The too many mouths and four lips mutations affect stomatal production in Arabidopsis.
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Stomata regulate gas exchange through the aerial plant epidermis by controlling the width of a pore bordered by two guard cells. Little is known about the genes that regulate stomatal development. We screened cotyledons from ethyl methanesulfonate-mutagenized seeds of Arabidopsis by light microscopy to identify mutants with altered stomatal morphology. Two mutants, designated too many mouths (tmm) and four lips (flp), were isolated with extra adjacent stomata. The tmm mutation results in stomatal clustering and increased precursor cell formation in cotyledons and a virtual absence of stomata in the inflorescence stem. The flp mutation results in many paired stomata and a small percentage of unpaired guard cells in cotyledons. The double mutant (tmm flp) exhibits aspects of both parental phenotypes. Both mutations appear to affect stomatal production more than patterning or differentiation. tmm regulates stomatal production by controlling the formation, and probably the activity, of the stomatal precursor cell. (+info)
Evidence that zeaxanthin is not the photoreceptor for phototropism in maize coleoptiles.
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The photoreceptor that mediates blue-light-induced phototropism in dark-grown seedlings of higher plants has not been identified, although the carotenoid zeaxanthin has recently been proposed as the putative chromophore. In the experiments described in this paper, we analyzed phototropism and a blue-light-induced protein phosphorylation that has been genetically and physiologically implicated in phototropism in wild-type maize (Zea mays L.) seedlings and compared the results with those from seedlings that are either carotenoid deficient through a genetic lesion or have been chemically treated to block carotenoid biosynthesis. The blue-light-dependent phototropism and phosphorylation responses of seedlings deficient in carotenoids are the same as those of seedlings containing normal levels of carotenoids. These results and those in the literature make it unlikely that zeaxanthin or any other carotenoid is the chromophore of the blue-light photoreceptor for phototropism or the blue-light-induced phosphorylation related to phototropism. (+info)
Asymmetric distribution of acetylcholinesterase in gravistimulated maize seedlings.
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Acetylcholinesterase (AChE) activity has previously been studied by this laboratory and shown to occur at the interface between the stele and cortex of the mesocotyl of maize (Zea mays L.) seedlings. In this work we studied the distribution of AChE activity in 5-d-old maize seedlings following a gravity stimulus. After the stimulus, we found an asymmetric distribution of the enzyme in the coleoptile, the coleoptile node, and the mesocotyl of the stimulated seedlings using both histochemical and colorimetric methods for measuring the hydrolysis of acetylthiocholine. The hydrolytic capability of the esterase was greater on the lower side of the horizontally placed seedlings. Using the histochemical method, we localized the hydrolytic capability in the cortical cells around the vascular stele of the tissues. The hydrolytic activity was inhibited 80 to 90% by neostigmine, an inhibitor of AChE. When neostigmine was applied to the corn kernel, the gravity response of the seedling was inhibited and no enzyme-positive spots appeared in the gravity-stimulated seedlings. We believe these results indicate a role for AChE in the gravity response of maize seedlings. (+info)
Asymmetric, blue light-dependent phosphorylation of a 116-kilodalton plasma membrane protein can be correlated with the first- and second-positive phototropic curvature of oat coleoptiles.
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The possible correlation between blue light-dependent phosphorylation of a 116-kD protein and phototropic responses of etiolated oat (Avena sativa L.) seedlings was tested by a micromethod for protein phosphorylation. Quantitation of the basipetal distribution of this protein showed that the in vitro 32p phosphorylation values declined exponentially from tip to node, with more than 50% of the total label being found in the uppermost 5 mm. Nonsaturating preirradiation of the coleoptiles in vivo resulted in partial phosphorylation with endogenous ATP. Subsequent in vitro phosphorylation under saturating irradiation allowed the determination of the degree of in vivo phosphorylation. Unilateral preirradiation resulted in higher in vivo phosphorylation on the irradiated than on the shaded side of the coleoptile. The fluence-response curve for the difference in phosphorylation between both sides of the coleoptile resembles the fluence-response curve for first-positive phototropic curvature, although it is shifted by two orders of magnitude to higher fluences. Possible reasons for this shift are discussed. In the coleoptile base the phosphorylation gradient across the coleoptile becomes larger with increasing time of irradiation at a constant fluence. Thus, phosphorylation of the 116-kD protein, in accordance with second-positive phototropic curvature, does not obey the Bunsen-Roscoe reciprocity law. (+info)
Exposure of oat seedlings to blue light results in amplified phosphorylation of the putative photoreceptor for phototropism and in higher sensitivity of the plants to phototropic stimulation.
