Phytochrome-mediated phototropism in de-etiolated seedlings : occurrence and ecological significance

Phototropic responses to broadband far red (FR) radiation were investigated in fully de-etiolated seedlings of a long-hypocotyl mutant (lh) of cucumber (Cucumis sativus L.), which is deficient in phytochrome-B, and its near isogenic wild type (WT). Continuous unilateral FR light provided against a background of white light induced negative curvatures (i.e. bending away from the FR light source) in hypocotyls of WT seedlings. This response was fluence-rate dependent and was absent in the lh mutant, even at very high fluence rates of FR. The phototropic effect of FR light on WT seedlings was triggered in the hypocotyls and occurred over a range of fluence rates in which FR was very effective in promoting hypocotyl elongation. FR light had no effect on elongation of lh-mutant hypocotyls. Seedlings grown in the field showed negative phototropic responses to the proximity of neighboring plants that absorbed blue (B) and red light and back-reflected FR radiation. The bending response was significantly larger in WT than in lh seedlings. Responses of WT and lh seedlings to lateral B light were very similar; however, elimination of the lateral B light gradients created by the proximity of plant neighbors abolished the negative curvature only in the case of lh seedlings. More than 40% of the total hypocotyl curvature induced in WT seedlings by the presence of neighboring plants was present after equilibrating the fluence rates of B light received by opposite sides of the hypocotyl. These results suggest that: (a) phytochrome functions as a phototropic sensor in de-etiolated plants, and (b) in patchy canopy environments, young seedlings actively project new leaves into light gaps via stem bending responses elicited by the B-absorbing photoreceptor(s) and phytochrome.     

苔藓植物蓝光/紫外光-A 信号传导的研究

种子植物含有5个已分离的光受体和至少1个未鉴定的蓝光/紫外光-A受体。隐花色素(CRY1、CRY2和CRY3) 调节植物的生长发育,而向光蛋白(PHOT1和PHOT2) 调节植物对光的营养反应。黄素可以吸收蓝光和紫外光-A,是生色团。对这些光受体的结构和作用模式已了解很多。苔藓植物小立碗藓中含有2个已分离的隐花色素(CRY1a和CRY1b),负责调节侧枝形成和生长素代谢;有4个向光蛋白(PHOTA1,PHOTA2,PHOTB1,PHOTB2) 调节叶绿体的运动。苔藓细胞内蓝光/紫外光-A刺激引发的信号转导有Ca2+参与。     

Action spectrum for cryptochrome-dependent hypocotyl growth inhibition in Arabidopsis

Cryptochrome blue-light photoreceptors are found in both plants and animals and have been implicated in numerous developmental and circadian signaling pathways. Nevertheless, no action spectrum for a physiological response shown to be entirely under the control of cryptochrome has been reported. In this work, an action spectrum was determined in vivo for a cryptochrome-mediated high-irradiance response, the blue-light-dependent inhibition of hypocotyl elongation in Arabidopsis. Comparison of growth of wild-type, cry1cry2 cryptochrome-deficient double mutants, and cryptochrome-overexpressing seedlings demonstrated that responsivity to monochromatic light sources within the range of 390 to 530 nm results from the activity of cryptochrome with no other photoreceptor having a significant primary role at the fluence range tested. In both green- and norflurazon-treated (chlorophyll-deficient) seedlings, cryptochrome activity is fairly uniform throughout its range of maximal response (390-480 nm), with no sharply defined peak at 450 nm; however, activity at longer wavelengths was disproportionately enhanced in CRY1-overexpressing seedlings as compared with wild type. The action spectrum does not correlate well with the absorption spectra either of purified recombinant cryptochrome photoreceptor or to that of a second class of blue-light photoreceptor, phototropin (PHOT1 and PHOT2). Photoreceptor concentration as determined by western-blot analysis showed a greater stability of CRY2 protein under the monochromatic light conditions used in this study as compared with broad band blue light, suggesting a complex mechanism of photoreceptor activation. The possible role of additional photoreceptors (in particular phytochrome A) in cryptochrome responses is discussed.     

New insights into resistance protein-mediated signaling against turnip crinkle virus in Arabidopsis

The Arabidopsis-turnip crinkle virus (TCV) system is one of the few tractable plant-virus systems that allow simultaneous characterization of host components required for basal- and/or resistance (R) protein-mediated defenses. Another unique feature is that hypersensitive response (HR) and resistance can be studied as two distinct phenotypes in this pathosytem. The R protein HRT confers HR to TCV but requires a recessive locus rrt to confer resistance. The pathways leading to HR and resistance are mutually exclusive. HRT interacts with EDS1, which potentiates HR to TCV and is also required for resistance signaling. HRT-mediated signaling is also dependent on the EDS1-interacting proteins PAD4 and SAG101, which form binary and ternary complexes with EDS1. HRT-mediated resistance is also dependent on light and more specifically on the blue-light photoreceptors, cryptochromes (CRY) and phototropins (PHOT). Of these, CRY2 and PHOT2 are required for the stability of HRT. HRT is degraded in a proteasome-dependent manner, which correlates with its interaction with the E3 ubiquitin ligase, COP1. Together, these results suggest that components of light signaling modulate plant defense against TCV by regulating the stability of, and signaling mediated by, the R protein HRT.     

