Differential Growth in Thigmotropism
While the turgor movements associated with thigmotropism result in temporary, rapid curvature of tendrils, a slower process of differential growth is also employed in many species of tendril-bearing plants. These changes in growth which occur as a result of mechanostimulation are often referred to as thigmomorphogenesis. Numerous studies over the past ten years have highlighted a numer of factors which may play a role in thigmomorphogenesis. Two such studies are summarized below.
Janet Braam - Touch genes
Dr. Braam has conducted research on the genetic factors which may be involved in the growth responses to a number of stimuli, including touch. To date she has found four "touch" genes (TCH1-4) which are intensely upregulated in response to touch. These genes were found to encode calmodulin and calmodulin-related proteins. She also found that touch, which results in the induction of the TCH genes, causes an immediate rise in the cytoplasmic Ca2+ levels. As a result of these findings, Dr. Braam has proposed that Ca2+ must play a major role in the signal transduction pathways activated by the inductive stimulus. Furthermore, increases in cytoplasmic Ca2+ levels induce the expression of TCH2, TCH3, and TCH4, suggesting that Ca2+ itself may regulate the expression of these TCH genes.
After conducting additional experiments on the function of the TCH4 gene product, Dr. Braam determined that TCH4 actually codes for a xyloglucan endotransglycosylase (XET). As we know, the major hemicellulose in dicotic cell walls is xyloglucan. This was a very important finding, as XET's have the ability to alter the structure of cell walls which contain xyloglucan. Particularly, XET's modify xyloglucan polymers by internal cleavage and linkage of the newly generated ends to other xyloglucan polymer ends. The modification of these xyloglucan polymers may control the strength and extensibility of the cell wall. As we know, cell growth requires modification of the cell wall, and the fact that XET's are upregulated upon mechanostimulation suggests a viable pathway by which this growth can occur. However, the particular regulatory pathway by which only select cells express the TCH4 gene has yet to be elucidated.
Weiler et. al - Jasmonates
Dr. Weiler and his colleagues have uncovered evidence for the involvement of particular compounds known as jasmonates in the tendril coiling response of Bryonia dioica. In particular, the researchers propose that the reaction of the tendril to touch is mediated by metabolites of alpha-linolenic acid, the most active being 12-oxo-phytodienoic acid and jasmonic acid. When these compounds were applied to the tendrils of B. dioica, a coiling response was induced which was indistinguishable from that induced by touch. Therefore, Dr. Weiler proposed that the two aforementioned compounds completely substitute for a mechanical stimulus. While this is a somewhat agressive statement, the results do suggest that 12-oxo-phytodienoic acid and jasmonic acid play very important roles in tendril coiling. In addition, the researchers also found that intracellular jasmonic acid levels rise several-fold during coiling. The results of this study strongly suggest that the alpha-linolenic acid cascade to jasmonic acid and other jasmonates may be the central transduction mechanism coupling the mechanical stimulus to the growth response.
In addition to testing the ability of jasmonates to induce coiling, the researchers also tested other hormonal growth-inducing factors, namely IAA and ethylene. It has been found that jasmonic acid stimulates ACC-dependent ethylene formation in tomato and rice. To test whether the coiling response was due to jasmonates or ethylene, the researchers treated the tendrils with jasmonates in the presence of an ACC-synthetase inhibitor (AVG), and found that this had no effect on tendril coiling. The same was observed for IAA, indicating that neither IAA nor jasmonate-induced ethylene was actually required for the coiling response. Therefore, it appears that jasmonates are crucial in touch-stimulus signal transduction, while the roles of endogenous IAA and ethylene in tendril coiling remain somewhat uncertain.
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