The temporal and spatlal expression of one member of the Arabidopsis 1-aminocyclopropane-1-carboxylate (ACC) synthase gene family (ACS7) was analyzed using a promoter-P-glucuronidase fusion. The expression of ACSl is under developmental control both in shoot and root. High expression was observed in young tissues and was switched off in mature tissues. ACS7 pmmoter activity was strongly correlated with lateral root formation. Dark-grown seedlings exhibited a different expression pattern from light-grown ones. The ACC content and the in vivo activity of ACC oxidase were determined. ACC content correlated with ACS7 gene activity. ACC oxidase activity was demonstrated in young Arabidopsis seedlings. Thus, the ACC formed can be converted into ethylene. In addition, ethylene production of immature leaves was fourfold higher compared to that of mature leaves. The possible involvement of ACSl in influencing plant growth and development is discussed. INTRODUCTIONEthylene is involved in several aspects of plant development from germination and seedling growth to flowering, fruit ripening, leaf abscission, and organ senescence. It also plays a key role in the response to environmental factors (Abeles, 1973; Yang and Hoffman, 1984; Moore, 1989; Van Der Straeten and Van Montagu, 1991).With the recent cloning of genes encoding the two most important enzymes in the biosynthesis of ethylene, l-aminocyclopropane-1-carboxylate (ACC) synthase (Nakajima et al., 1990; Van Der Straeten et al., 1990; Dong et al., 1991;Huang et al., 1991; Olson et al., 1991; and ACC oxidase (ethylene-forming enzyme or EFE) (Hamilton et al., 1990; Spanu et al., 1991;Wang and Woodson, 1991), more information about the molecular regulation of the synthesis of this hormone has been made available. The main focus has been on the regulation of ACC synthase gene expression because of its key regulatory role in the pathway. This implies that diverse inducers of ethylene production are also inducers of de novo synthesis of ACC synthase (Yang and Hoffmann, 1984; Van Der Straeten and Van Montagu, 1991).ACC synthases have been cloned from tomato (Van Der Straeten et al., 1990; Olson et al., 1991; To whom correspondence should be addressed.Yip et al., 1992), winter squash (Nakajima et al., 1990), zucchini (Huang et al., 1991), Arabidopsis (Liang et al., 1992; Van Der Straeten et al., 1992), mung bean (Botellaet al., 1992; Kim et al., 1992), carnation (Park et al., 1992), orchid (ONeill et al., 1993), tobacco (Bailey et al., 1992), and apple (Dong et ai., 1991; Kim et al., 1992). In several cases, it has been shown that ACC synthase genes belong to a multigene family and that they are differentially responsive to various ethyleneinducing factors and conditions such as wounding, fruit ripening, and auxins, although in certain cases some leve1 of coordination was demonstrated (Nakajima et al., 1990; Van Der Straeten et al., 1990; Dong et al., 1991; Olson et al., 1991; ; Yip et ai., 1992). These studies have provided information about general aspects of AC...
Abstract— Kalanchoë blossfeldiana seeds are light‐requiring for seed germination. On water or KNO3 solution and irradiated with several daily red (R) irradiations, the seeds show a low‐fluence (LF) response which is far‐red (FR) reversible. Incubated on gibberellic acid (GA3) the seeds show a very‐low‐fluence (VLF) response which can be saturated with red as well as with far red light. As germination is a quantal response, the sub‐optimal segments of the dose‐response curves are analysed by means of probit analysis in order to calculate the seed population parameters. There is a linear relation between the probit of the germination response and the logarithm of the fluence. Moreover, the slope for the VLF as well as for the LF response is the same. The VLF requires about 8 × 104 times less fluence than the LF. VLF saturation with FR requires about 200 times more fluence than with R. Although, GA3 and KNO3 modulate VLF and LF, respectively, there is no direct influence on the phytochrome‐phototransformations. Once Pfr is formed (in VLF or LF, or preserved in dry seeds) germination is proportional to the GAS concentration (for VLF and dark germination) or proportional to the KNO, concentration (for LF). The non‐photochemical events leading to germination seem to be triggered by a similar action mechanism for both GA, and KNO3.
