Plant actins are encoded by a gene family. Despite the crucial significance of the actin cytoskeleton for plant structure and function, the importance of individual actin isotypes and their specific roles in various plant tissues or even single cells is rather poorly understood. This review summarizes our current knowledge about the plant actin gene family including its evolution, gene and protein structure, and the expression profiles and regulation. Based on this background information, we review mutant and complementation analyses in Arabidopsis to draw an emerging picture of overlapping and specific roles of plant actin isotypes. Finally, we examine hypotheses explaining the mechanisms of isotype-specific functions.

Background: Somatic embryogenesis in spruce is a process of high importance for biotechnology, yet it comprises of orchestrated series of events whose cellular and molecular details are not well understood. In this study, we examined the role of actin cytoskeleton during somatic embryogenesis in Norway spruce line AFO 541 by means of anti-actin drugs. Results: Application of low doses (50-100 nM) of latrunculin B (Lat B) during the maturation of somatic embryos predominantly killed suspensor cells while leaving the cells in meristematic centres alive, indicating differential sensitivity of actin in the two cell types. The treatment resulted in faster development of more advanced embryos into mature somatic embryos and elimination of insufficiently developed ones. In searching for the cause of the differential actin sensitivity of the two cell types, we analysed the composition of actin isoforms in the culture and isolated four spruce actin genes. Analysis of their expression during embryo maturation revealed that one actin isoform was expressed constitutively in both cell types, whereas three actin isoforms were expressed predominantly in suspensor cells and their expression declined during the maturation. The expression decline was greatly enhanced by Lat B treatment. Sequence analysis revealed amino-acid substitutions in the Lat B-binding site in one of the suspensor-specific actin isoforms, which may result in a different binding affinity for Lat B. Conclusions: We show that manipulating actin in specific cell types in somatic embryos using Lat B treatment accelerated and even synchronized the development of somatic embryos and may be of practical use in biotechnology.

The Arp2/3 complex is an actin nucleator shown to be required throughout plant morphogenesis, contributing to processes such as cell expansion, tissue differentiation or cell wall assembly. A recent publication demonstrated that plants lacking functional Arp2/3 complex also present defects in auxin distribution and transport. This work shows that Arp2/3 complex subunits are predominantly expressed in the provasculature, although other plant tissues also show promoter activity (e.g., cotyledons, apical meristems, or root tip). Moreover, auxin can trigger subunit expression, indicating a role of this phytohormone in mediating the complex activity. Further investigation of the functional interaction between Arp2/3 complex and auxin signaling also reveals their cooperation in determining pavement cell shape, presumably through the role of Arp2/3 complex in the correct auxin carrier trafficking. Young seedlings of arpc5 mutants show increased auxin-triggered proteasomal degradation of DII-VENUS and altered PIN3 distribution, with higher levels of the protein in the vacuole. Closer observation of vacuolar morphology revealed the presence of a more fragmented vacuolar compartment when Arp2/3 function is abolished, hinting a generalized role of Arp2/3 complex in endomembrane function and protein trafficking.

Microtubules of all eukaryotic cells are formed by α- and β-tubulin heterodimers. In addition to the well known cytoplasmic tubulins, a subpopulation of tubulin can occur in the nucleus. So far, the potential function of nuclear tubulin has remained elusive. In this work, we show that α- and β-tubulins of various organisms contain multiple conserved nuclear export sequences, which are potential targets of the Exportin 1/CRM1 pathway. We demonstrate exemplarily that these NES motifs are sufficient to mediate export of GFP as model cargo and that this export can be inhibited by leptomycin B, an inhibitor of the Exportin 1/CRM1 pathway. Likewise, leptomycin B causes accumulation of GFP-tagged tubulin in interphase nuclei, in both plant and animal model cells. Our analysis of nuclear tubulin content supports the hypothesis that an important function of nuclear tubulin export is the exclusion of tubulin from interphase nuclei, after being trapped by nuclear envelope reassembly during telophase.