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Dark recovery of blue light-induced in vitro phosphorylation in oat (Avena sativa L.) seedlings after in vivo preirradiation with blue light revealed different recovery kinetics for the coleoptile base and tip. Although, in both cases, maximum in vitro phosphorylation was observed 90 min after in vivo blue light treatment, the phosphorylation levels for the entire base were about 3-fold higher than those found in nonpreirradiated plants. The tip response only slightly exceeded that of the dark controls. The fluence applied during preirradiation determined the extent of the increase in phosphorylation. Consequently, unilateral irradiation and subsequent dark incubation resulted in a more pronounced increase in phosphorylation in the irradiated than in the shaded side of the coleoptile base. Furthermore, blue light-irradiation conditions, known to induce neither first- nor second-positive curvature in nonpreirradiated plants, stimulated both asymmetric distribution of protein phosphorylation and second-positive phototropic curvature in the coleoptile base when administered to blue light-pretreated plants. Based on these data, we conclude that photosensitivity of the coleoptile base increases upon exposure to blue light in a time-and fluence-dependent manner, providing an excellent explanation of the invalidity of the Bunsen-Roscoe reciprocity law for second-positive phototropism. (+info)
Restoration of phototropic responsiveness in decapitated maize coleoptiles.
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The literature indicates that the tip of maize (Zea mays L.) coleoptiles has the localized functions of producing auxin for growth and perceiving unilateral light stimuli and translocating auxin laterally for phototropism. There is evidence that the auxinproducing function of the tip is restored in decapitated coleoptiles. We examined whether the functions for phototropism are also restored by using blue-light conditions that induced a first pulse-induced positive phototropism (fPIPP) and a time-dependent phototropism (TDP). When the apical 5 mm, in which photosensing predominantly takes place, was removed, no detectable fPIPP occurred even if indole-3-acetic acid (lanolin mixture) was applied to the cut end. However, when the blue-light stimulation was delayed after decapitation, fPIPP became inducible in the coleoptile stumps supplied with indole-3-acetic-acid/lanolin (0.01 mg g-1), indicating that phototropic responsiveness was restored. This restoration progressed 1 to 2 h after decapitation, and the curvature response became comparable to that of intact coleoptiles. The results for TDP were qualitatively similar, but some quantitative differences were observed. It appeared that the overall TDP was based on a major photosensing mechanism specific to the tip and on at least one additional mechanism not specific to the tip, and that the tip-specific TDP was restored in decapitated coleoptiles with kinetics similar to that for fPIPP. It is suggested that the photoreceptor system, which accounts for fPIPP and a substantial part of TDP, is regenerated in decapitated coleoptiles, perhaps together with the mechanism for lateral auxin translocation. (+info)
Phytochrome induces changes in the immunodetectable level of a wall peroxidase that precede growth changes in maize seedlings.
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The regulatory pigment phytochrome induces rapid and opposite growth changes in different regions of etiolated maize seedlings: it stimulates the elongation rate of coleoptiles and inhibits that of mesocotyls. As measured by a quantitative immunoassay, phytochrome also promotes rapid and opposite changes in the extractable content of a Mr 98,000 anionic isoperoxidase in the cell walls of these same organs: it induces a decrease of this peroxidase in coleoptiles and an increase in mesocotyls. The peroxidase changes precede the growth changes. As measured by video stereomicroscopy or a position transducer, red light (R), which photoactivates phytochrome, stimulates coleoptile elongation with a lag of about 15-20 min and suppresses mesocotyl growth with a lag of 45-50 min. R also induces a 50% reduction in the extractable level of the anionic peroxidase in coleoptile walls in less than 10 min and a 40% increase in the level of this peroxidase in mesocotyl walls within 30 min. Ascorbic acid, an inhibitor of peroxidase activity, blocks the effects of R on mesocotyl section growth. These results are relevant to hypotheses that postulate that certain wall peroxidases can participate in light-induced changes in growth rate by their effects on wall extensibility. (+info)
Growth inhibition, turgor maintenance, and changes in yield threshold after cessation of solute import in pea epicotyls.
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The dependence of stem elongation on solute import was investigated in etiolated pea seedlings (Pisum sativum L. var Alaska) by excising the cotyledons. Stem elongation was inhibited by 60% within 5 hours of excision. Dry weight accumulation into the growing region stopped and osmotic pressure of the cell sap declined by 0.14 megapascal over 5 hours. Attempts to assay phloem transport via ethylenediaminetetraacetate-enhanced exudation from cut stems revealed no effect of cotyledon excision, indicating that the technique measured artifactual leakage from cells. Despite the drop in cell osmotic pressure, turgor pressure (measured directly via a pressure probe) did not decline. Turgor maintenance is postulated to occur via uptake of solutes from the free space, thereby maintaining the osmotic pressure difference across the cell membrane. Cell wall properties were measured by the pressure-block stress relaxation technique. Results indicate that growth inhibition after cotyledon excision was mediated primarily via an increase in the wall yield threshold. (+info)