Confined-microtubule assembly shapes three-dimensional cell wall structures in xylem vessels

Abstract

Properly patterned deposition of cell wall polymers is prerequisite for the morphogenesis of plant cells. A cortical microtubule array guides the two-dimensional pattern of cell wall deposition. Yet, the mechanism underlying the three-dimensional patterning of cell wall deposition is poorly understood. In metaxylem vessels, cell wall arches are formed over numerous pit membranes, forming highly organized three-dimensional cell wall structures. Here, we show that the microtubule-associated proteins, MAP70-5 and MAP70-1, regulate arch development. The map70-1 map70-5 plants formed oblique arches in an abnormal orientation in pits. Microtubules fit the aperture of developing arches in wild-type cells, whereas microtubules in map70-1 map70-5 cells extended over the boundaries of pit arches. MAP70 caused the bending and bundling of microtubules. These results suggest that MAP70 confines microtubules within the pit apertures by altering the physical properties of microtubules, thereby directing the growth of pit arches in the proper orientation. This study provides clues to understanding how plants develop three-dimensional structure of cell walls.

Protoxylem microtubule patterning requires ROP pattern co-alignment, realistic microtubule-based nucleation, and moderate microtubule flexibility

“…Therefore, metaxylem patterning may require additional proteins to help the microtubules form this structure that are absent or less important in protoxylem patterning. Recent evidence indicated the involvement of microtubule-associated protein MAP70-5, which lowers microtubule persistence lengths, enabling the formation of microtubule loops [46,61]. This is consistent with earlier observations that MAP70-5 lines the borders of the pits and its overexpression increases the ratio of pitted to spiral cell wall patterns [62].…”

“…The values they use are similar to the values found in vitro for microtubules with reduced persistence length due to the addition MAP70-5 [41], suggesting that persistence length values from this context are too low for transverse arrays and protoxylem bands. MAP70-5 is thought to be essential for the formation of microtubule rings around the ellipsoid gaps in metaxylem and induces spontaneous formation of microtubule rings in vitro [41]. The effect of flexibility on protoxylem patterning remains an open question.…”

“…There is a large gap between the microtubule persistence lengths of 30-100 µm measured in gliding assays with and without Map70-5 [41], which are comparable to the 20-30 µm measured in animal cells [48,49] and those in the millimetre range measured in flow cells [41]. This difference suggests a large uncertainty in the value of the microtubule persistence length in plant cells, as well as a possibility for cells to modulate it.…”

“…Possibly, microtubule persistence lengths may have lower values in (often smaller) animal cells. Evidence from in vitro measurements in gliding assays suggests persistence lengths of 60 µm and 100 µm for microtubules with many and few interactions, respectively, while measurements of microtubules suspended in flow cells gave persistence lengths of around 2 mm for noninteracting microtubules [41]. Since the deflections due to microtubule interactions are modelled directly in our simulations, it is not just the 'intrinsic' persistence length that contributes to the measured persistence length and so the measured persistence length will be smaller than the value we use to determine deflections.…”

“…Unfortunately, the actual persistence length in plant cells has never been quantified yet, and the recent studies that do model microtubules as semiflexible polymers [46,47], base themselves on measurements from animal cells [48,49]. The values they use are similar to the values found in vitro for microtubules with reduced persistence length due to the addition MAP70-5 [41], suggesting that persistence length values from this context are too low for transverse arrays and protoxylem bands. MAP70-5 is thought to be essential for the formation of microtubule rings around the ellipsoid gaps in metaxylem and induces spontaneous formation of microtubule rings in vitro [41].…”