of acentrosomal ACD in animals, involves microtubules and actin filaments, and sets the division site in plants.

Kinetochore protein depletion underlies cytokinesis failure and somatic polyploidization in the moss Physcomitrella patens

Lagging chromosome is a hallmark of aneuploidy arising from errors in the kinetochore–spindle attachment in animal cells. However, kinetochore components and cellular phenotypes associated with kinetochore dysfunction are much less explored in plants. Here, we carried out a comprehensive characterization of conserved kinetochore components in the moss Physcomitrella patens and uncovered a distinct scenario in plant cells regarding both the localization and cellular impact of the kinetochore proteins. Most surprisingly, knock-down of several kinetochore proteins led to polyploidy, not aneuploidy, through cytokinesis failure in >90% of the cells that exhibited lagging chromosomes for several minutes or longer. The resultant cells, containing two or more nuclei, proceeded to the next cell cycle and eventually developed into polyploid plants. As lagging chromosomes have been observed in various plant species in the wild, our observation raised a possibility that they could be one of the natural pathways to polyploidy in plants.

Plants and animals, like all living things, are made of self-contained units called cells that are able to grow and multiply as required. Each cell contains structures called chromosomes that provide the genetic instructions needed to perform every task in the cell. When a cell is preparing to divide to make two identical daughter cells – a process called mitosis – it first needs to duplicate its chromosomes and separate them into two equal-sized sets. This process is carried out by complex cell machinery known as the spindle.

Structures called kinetochores assemble on the chromosomes to attach them to the spindle. Previous studies in animal cells have shown that, if the kinetochores do not work properly, one or more chromosomes may be left behind when the spindle operates. These ‘lagging’ chromosomes may ultimately land up in the wrong daughter cell, resulting in one of the cells having more chromosomes than the other. This can lead to cancer or other serious diseases in animals. However, it was not known what happens in plant cells when kinetochores fail to work properly.

To address this question, Kozgunova et al. used a technique called RNA interference (or RNAi for short) to temporarily interrupt the production of kinetochores in the cells of a moss called Physcomitrella patens. Unexpectedly, the experiments found that most of the moss cells with lagging chromosomes were unable to divide. Instead, they remained as single cells that had twice the number of chromosomes as normal, a condition known as polyploidy. After the effects of the RNAi wore off, these polyploid moss cells were able to divide normally and were successfully grown into moss plants with a polyploid number of chromosomes.

Polyploidy is actually widespread in the plant kingdom, and it has major impacts on plant evolution. It is also known to increase the amount of food that crops produce. However, it is still unclear why polyploidy is so common in plants. By showing that errors in mitosis may also be able to double the number of chromosomes in plant cells, the findings of Kozgunova et al. provide new insights into plant evolution and, potentially, a method to increase polyploidy in crop plants in the future.

Multiple kinesin-14 family members drive microtubule minus end-directed transport in plant cells

Minus end-directed cargo transport along microtubules (MTs) is exclusively driven by the molecular motor dynein in a wide variety of cell types. Interestingly, during evolution, plants have lost the genes encoding dynein; the MT motors that compensate for dynein function are unknown. Here, we show that two members of the kinesin-14 family drive minus end-directed transport in plants. Gene knockout analyses of the moss Physcomitrella patens revealed that the plant-specific class VI kinesin-14, KCBP, is required for minus end-directed transport of the nucleus and chloroplasts. Purified KCBP directly bound to acidic phospholipids and unidirectionally transported phospholipid liposomes along MTs in vitro. Thus, minus end-directed transport of membranous cargoes might be driven by their direct interaction with this motor protein. Newly nucleated cytoplasmic MTs represent another known cargo exhibiting minus end-directed motility, and we identified the conserved class I kinesin-14 (ATK) as the motor involved. These results suggest that kinesin-14 motors were duplicated and developed as alternative MT-based minus end-directed transporters in land plants.