Ethylene regulates fast apoplastic acidification and expansin A transcription during submergence-induced petiole elongation in Rumex palustris.

The semi-aquatic dicot Rumex palustris responds to complete submergence by enhanced elongation of young petioles. This elongation of petiole cells brings leaf blades above the water surface, thus reinstating gas exchange with the atmosphere and increasing survival in flood-prone environments. We already know that an enhanced internal level of the gaseous hormone ethylene is the primary signal for underwater escape in R. palustris. Further downstream, concentration changes in abscisic acid (ABA), gibberellin (GA) and auxin are required to gain fast cell elongation under water. A prerequisite for cell elongation in general is cell wall loosening mediated by proteins such as expansins. Expansin genes might, therefore, be important target genes in submergence-induced and plant hormone-mediated petiole elongation. To test this hypothesis we have studied the identity, kinetics and regulation of expansin A mRNA abundance and protein activity, as well as examined pH changes in cell walls associated with this adaptive growth. We found a novel role of ethylene in triggering two processes affecting cell wall loosening during submergence-induced petiole elongation. First, ethylene was shown to promote fast net H(+) extrusion, leading to apoplastic acidification. Secondly, ethylene upregulates one expansin A gene (RpEXPA1), as measured with real-time RT-PCR, out of a group of 13 R. palustris expansin A genes tested. Furthermore, a significant accumulation of expansin proteins belonging to the same size class as RpEXPA1, as well as a strong increase in expansin activity, were apparent within 4-6 h of submergence. Regulation of RpEXPA1 transcript levels depends on ethylene action and not on GA and ABA, demonstrating that ethylene evokes at least three, parallel operating pathways that, when integrated at the whole petiole level, lead to coordinated underwater elongation. The first pathway involves ethylene-modulated changes in ABA and GA, these acting on as yet unknown downstream components, whereas the second and third routes encompass ethylene-induced apoplastic acidification and ethylene-induced RpEXPA1 upregulation.

Plant expansins are a complex multigene family with an ancient evolutionary origin.

Expansins are a group of extracellular proteins that directly modify the mechanical properties of plant cell walls, leading to turgor-driven cell extension. Within the completely sequenced Arabidopsis genome, we identified 38 expansin sequences that fall into three discrete subfamilies. Based on phylogenetic analysis and shared intron patterns, we propose a new, systematic nomenclature of Arabidopsis expansins. Further phylogenetic analysis, including expansin sequences found here in monocots, pine (Pinus radiata, Pinus taeda), fern (Regnellidium diphyllum, Marsilea quadrifolia), and moss (Physcomitrella patens) indicate that the three plant expansin subfamilies arose and began diversifying very early in, if not before, colonization of land by plants. Closely related "expansin-like" sequences were also identified in the social amoeba, Dictyostelium discoidium, suggesting that these wall-modifying proteins have a very deep evolutionary origin.