The Zygnematophyceae have recently been shown to most likely be the closest living ancestors of land plants (Wodniok et al., 2011; Delwiche and Cooper, 2015). Unicellular members of the inclusive placoderm desmid group have become important models for elucidating many fundamental principles of plant cell biology and development. Large cell size, notable symmetry/shape, distinct cell wall architecture and elaborate endomembrane/cytoskeletal systems are just a few of the characteristics that make them excellent cell systems for studying cell physiology and development. The recent establishment of stable transformed lines and soon-to-be sequenced genomes/transcriptomes of select desmids have further enhanced their value in botanical studies. Two taxa, Micrasterias and Penium, have emerged as the most well studied of the desmids.

Micrasterias has been the most popular desmid for cell biology research for the past 50 years due to its unique features (Meindl, 1993; Lutz-Meindl, 2016). The Micrasterias cell exhibits a bilateral symmetry that is often highlighted by a spectacularly dissected (i.e., multilobed) periphery. New daughter semicells produced by cell division do not have this complex morphology but rather are spherical in shape. A complex post-cytokinetic developmental program that employs multiple sets of highly coordinated subcellular mechanisms subsequently generates the multilobed phenotype during daughter semicell expansion. This program also is responsible for deposition of polymers for the production of both a primary and secondary cell wall. These events are centered on a large network of Golgi bodies and associated vesicles that yield a complex secretory mechanism. This process, in turn, is targeted to specific cell surface sites that generate a multipolar cell expansion and the concurrent secretion of cell wall macromolecules and extracellular mucilage (Kim et al., 1996; Lutz-Meindl and Brosch-Salomon, 2000; Oertel et al., 2004; Aichinger and Lutz-Meindl, 2005). The delivery of secretory components to precise cell surface loci requires an elaborate actinomyosin-based cytomotile system and is controlled by several signal transduction cascades (Meindl et al., 1994; Oertel et al., 2003). Furthermore, cell morphogenesis is highly sensitive to external stress (e.g., oxidative stress, salinity, heavy metals) that leads to major changes in cell differentiation (Darehshouri et al., 2008; Affenzeller et al., 2009; Andosch et al., 2012). Micrasterias is an outstanding organism for analyzing these subcellular features as it is easy to maintain and manipulate in the laboratory and it lends itself especially well for acquisition of high resolution imaging using light and electron microscopy including immunocytochemistry, Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) and Electron Energy Loss Spectroscopy (EELS) imaging (Lutz-Meindl, 2007; Eder and Lutz-Meindl, 2008, 2010; Wanner et al., 2013; Lutz-Meindl et al., 2015). Furthermore, initial molecular analyses including the production of transformed cell lines have further enhanced the use of Micrasterias especially in the molecular dynamics of cell wall processing (Vannerum et al., 2010, 2011, 2012).

Over the past decade, the desmid, Penium margaritaceum, has also become a valuable model organism (Domozych et al., 2009, 2014; Sørensen et al., 2014; Rydahl et al., 2015; Worden et al., 2015). Unlike Micrasterias, Penium has a simple cylindrical shape, possesses only a primary cell wall and deposits wall polymers at two specific loci of the cell surface during expansion (Domozych et al., 2011). This relative simplicity is highly attractive for elucidating fundamental principles of plant cell development including cell wall development, cell expansion and secretion dynamics (Domozych et al., 2005, 2014; Ochs et al., 2014). One of its main attributes is that it has wall polymers similar to those present in many land plants (e.g., cellulose, pectins, hemicelluloses) and that these polymers can be labeled with monoclonal antibodies. After labeling of live cells, these can be returned to culture where subsequent cell expansion and wall deposition events can be monitored (Domozych et al., 2009; Rydahl et al., 2015). Penium is also easily maintained in the laboratory and its fast growth rate under precisely controlled conditions makes it an excellent specimen for large-scale concurrent microarray screenings of many chemical agents (by growth in multi-well plates) and for assessment of their specific effects on expansion/wall development (Worden et al., 2015). This significantly aids in revealing the role of specific subcellular components and processes in the expansion/differentiation. Recently, the isolation of stable transformed lines (Sørensen et al., 2014) has further enhanced the potential of this alga in future plant cell studies.

Species of Chara and Nitella (Charophyceae) have long been used as specimens for a variety of biological investigations especially those dealing with cellular dynamics, expansion and cytoplasmic streaming (Green, 1954; Probine and Preston, 1962). Their macroscopic thalli are distinguished by nodes, where branches and gametangia arise, and internodal regions that consist of exceptionally elongate cells. Internodal cells exhibit a clearly notable stratification of subcellular components whereby helical-oriented chloroplasts define a stationary cortex that surrounds an endoplasm where the fastest recorded actinomyosin-generated cytoplasmic streaming occurs. The cytoplasm contains diverse endomembrane components that participate in dynamic membrane trafficking networks (e.g., endocytosis and exocytosis) that are controlled by complex signal transduction cascades (Foissner and Wastenys, 2000, 2012, 2014; Sommer et al., 2015; Foissner et al., 2016). The internodal cells possess cell walls rich in cellulose and the pectin, homogalacturonan (HG). Turgor is the main driving force for expansion and pectin is most likely the load-bearing component controlling wall expansion. Cyclic, non-enzymatic modulation and concurrent calcium complexing of the HG is the mechanism responsible for controlling wall modulations that lead to cell expansion (Proseus and Boyer, 2007, 2008, 2012; Boyer, 2016). One of the technical advantages of Chara and Nitella in cellular research is that internodal cells can be individually removed; endoplasm and cell walls can be “dissected out” and then used as acellular systems for experimental manipulation.

