Analysis of a novel mechanism that regulates microtubule severing in

植物细胞的生长和形状是由细胞膨胀的方向决定的。这种扩张是显著的，因为植物细胞可以膨胀到原来的1000倍。对于许多细胞，如茎或根中的细胞，为了使植物正常生长，需要在一个方向上（即向上）进行扩张。为了实现这种定向膨胀，植物细胞需要组织它们的细胞壁，特别是纤维素，一种非常强的纤维聚合物，对细胞壁的机械性能有很大的影响。纤维素通常垂直于细胞膨胀的方向沉积。在细胞内发现的蛋白质支架，被称为微管网络，决定了细胞壁内纤维素的组织。微管能够采用不同的模式。在单一方向上的膨胀需要微管排列的模式，而在几个方向上的膨胀由微管的网状或网状模式产生。我们之前的工作已经发现，微管通过切掉任何未排列的微管，将自己组织成一个排列的结构。如果没有这种切割，微管就会采用网状结构。切割机械通过仅切割那些与现有微管交叉的微管来识别未对齐的微管。越来越多的证据表明，对植物正常生长的许多方面至关重要的微管重排依赖于微管的切断。一种叫做katanin的酶进行微管切断。Katanin突变体只有“网状”阵列。我们最近发现植物中含有第二种蛋白质的缺陷，称为SPIRAL2 (SPR2)，在通常采用这种模式的细胞中不能形成网状阵列。相反，它们形成了一个主要对齐的阵列。而katanin突变体不切割微管，螺旋突变体显示高速率的微管切断。这表明，微管交叉处的切割量最终决定了微管的组织方式。这也表明SPIRAL2是一个重要的因子，可以改变katanin的活性。SPIRAL2蛋白存在于所有细胞中，但在某些细胞类型中，它仍然附着在微管交叉点上，而在其他细胞类型中，它不断沿着微管移动，从而切断微管。SPR2在交叉点的固定结合似乎可以防止断裂。我们现在想知道更多关于是什么控制着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是结合微管交叉点还是沿着微管移动，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对齐。我们提出的工作将探讨决定SPR2本地化和移动性的因素。这项工作不仅回答了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），而且可能使我们能够预测改变微管模式。在未来，这可能使我们能够操纵植物的发育，并设计出更有效的冠层或更短的茎。它还可以提供一种增加植物生物量的手段，以提高作物产量或为生物能源生产生产材料。

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.