20071027根為什麼往下長

植物的地上部份會往有光的地方長，而根就往地底鑽，這就好像1+1=2一樣理所當然。但就像所有事情一樣，當我們仔細去想要找出機制時，事情就變得異常複雜。

根部向地心方向生長的現象稱為向地性，其實應該稱為向重力性(gravitropism)。

以往都是說根在水平狀態時，上側的生長素少，下側的生長素多，由於高濃度生長素會抑制根部細胞延長，因此根的下側的細胞生長較慢而上側生長較快，於是根便向地心彎曲，表現向地性。

這樣的說法建立在生長素會因為重力而累積在下方的觀點上，不過生長素是小分子物質不易受地心引力作用而下移，所以這樣的說法還是有待商榷的。就如同橫放的莖為何會背地向上生長，現在認為可能是地心引力造組織內部發生改變，影響生長素運送，使生長素集中於莖的下方所致


不過現在目前還有其他的幾種理論被提出：
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1根冠會產生離層素(abscisic acid)，以抑制根的下側細胞的生長，於是下側生長慢而上側生長快，故根向地心彎曲。

2根冠細胞內內的中央細胞中含有又大又重澱粉性質體(amyloplast)組成的平衡石(Atatolith)，平衡石會因重力下沉，壓迫內質網藉以傳導重力訊息，使根向重力方向生長。

3澱粉性質體中有高濃度的根抑制激素(root-inhibiting hormone)，會抑制與質體接觸的細胞延長，因此根向下彎曲。

4平衡石的向下移動刺激近地側的細胞產生較多的生長激素，高濃度生長激素抑制近地側的細胞延長。

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我在Wake Forest University 的Muday實驗室看到一個網頁是在說明生長素對根向地性的調控，也許可以提供一些想法

The gravitropic bending of roots has previously been suggested to be mediated by redistribution of the plant hormone auxin from the normal polar transport stream to lateral transport across the root (as reviewed in Muday, 2001).以前，根的向地性被認為是由於生長素的極性運輸改為橫向運輸，The elevated auxin on the lower root would then inhibit growth on the lower side and allow bending to occur.導致近地側的生長素濃度較高，抑制生長造成彎曲所致。 Auxin transport inhibition by application of auxin transport inhibitors or genetic lesions in genes encoding auxin transport proteins lead to a loss of root gravitropism. T利用生長素運輸抑制劑或利用遺傳方法將生長素運輸蛋白基因去除，都會造成根部喪失向地性。The best visual evidence in support of this model are gradients in auxin that can be indirectly detected in roots by promoters that respond to auxin fused to reporter enzymes. 最好的證據就是生長素的濃度可以間接地利用reporter enzyme來觀察。The image shows expression in vertical and gravity stimulated roots.圖片顯示直立及重力方向改變的根間細胞中DR5-GUS 表現的狀況。

In Arabidopsis roots, there are two polarities of auxin movement that are linked to different physiological processes. 在阿拉伯芥的根部，生長素的極性有兩種。 Basipetal movement of IAA from the root tip back has been linked to gravity response (Rashotte et al., 2000).吲哚乙酸IAA由根冠回流的Basipetal 運動與植物的重力反應有關。 Current studies are focused on understanding how basipetal auxin transport is regulated during gravitropic bending. 目前的研究聚焦在探討Basipetal的生長素在向地生長中如何被調整。The role of phosphorylation in regulation of auxin transport is one area of study (Rashotte et al., 2001). 蛋白質磷酸化改變生長素運輸狀態是研究方向之一。Additionally, the ability of flavonoids,which appear to act as endogenous regulators of auxin movement (Brown et al. 2001), to control root gravitropic bending has also been examined (Buer and Muday, 2004).植物本身即具有的 flavonoids 分子是生長素運動的內源性調節因子，而其對向地性的影響目前也正在檢驗中。 We are also examining the role of the gaseous plant hormone, ethylene, in regulation of auxin transport and gravitropic bending and have found that ethylene negatively regulates root (Buer et al. 2006) and shoot gravitropism (Muday et al. 2006), through well established ethylene signaling pathways. 現在我們也正在透過乙烯訊號系統探討氣態植物激素--乙烯對生長素運輸及向地性的影響，已經知道乙烯對根及莖的重力向性的調控是負向的。

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Buer, CS, and Muday, GK (2004) The transparent testa4 mutation prevents flavonoid synthesis and alters auxin transport and the response of Arabidopsis roots to gravity and light. Plant Cell, 16: 1191-1205.

Buer, CS, Sukumar, P, and Muday, GK (2006) Ethylene induced flavonoid synthesis modulates root gravitropism. Plant Physiology: 140: 1384-1396

Rashotte, AM, DeLong, A, and Muday, GK (2001) Genetic and chemical reductions in protein phosphatase activity alter auxin transport, gravity response and lateral root elongation. Plant Cell 13: 1683-1697

Brown, DE, Rashotte, AM, Murphy, AS, Normanly, J, Tague, BW, Peer , WS, Taiz ,L , and Muday, GK (2001) Flavonoids act as negative regulators of auxin transport in vivo in Arabidopsis. Plant Physiol 126: 524-535

Muday, GK (2001) Auxin and Tropisms. Journal of Plant Growth Regulation. 20:226-243

Muday, GK, Brady1, SR, Argueso, C., Deruère, J, Kieber, JJ, and DeLong, A. (2006) RCN1-regulated phosphatase activity and EIN2 modulate hypocotyl gravitropism by a mechanism that does not require ethylene signaling. Plant Physiology: 141: 1617-1629

Muday, GK and Rahman, A (2006) Auxin transport and the integration of gravitropic growth. In Plant Tropisms: eds Gilroy, S and Masson, P, Blackwell Publishing; In press

Rashotte AM, Brady SR, Reed RC, Ante SJ, Muday GK (2000) Basipetal auxin transport is required for gravitropism in roots of Arabidopsis. Plant Physiol 122: 481-490
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這也看出植物體內所有激素彼此交錯影響的複雜程度有多高了。