Abstract
The riverplain species Rumex palustris shows a remarkable response upon submergence. The petioles and leafblades change from a rather horizontal, to a vertical position (hyponastic growth), followed by enhanced elongation of the petioles. The hyponastic and elongation growth of R. palustris petioles could result in restored contact of the plant
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with the air atmosphere and enhanced survival due to restored gas exchange.
The submergence-induced petiole elongation of R. palustris is achieved solely by cell elongation. A protein shown to be of major importance in regulating plant cell elongation is expansin. Vriezen et al. (2000) showed with Northern blot analysis an increase of expansin transcripts in submerged R. palustris petioles. This thesis continues the work started by Vriezen et al. and describes an in depth study to the role of expansins in the submergence-induced petiole elongation of R. palustris.
At least 20 different expansins genes are present in R. palustris. The transcript levels of five of these genes (RpEXP1, 8, 10, 15 and 18) were studied in their response to submergence. Only one of these five genes, RpEXP1, showed enhanced mRNA abundance in petioles upon submergence. Next to increased RpEXP1 transcript levels, submergence also induced a higher abundance of expansin proteins, of the size class containing RpEXP1. Expansin activity, measured as the acid-induced extension of frozen/thawed petioles, also showed a remarkable rise upon submergence. Since the increase of both RpEXP1 and the acid-induced activity correlated closely with the onset of submergence-induced elongation, RpEXP1 is likely to be involved in the submergence-induced petiole elongation of R. palustris. However, high expansin levels did not always coincide with enhanced petiole elongation. This lack of correlation, together with the observation that petiole elongation could be induced by hormones, without a concerted rise in RpEXP1, led to the conclusion that more factors are involved leading to enhanced petiole elongation in R. palustris.
A primary hormone in the signal transduction pathway leading to enhanced petiole elongation upon submergence is the gaseous hormone ethylene. Ethylene induces a signaling cascade, involving ea. gibberellic acid and abscisic acid. However, none of the phytohormones known to affect petiole elongation, other than ethylene, showed a clear regulatory role in RpEXP1 transcript levels. Therefore, expression of RpEXP1 seems to be under direct regulation of ethylene.
Cell wall pH, a property regulating expansin action, decreased upon submergence. This acidification could be inhibited by the ethylene antagonist 1-MCP. The submergence-induced, ethylene dependent acidification preceded the enhanced elongation. Apoplastic acidification is therefore thought to facilitate the action of cell wall loosening mechanisms, such as expansin action, and not to have a direct regulatory function in determining the onset of submergence-induced petiole elongation.
Submerged R. palustris petioles that are fixed in an horizontal position show a reduced elongation response. Whether this growth reduction is due to lower levels of RpEXP1 remains to be elucidated. When the petioles are fixed at angles below their original angles, the petioles do show enhance elongation when submerged. This enhanced elongation is probably due to enhanced synthesis of expansins at the abaxial side of the petiole.
The results presented in this study suggest a role for RpEXP1 in the initiation of submergence-induced petiole elongation of R. palustris. The transcription of RpEXP1 is under direct control of ethylene, a prime hormone in the signal transduction pathway. Ethylene is also involved in regulating submergence-induced cell wall acidification. This acidification is likely to enhance the activity of expansins and other cell wall proteins that might be involved in the submergence-induced petiole elongation of R. palustris.
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