Stomata are pores on the leaf surface, bounded by two guard cells, which control the uptake of CO(2) for photosynthesis and the concomitant loss of water vapor. In 1898, Francis Darwin showed that stomata close in response to reduced atmospheric relative humidity (rh); however, our understanding of the signaling pathway responsible for coupling changes in rh to alterations in stomatal aperture is fragmentary. The results presented here highlight the primacy of abscisic acid (ABA) in the stomatal response to drying air. We show that guard cells possess the entire ABA biosynthesis pathway and that it appears upregulated by positive feedback by ABA. When wild-type Arabidopsis and the ABA-deficient mutant aba3-1 were exposed to reductions in rh, the aba3-1 mutant wilted, whereas the wild-type did not. However, when aba3-1 plants, in which ABA synthesis had been specifically rescued in guard cells, were challenged with dry air, they did not wilt. These data indicate that guard cell-autonomous ABA synthesis is required for and is sufficient for stomatal closure in response to low rh. Guard cell-autonomous ABA synthesis allows the plant to tailor leaf gas exchange exquisitely to suit the prevailing environmental conditions.
To elucidate the role of electrical signaling in the phloem of maize the tips of attached leaves were stimulated by chilling and wounding. Two different signals were detected in the phloem at the middle of the leaf using the aphid stylet technique: (1) action potentials (AP) arose in the phloem after chilling; and (2) variation potentials (VPs) were evoked after wounding the leaf tip. Combined electric potential and gas exchange measurements showed that while the wound-induced VP moved rapidly towards the middle of the leaf to induce a reduction in both the net-CO2 uptake rate and the stomatal conductance, there was no response in the gas exchange to the cold-induced AP. To determine if electrical signaling had any impact on assimilate transport the middle of the leaf was exposed to 14CO2. Autoradiography of labeled assimilates provided evidence that phloem and intercellular transport of assimilates from mesophyll to bundle sheath cells was strongly reduced while the cold-induced AP moved through. In contrast, wound-induced VP did not inhibit assimilate translocation but did reduce the amount of the labeled assimilate in phloem and bundle sheath cells. Biochemical analysis revealed that callose content increased significantly in chilled leaves while starch increased in chilled but decreased in wounded leaves. The results led to the conclusion that different stimulation types incite characteristic phloem-transmitted electrical signals, each with a specific influence on gas exchange and assimilate transport.
ABSTRACIEvents of reactivation by re-illumination were studied in predarkened detached mature maize leaves, which were arranged as distal sources and proximal sinks; the latter were kept in COrfree atmosphere and were either illuminated or darkened. Adenine nucleotide (AdN) content and orthophosphate (Pi) concentrations were measured 10 minutes, 30 minutes, and 2, 7, and 14 hours after the onset of re-illumination. For comparison, mature leaves attached to the plant were analyzed.The sum of AdN increased up to 7 hours of re-illumination, then dark sinks and their sources showed decreasing amounts of AdN, while the increase continued up to 14 hours in sources and illuminated sinks. In leaves attached to the plant, no further increase in AdN level followed the 7-hour mark. The amount of individual AdN (ATP, ADP, AMP) differed considerably in sources and sinks of the detached leaves. Although both the source supplying the illuminated sink and the source supplying the dark sink were treated the same, they showed striking differences in AdN contents. Such relations were also observed, when ATP/ADP ratios and Pi concentrations were compared. The influence a sink can exert on its source suggests a participation of the physiological events in the sink on the regulation of AdN and Pi metabolism in the source.Translocation of assimilates in higher plants involves phloem loading, transport of sucrose in the sieve tubes, and phloem unloading. Whereas the movement of sucrose inside the sieve tubes may be passive and independent of energy supply (1, 16), the loading of sucrose into the sieve tubes requires energyconsuming membrane transport processes. The accumulation of a sugar against its concentration gradient has been shown to operate with a protonated carrier (12). According to this model, phloem loading by sucrose/H+ cotransport received strongest evidence (8,10,13).In several publications (2, and literature cited therein), ATPase activity was found associated with the plasmalemma of sieve tubes. In a previous communication, such ATPase activity was shown to be concentrated in sieve elements of differentiating root zones, where phloem unloading was recognized by microautoradiography (3). Following the course of photosynthetically produced ['4C]sucrose from its source to the sink regions within a mature maize leaf, the unloading of sucrose was found to be followed by ATP-requiring phosphorylations of the hexose moieties (4).In illuminated green leaves, photophosphorylations are known to provide most of the ATP, which can be synthesized independ-'Supported by Deutsche Forschungsgemeinschaft. ent of C02-reduction processes. During the night, ATP is produced solely by oxidative phosphorylations. Since neither phloem transport nor growth processes are interrupted during the night, metabolic energy supplied by oxidative phosphorylation must satisfy both phloem loading and phloem unloading. Respiration is often assumed to be suppressed in the light to prevent useless consumption of ATP by futile cycles (9). If so, metabolic en...
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