Modulation of the root-sourced ABA signal along its way to the shoot in Vitis riparia x Vitis labrusca under water deficit.

The intensity of the root-sourced abscisic acid (ABA) signal has long been thought to decrease along its long-distance transport pathway, and hence the shoot responses to the ABA signal would be expected to become less sensitive with the increase in plant height. It is reported here that there is a significant modification of the ABA signal intensity in its pathway to leaves in grapevine (Vitis riparia×Vitis labrusca), but in contrast to the expectation that the ABA signal intensity may decrease along its long-distance transport pathway, it was found that the root-sourced ABA signal is gradually intensified along a vine for as long as 3 m under both water-stressed and non-stressed conditions. Consistent with the alterations in ABA signal intensity, stomatal sensitivity to a root-sourced ABA signal was also gradually increased from the base to the apex. Leaf stomatal conductance near the apex was more severely inhibited than in the leaves at the base of the vine. It was observed that xylem pH was significantly increased from the base to the apex, and that artificially changing the xylem sap pH to be more alkaline by feeding with buffers increased the xylem ABA concentration. Our results suggest that the pH gradient along the stem may play a role in the modification and enhancement of ABA signal intensity such that the stomata at the top of canopy can be more sensitively regulated in response to soil drying.

Excretion and folding of plasmalemma function to accommodate alterations in guard cell volume during stomatal closure in Vicia faba L.

Stomatal movement results in large and repetitive changes in cell volume and consequently surface area. While endocytosis has been extensively studied and is thought to be a major mechanism for accommodating the volume changes as evidenced mainly by fluorescent labelling and confocal imaging, studies at the ultrastructural level in intact guard cells of stomata regulated by natural factors have never been reported. Here, it is reported that excretion and folding of the plasmalemma are critical for accommodating the volume alterations in intact guard cells in Vicia faba L. Using transmission electron microscopy in combination with laser confocal microscopy, it was observed that in fully opened stomata the plasmalemma was smooth and tightly adhered to the cell walls while a whole large vacuole appeared in the cell. In the closed stomata, besides vacuole fragmentation, endocytosis of the tonoplast rather than the plasmalemma commonly occurred. Importantly, in stomata where pore closure was induced by circadian rhythm or CO(2), numerous tiny vesicles were found outside the plasmalemma and, moreover, extensive folding of the plasmalemma could also be found in some regions of the cells. Additionally, an unknown structure was found at the interface between the plasmalemma and cell walls, especially in those areas of the cell where extensive folding occurred, suggesting that plasmalemma turnover is possibly associated with an interaction between the plasmalemma and cell walls. Collectively, the results strongly indicate that excretion and folding of the plasmalemma are critical for the accommodation of the cell volume alterations during stomatal movement.