Analysis of a novel mechanism that regulates microtubule severing in

植物细胞的生长和形状由细胞扩张的方向决定。这种扩张是值得注意的，因为植物细胞可以扩张到原来大小的1000倍。对于许多细胞，如茎或根中的细胞，需要在单一方向(即向上)发生扩张，才能使植物正常生长。为了实现这种定向扩张，植物细胞需要组织它们的细胞壁，特别是纤维素，这是一种非常强大的纤维状聚合物，对细胞壁的机械性能有很大影响。纤维素通常垂直于细胞扩张的方向沉积。在细胞内发现的一种蛋白质支架，称为微管网络，决定了壁内纤维素的组织。微管可以采用不同的模式。在单个方向上的膨胀需要排列的微管图案，而在多个方向上的扩张是由网状或网状的微管图案引起的。我们先前的工作发现，通过切断任何未对准的微管，微管将自己组织成排列的结构。如果没有这种切割，微管就会采用网状结构。切割机器只切割那些与现有微管交叉的微管，从而识别未对齐的微管。越来越多的证据表明，微管重排对植物正常生长的许多方面都是必不可少的，这依赖于微管的切断。一种名为katanin的酶可以切断微管。我们最近发现，含有第二种蛋白质SPIRAL2(SPR2)缺陷的植物，在通常采用这种模式的细胞中无法形成网状阵列。相反，它们形成了一个主要排列的阵列。然而，katanin突变体不会切断微管，而螺旋2突变体显示出高比例的微管切断。这表明，微管交叉处的切割量最终决定了微管的组织方式。它还指出SPIRAL2是修饰katanin活性的一个重要因素。SPIRAL2蛋白存在于所有细胞中，但在某些类型的细胞中，它仍然附着在微管交叉点上，而在其他类型的细胞中，它不断地沿着微管移动，从而使微管被切断。SPR2在交叉点的固定结合似乎阻止了断裂。我们现在想知道更多的是什么控制着SPR2的活性，了解它是如何识别和结合微管交叉点的，是什么决定了SPR2的行为，即SPR2是与微管交叉点结合还是沿着微管移动，以及它需要哪些其他因素来帮助它调节微管切断和微管阵列排列。我们拟议的工作将探索是什么决定了SPR2的定位和移动性。这项工作不仅将回答关于细胞生长和微管模式的重要基本问题(包括确定导致扭曲生长的潜在机制)，而且还将潜在地使我们有能力可预测地改变微管模式。在未来，这可能允许我们操纵植物的发展，并设计更高效的树冠或更强壮的短茎。它还可以提供一种增加植物生物量的手段，要么是为了提高作物产量，要么是为了生产用于生物能源生产的材料。

The growth and shape of a plant cell is determined by the direction of cell expansion. This expansion is remarkable as plant cells can expand up to 1000 times their original size. For many cells, such as those in the stem or root, expansion needs to occur in a single direction (i.e. upwards) for proper plant growth to occur. To achieve this directional expansion, plant cells need to organise their cell wall and particularly cellulose a very strong fibrillar polymer that has a big influence of cell wall mechanical properties. Cellulose is normally deposited perpendicular to the direction of cell expansion. A protein scaffold, referred to as the microtubule network that is found within the cells, dictates the organisation of cellulose within the wall. Microtubules are able to adopt different patterns. Expansion in a single direction requires a pattern of aligned microtubules whereas expansion in several directions results from a net-like or mesh pattern of microtubules.Our previous work has found that the microtubules organise themselves into an aligned configuration by cutting away any unaligned microtubules. Without this cutting, microtubules instead adopt a net-like configuration. The cutting machinery recognises unaligned microtubules by only cutting those that crossover existing microtubules. There is increasing evidence that microtubule rearrangements that are essential for many aspects of normal plant growth depend upon microtubule severing. An enzyme called katanin carries out microtubule severing. Katanin mutants have only "net-like" arrays.We have recently found that plants containing defects in a second protein, called SPIRAL2 (SPR2), that fail to form net-like arrays in cells that normally adopt this pattern. Instead they form a predominantly aligned array. Whereas a katanin mutant does not cut microtubules, a spiral2 mutant shows high rates of microtubule severing. This suggests that the amount of cutting at microtubule crossovers ultimately determines how the microtubules will be organised. It also points to SPIRAL2 being an important factor that modifies the activity of katanin. The SPIRAL2 proteins is present in all cells, but in some cell types it remains attached to microtubules crossover points, while it is constantly moving along microtubules in other cell types allowing severing of microtubules. Stationary binding of SPR2 at crossover points is what appears to prevent severing. We would now like to know more about what controls the activity of SPR2 and understand how it is able to recognise and bind to microtubule crossover points, what determines SPR2 behaviour, i.e. whether SPR2 binds to microtubule crossover points or moves along microtubules and what other factors it need to help it regulate microtubule severing and microtubule array alignment.Our proposed work will probe what determine SPR2 localisation and mobility. Not only will this work answer important fundamental questions about cell growth and microtubule patterning (including identifying the underlying mechanism that gives rise to twisted growth), but will potentially give us the ability to predictably alter microtubule patterns. In the future this may allow us to manipulate plant development and engineers more efficient canopies or stronger shorter stems. It may also provide a means of increasing plant biomass either for better crop yields or to generate material for bioenergy production.