Abstract
BackgroundThe cortical microtubules (CMTs) that line the plasma membrane of interphase plant cells are extensively studied owing to their importance in forming cell walls, and their usefulness as a model system for the study of MT dynamic instability and acentrosomal MT organization. CMTs influence the orientation and structure of cellulose microfibrils in the cell wall by cooperatively forming arrays of varied patterns from parallel to netted. These CMT patterns are controlled by the combined activities of MT dynamic instability and MT-MT interactions. However, it is an open question as to how CMT patterns may feedback to influence CMT dynamics and interactions.ResultsTo address this question, we investigated the effects of CMT array patterning on encounter-based CMT catastrophe, which occurs when one CMT grows into another and is unable to cross over. We hypothesized that the varied CMT angles present in disordered (mixed CMTs) arrays will create more opportunities for MT-MT interactions, and thus increase encounter-based catastrophe rates and distribution. Using live-cell imaging of Arabidopsis cotyledon and leaf epidermal cells, we found that roughly 87 % of catastrophes occur via the encounter-based mechanism, with the remainder occurring without encounter (free). When comparing ordered (parallel) and disordered (mixed orientation) CMT arrays, we found that disordered configurations show higher proportions of encounter-based catastrophe relative to free. Similarly, disordered CMT arrays have more catastrophes in general than ordered arrays. Encounter-based catastrophes were associated with frequent and sustained periods of pause prior to depolymerization, and CMTs with tight anchoring to the plasma membrane were more prone to undergo encounter-based catastrophe than weakly-attached ones. This suggests that encounter-based catastrophe has a mechanical basis, wherein MTs form physical barriers to one another. Lastly, we show that the commonly used measure of catastrophe frequencies (Fcat) can also be influenced by CMT ordering and plasma membrane anchoring.ConclusionsOur observations add a new layer of complexity to our current understanding of MT organization in plants, showing that not only do individual CMT dynamics influence CMT array organization, but that CMT organization itself has a strong effect on the behavior of individual MTs.
Highlights
The cortical microtubules (CMTs) that line the plasma membrane of interphase plant cells are extensively studied owing to their importance in forming cell walls, and their usefulness as a model system for the study of MT dynamic instability and acentrosomal MT organization
Encounter-based catastrophes constitute the dominant source of CMT catastrophe To visualize CMT dynamics, we performed confocal imaging using cotyledons and leaves of Arabidopsis seedlings constitutively expressing green fluorescent protein (GFP) fused to Arabidopsis TUBULIN BETA6 (GFP-TUB6) [46]
This apparent contradiction can be reconciled by our data, which suggest that the clasp-1 reduction in frequency of catastrophe (Fcat) results at least in part from the enhanced MT cross-overs associated with weakly cortex-bound CMTs
Summary
The cortical microtubules (CMTs) that line the plasma membrane of interphase plant cells are extensively studied owing to their importance in forming cell walls, and their usefulness as a model system for the study of MT dynamic instability and acentrosomal MT organization. In many cells, MTOCs are absent but MTs still obtain global ordering through self-organization, a process wherein global order emerges in a system from interactions between individual elements [18, 19] This “acentrosomal” MT organization is intensely studied in the interphase cortical array of plants, which consists of MTs that are laterally attached to the plasma membrane and undergo continuous dynamic behaviours. These Cortical MTs (CMTs) arrange in various orientations ranging from random/netted, parallel, and highly bundled. These MT-MT interactions include bundle formation, collision-induced catastrophe [21], severing [23, 24], and nucleation of MTs from pre-existing MTs [25,26,27]
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