Other thallus components like rhizoids have organized polar organization of the cytoplasm. These cells contain sedimentable, mineral-rich statoliths located at the apex that function in gravity perception (Hodick et al., 1998; Braun, 2002; Braun and Limbach, 2006). This feature has made Chara the first charophyte in space. During TEXUS rocket parabolic flights, it was shown that the statoliths exert tensional forces on actin filaments. A balance of forces (i.e., gravity and the counteracting force of actin filaments) is responsible for the correct positioning of the statoliths in the rhizoid that, in turn, guarantee the ability to respond to the gravity vector (Braun and Limbach, 2006). The use of laser tweezers and slow rotating centrifuge microscopy has demonstrated that statolith sedimentation is not sufficient to cause gravitropic bending of the rhizoid tip. Rather, the mineral-rich statoliths must settle onto specific regions of the plasma membrane for gravitropic morphological effects to proceed (Braun, 2002). Chara has also been used to study other phenomena including electrophysiology and the role and movement of hormones (Belby, 2016).

The formation of a multicellular thallus (i.e., tissue, organ) of a plant requires precise, spatially regulated cell division planes that are controlled by multiple sets of genes expressed at specific points during development (Besson and Dumais, 2011; Umen, 2014). Directed in both 3-dimensional space and time, thallus morphogenesis also modulates in response to external stress factors. In plants, these developmental programs are made even more complex by the presence of cell walls that do not allow for cell migration or tissue flexibility. The elucidation of the specific events that are central to the manifestation of thallus shape and size is often difficult to decipher in land plants where resolution of specific cell behaviors are often poorly resolved when embedded in highly complex and expansive tissues/organs. The use of organisms with simpler thallus designs has been advantageous for understanding basic developmental phenomena and the charophyte genus Coleochaete is one such example. Certain species (e.g., C. orbicularis, C. scutatum) produce a parenchymatous discoid thallus that grows outward by a combination of anti- or peri- clinal cell divisions and subsequent expansion of its outermost cells (Brown et al., 1994; Cook, 2004). The plane of division of any cell follows simple rules that are based upon cell size, cell shape and geographic location in the thallus (Dupuy et al., 2010; Besson and Dumais, 2011). This characteristic allows for the construction of mathematical models that can then be used to interpret multicellular morphogenesis (Domozych and Domozych, 2014; Umen, 2014) and yield critical insight into the evolution, biomechanics and physiology of organs and whole organisms. Furthermore, some Coleochaete species have been extensively studied to determine the structural and functional modulations that occur to a thallus under desiccation conditions (Graham et al., 2012), i.e., key features in understanding early land plant evolution and plant growth dynamics during droughts.

Similarly to many other technologies, molecular tools have the ability to impart new information of importance for many areas of research, including physiology and development. For example members of the MADS-box gene family have been isolated and characterized from three charophytes, Chara globularis, Coleochaete scutata, and Closterium peracerosum-strigosum-littorale (Tanabe et al., 2005). The expression pattern of the MADS-box genes in the charophytes suggests that they play a role in haploid development and reproduction (Tanabe et al., 2005). They are thought to have been recruited into the diploid generation, and their roles in development further diversified, during land plant evolution (Tanabe et al., 2005). To date Klebsormidium flaccidum is the only charophyte for which a near complete nuclear genome is available (Hori et al., 2014); although chloroplast and mitochondrial genomes have been sequenced for representatives of each of the six classes of charophytes and ESTs (expressed sequence tags) are available for some charophytes (Delwiche, 2016). The Klebsormidium genome has already revealed the presence of genes for the synthesis of several plant hormones and signaling intermediates, mechanisms for protection against high light intensity (Hori et al., 2014), and the presence of Group IIb WRKY transcription factors that were previously thought to have first appeared in mosses (Rinerson et al., 2015). Several charophyte genome sequencing projects are currently underway (Delwiche, 2016) and will enable more detailed comparative analyses including a larger number of charophytes and potentially give new insight into the evolutionary relationships between species and genes. The unique position of charophytes as the earliest diverging group of Streptophytes makes them a particularly powerful tool for evolutionary-developmental studies and emerging technologies such as the recent achievement of stable transformation in Penium (Sørensen et al., 2014) will enable the function of specific genes to be explored.