Geotropismo

African Journal of Agricultural Research Vol. 1 (4), pp. 078-084, November 2006 Available online at http://www.academicjournals.org/AJAR ISSN 1991- 637X © 2006 Academic Journals

Review

A review on mechanism of plant geotropism: developing trend in research on pine root geotropism
Dexian He1 and David B South2*
2

College of Agriculture, Henan Agricultural University, 95 Wenhua Rd.,Zhengzhou, Henan 450002, China. School of Forestry and Wildlife Sciences and Alabama Agricultural Experiment Station, Auburn University, Auburn, Alabama 36849-5418, USA.
Accepted 2 November, 2006

1

Despite the fundamental importance of gravity-driven growth response in plants, the mechanisms that result in root geotropism are poorly understood and the signaling pathways involved remain elusive.Therefore, we reviewed root gravisensing structures and theories (or propositions) explaining root growth geotropism in directions of genetics, physiology, biochemistry, and environmental influence. The available data show that the mechanism(s) of root geotropism and root-gravisensing sites depends on the plant species studied. Based upon such a review, key academic controversies on mechanism ofroot geotropism were pinpointed, and developing trend of research on root geotropism was addressed. Key words: developing trend, hypothesis, mechanism, root geotropism. INTRODUCTION Root geotropism; the downward curvature of the root tip toward the center of gravity is an essential characteristic for plant survival. Studies in this area can be traced back to Charles Darwin (1880) who gave adescription of geotropism and showed that the root cap is essential for root geotropism. A plethora of studies that extend throughout the entire 20th century revealed that perception of a change in root orientation (gravistimulation) by gravity-perceiving cells results in the formation of a signal that will have to be transmitted to a site of that organ where a curvature response can develop (Blancaflorand Masson, 2003). Recent results obtained by different plant species raise critical questions regarding the mechanism of geotropism and several hypotheses have been given on (i) which organ or structure of the root perceives gravity, (ii) how the root responds to gravity, and (iii) how environmental factors affect geotropism. However, no single hypothesis has been accepted regarding the wholemechanism of root geotropism (Sievers and Braun, 1996). Gravity-sensitive structures in plant roots Although there are no convincing conclusions on where the exact gravisensing site is located, scientists are in agreement that the gravity sensitive structures are contained within the most distal segment of the plant root. Researchers refer to this root segment using different terms such as root cap,root apex, and root tip. However, the rootcap is an anatomical root tissue, whereas the root apex indicates a root part distinct from the cap. For some authors, the root tip includes a rootcap, the meristematic zone, the elongation zone, and the maturation zone (Li, 1984). For others, it only includes a rootcap, the quiescent center (QC), and some meristematic cells (Suzuki et al., 1994). Therootcap has widely been regarded as the gravitysensitive site in various plant species. It is reported that during their initial differentiation stages, rootcap cells can perceive gravity and cause positive orthogeotropic growth of roots (Hensel, 1986). Ablation of the rootcap results in a loss of geotropism (Pilet, 1972; Barlow, 1974; Moore and McClelen, 1989) but if rootcaps are replaced, a strongresponse to gravity is restored (Pilet and Elliott, 1981; Moore and McClelen, 1989). Half-decapitated maize (Zea mays L.) roots bent towards the remaining tip. This suggests that rootcap cells might produce growth inhibitors that

*Corresponding author’s E-mail: southdb@auburn.edu. Tel: +01 334 844 1022, Fax: +01 334 844 1084.

Dexian and South

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influence root gravicurvature...