- © 2015 American Society of Plant Biologists. All rights reserved.
Plants sip water and dissolved minerals through their straw-like tracheary elements (TEs), a process mainly driven by transpirational pull and positive root pressure. These forces are strongest when the decreasing water potential between the soil and the atmosphere is greatest, such as during dry, sunny days. TEs are not simply empty tubes, however. These dead, hollow xylem cells are lined with lignified secondary cell wall thickenings that hold onto water when the forces pulling it through the plant are weak, thus maintaining water flow and preventing deadly air bubble formation. These wall thickenings also provide structure to the TEs to help them withstand strong negative pressure (ever had a cheap straw collapse on you?). TEs that form during primary plant growth are lined with ring-like or helical thickenings, which allow cell growth to continue. By contrast, TEs that form in mature tissues have sturdy ladder-like or net-like reticulate linings, as well as pits, which overlap with pits from other TEs to allow water to pass horizontally from cell to cell (reviewed in Ménard and Pesquet, 2015).
Proper formation of these distinctive patterns, which is crucial for plant survival, depends on the work of underlying microtubule templates, whose organization guides cell wall deposition (Pesquet and Lloyd, 2011). Golgi-derived secretory vesicles containing plasma membrane- and microtubule-associated cellulose synthase compartments are shuttled along these microtubules to deliver their cell wall-producing cargo exactly where it’s needed. While CELLULOSE SYNTHASE-INTERACTING PROTEIN1 (CSI1) and CSI3 are known to direct this process during primary cell wall synthesis (Lei et al., 2013), until recently, little was known about the many microtubule-associated proteins (MAPs) that help determine microtubule patterning and guide secondary cell wall construction.
Derbyshire et al. (2015) undertook the daunting task of identifying the entire microtubule interactome and how it changes during TE formation. They made use of a powerful system: Arabidopsis thaliana suspension cell cultures that can be induced to undergo synchronous TE formation. The authors grew these cultures in medium containing 15N and performed microtubule pull-down experiments at various stages of TE differentiation, followed by quantitative proteomic analysis of labeled microtubule-interacting proteins. A whopping 605 proteins were identified, many with stage-specific changes in abundance, including numerous MAPs and their interactors, as revealed by in silico analysis. The authors then focused on classic and newly identified putative MAPs that function during secondary cell wall deposition using both RT-qPCR and RNAi analysis, finding that this process depends on the coordinated activities of many MAPs with nonoverlapping functions. For example, knockdown of CSI1 led to random misdeposition of patches of secondary cell wall material (see figure), uncovering an important role for CSI1 in secondary (in addition to primary) cell wall formation. Finally, Derbyshire et al. used real-time live imaging and time-lapse photography to capture details of the TE differentiation process and to illustrate the impact of oryzalin-induced microtubule destabilization and RNAi knockdown on secondary cell wall patterning, allowing us to grab a glimpse of this fascinating process.
Misdeposition in Calcofluor-stained cell walls. The secondary walls in these maximum-intensity half-cell projections are evenly stained by Calcofluor between TE thickenings in the control (left), but misdepositions are observed on and between thickenings in the CSI1-silenced line (right). Arrows indicate large, uneven distribution of secondary wall within and between individual thickenings caused by large misdepositions. Bars = 3 µm. (Reprinted from Derbyshire et al. [2015], Figures 5I and 5J.)