The fluence-response curves for the effect of two red pulses separated by 24 hours on the germination of Kalanchoe blossfeldiana Poelln. cv Vesuv seeds, incubated on gibberellic acid (GA3) are biphasic for suboptimal concentrations. The response in the low fluence range corresponds with a classical red/far-red reversible phytochrome mediated reaction. GA3 induces an additional response in the very low fluence range, which is also phytochrome mediated. The sensitivity to phytochrome-far-red absorbing form (Pfr), however, is increased about 20,000-fold, so that even far-red fluences become saturating. Both in the very low and low fluence response range, the maximal responses induced by saturating fluences are modulated by the GA3 concentration. GA3 having no direct influence on the phytochrome phototransformations, alters the Pfr requirement and determines the responding seed population fraction in the very low and low fluence range. The effet of GA3 appears to be on the transduction chain of the phytochrome signal.The germination of Kalanchoe seeds is light-requiring. Incubated on water or KNO3 solution, germination requires several daily light pulses given from the first day on after sowing, to prevent induction of secondary dormancy (8,20). The seed population responds in the LF2 range (3xl0-5 to 3x10-2 mol/ mi2, R) and reversion ofa promoting light pulse by FR is complete (8, 11). The KNO3 concentration determines the responding population fraction in the LF range (8).In the presence of GA3, germination can be induced with a single light pulse. This irradiation was given on d 7 after sowing, the moment of highest response in the presence of GA3 (6). The seed population responds in the VLF range (10-9_10-6 mol/m2, R) and a single FR pulse is inductive (8). For a single R or FR pulse, the GA3 concentration determines the responding population fraction in the VLF range, in an identical way (8). Both in the presence and absence of GA3, preliminary action spectra point to phytochrome as the photoreceptor (7). GA3 increases ' Supported by the Belgian National Science Foundation Grant FKFO-2.0083.83. GA3 concentrations, the effect of R is higher than that of FR in the 0.1 to 0.01mM concentration range (6). These results suggest the possibility of a LF response, additional to the VLF response, which is induced by two R but not by two FR irradiations at suboptimal GA3 concentrations.Fluence-response curves for two R or FR pulses are presented, allowing the quantification and correlation of both the VLF and the LF response types in the same experimental conditions.We also studied the relation between exogenously applied GA3 and the maximal germination induced with VLF of R and FR, saturating the VLF response and with low R fluences, saturating the LF response. Probit analysis is used to calculate the population parameters (see "Materials and Methods"). From the LDP equations and from the zero response and the maximal response, the sigmoid curves fitting the experimental data are generated. The phytochrome...
Germination of Kalanchoe blossfeldiana Poelin. seeds is absolutely light-requiring. Germination of one seed is the result of one out of three reactions, viz. the very low fluence response (VLFR), the low fluence response (LFR) and the high fluence response/high irradiance response. In order to demonstrate the involvement of phytochrome for both photoresponses, i.e. VLFR and LFR, action spectra for induction were determined. Fluence-response data are analyzed by means of probit analysis in order to calculate the seed population parameters, with special attention to t, or the fluence for half-maximal induction, and B, the slope in the probit diagram. Laser light was used between 620 and 800 nanometers to analyze the VLFR. Phytochrome is responsible for both photoresponses: the VLFR action spectrum demonstrates an exponential decrease in apparent photoconversion cross-section (Pr -_ Pft) up to about 800 nanometers. Assuming that Pr:Pfr-X and Pfr:Pfr-X are the effectors for the VLFR and the LFR, respectively, we estimate an average induction threshold of about 0.003% P,:P, for the VLFR and about 1% Pfr:Pf, for the LFR among individuals of the seed population.
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