The Muday Laboratory, Wake Forest University

Auxin Transport and Root Gravitropism

One of the interests of the Muday laboratory is the role of auxin transport in plant gravitropism. 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). The elevated auxin on the lower root would then inhibit growth on the lower side and allow bending to occur. 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. 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. The image shows DR5-GUS expression in vertical and gravity stimulated roots. A number of current studies in the laboratory focus on how auxin transport is regulated during gravitropic bending.

In Arabidopsis roots, there are two polarities of auxin movement that are linked to different physiological processes.  Basipetal movement of IAA from the root tip back has been linked to gravity response (Rashotte et al., 2000). Current studies are focused on understanding how basipetal auxin transport is regulated during gravitropic bending. The role of phosphorylation in regulation of auxin transport is one area of study (Rashotte et al., 2001).  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).  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. The experiments in each of these areas are summarized separately. Together these experiments and many performed in other laboratories have provided new insight into how auxin transport changes in response to alterations in the gravity vector, thereby allowing redirection of plant growth orientation (Muday and Rahman, 2006).

(graduate student names in bold, undergraduates underlined)

Lewis, DR, Ramirez, MV, Miller, ND, Keith, R, Helm, R, Winkel, BSJ, Muday, GK (In review) Auxin and ethylene induce distinct flavonol accumulation patterns through independent transcriptional networks: Plant Physiol

Lewis, DR and Muday, GK (2009) Measurement of auxin transport in Arabidopsis thaliana Nature Protocols: 4: 437-451.

Sukumar, P, Edwards, KS, Rahman, A, and DeLong, A, and Muday, GK (2009) PINOID kinase regulates root gravitropism through modulation of PIN2-dependent basipetal auxin transport in Arabidopsis thaliana. Plant Physiol: 150: 722-735.

Chen, Z, Noir, S. Kwaiitaal, M, Hartmann, HA, Wu, M-J, Mudgel, Y, Sukumar, P, Muday, GK, Panstruga, R, and Jones, AM (2009) Two seven-transmembrane domain MLO proteins co-function in root Thigmomorphogenesis. Plant Cell: 21:1972-1991.

Muday, GK and Rahman, A (2007) Auxin transport and the integration of gravitropic growth. In Plant Tropisms: eds Gilroy, S and Masson, P, Blackwell Publishing pp 47-78.

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

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 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

see also [Pubmed List of Papers]

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