Seed germination in a tomato male-sterile mutant is resistant to osmotic, salt and low-temperature stresses

Seed germination in a male-sterile 7B-1 mutant in tomato is reletively more resistant to the inhibitory effects of a high osmoticum induced by mannitol and polyethylene glycol, to various salts, including NaCl, Na₂SO₄, KCl and K₂SO₄, and to low-temperature stress, compared to the wild-type (WT) seeds. The inhibitory effects of various stresses could be partly or completely overcome by fluridone (FLU), an inhibitor of abscisic acid (ABA) biosynthesis. However, lower concentration of fluridone was required for the 7B-1 mutant than for WT seeds, and the mutant seeds were more sensitive to the inhibitory effects of exogenous ABA. The data suggest that 7B-1 seed has a pre-existing level of elevated ABA which imparts resistance to the various stresses. The ability to regulate male sterility in the 7B-1 mutant by photoperiod, as previously reported by Sawhney (1997), and its resistance to abiotic stresses, as reported here, makes this a useful system for tomato breeding and in hybrid programs. Received: 11 May 2000 / Accepted: 4 June 2000     

Is the rate of photosynthesis under blue light altered in the 7B-1 tomato mutant?

Changes of photosynthesis under blue light were examined in the ABA-overproducing 7B-1 mutant in tomato. Net photosynthetic rate (P _N), stomatal conductance (g _s), intrinsic water-use efficiency (WUE_i) and chlorophyll (a+b) [Chl (a+b)] content in leaves of different insertion (1^st, 4^th and 9^th ones) were measured in 5-, 7- and 9-week-old plants. P _N, g _s, and Chl (a+b) content were mostly similar in young leaves of 7B-1 and wild type (WT) plants. With the aging of leaves, a blue-light-induced increase in P _N and g _s to steady-state was delayed and steady-state values of P _N and g _s were lower in 7B-1 plants compared with WT. Steady-state values of WUE_i were increased in 4^th and 9^th leaves of 7B-1 plants compared with WT. The results can be explained by the higher endogenous level of ABA in 7B-1 plants and their lower sensitivity to ABA in earlier growth stage.     

Effects of light quality on the chloroplastic ultrastructure and photosynthetic characteristics of cucumber seedlings

To understand how light quality influences plant photosynthesis, we investigated chloroplastic ultrastructure, chlorophyll fluorescence and photosynthetic parameters, Rubisco and chlorophyll content and photosynthesis-related genes expression in cucumber seedlings exposed to different light qualities: white, red, blue, yellow and green lights with the same photosynthetic photon flux density of 100 μmol m^?2 s^?1. The results revealed that plant growth, CO₂ assimilation rate and chlorophyll content were significantly reduced in the seedlings grown under red, blue, yellow and green lights as compared with those grown under white light, but each monochromatic light played its special role in regulating plant morphogenesis and photosynthesis. Seedling leaves were thickened and slightly curled; Rubisco biosynthesis, expression of the rca, rbcS and rbcL, the maximal photochemical efficiency of PSII (Fv/Fm) and quantum yield of PSII electron transport (Ф_PSII) were all increased in seedlings grown under blue light as compared with those grown under white light. Furthermore, the photosynthetic rate of seedlings grown under blue light was significantly increased, and leaf number and chlorophyll content of seedlings grown under red light were increased as compared with those exposed to other monochromatic lights. On the contrary, the seedlings grown under yellow and green lights were dwarf with the new leaves etiolated. Moreover, photosynthesis, Rubisco biosynthesis and relative gene expression were greatly decreased in seedlings grown under yellow and green light, but chloroplast structural features were less influenced. Interestingly, the Fv/Fm, Ф_PSII value and chlorophyll content of the seedlings grown under green light were much higher than those grown under yellow light.     

Anthocyanin Accumulation Mediated by Blue Light and Cytokinin in Arabidopsis Seedlings

It has been reported that pigmentation In plants Is stimulated by light and cytoklnln （CTK）; however, the signaling pathways and the relationship between light and CTK Involved In the regulation of anthocyanln accumulation remain to be elucidated. We Investigated （i） the role of blue light （BL） and CTK In anthocyanln accumulation ; and （ii） the relationship between BL and CTK In wild type （WT） and by4 mutants of Arabidopsis thaiiana. Two-d-old seedlings grown on medium with or without klnetln （KT） or zeatln （ZT） In darkness were Irradiated using BL at different fluence rates for 3 d before the anthocyanln content was determined using a spectrophotometrlc method. Anthocyanln accumulation was strongly Induced by BL In WT seedlings but not In hy4 seedlings, which demonstrated that CRY1 Is the main photoreceptor for BL. Both KT and ZT enhanced the response of the WT seedlings to BL In a dose-dependent manner, whereas they were not sufficient to promote anthocyanln eccumulatlon In darkness. In addition, data from experiments using the hy4 mutant showed that the CTK effect of BL was also CRYl-dependent. The results from experiments with three different treatment programs showed that the relationship between BL and KT In anthocyanln accumulation of Arabidopsis seedlings seems neither muItlpllcatlve nor additive coactlon, but rather Interaction. BL Is necessary for anthocyanln accumulation, and KT might be Involved In the BL signaling pathway.     

Cloning,characterization and functional analysis of the role MhNCED3, a gene encoding 9-cis-epoxycarotenoid dioxygenase in Malus hupehensis Rehd., plays in plant tolerance to osmotic and Cd2+ stresses

Aims

Abscisic acid (ABA) plays an important role in the stress tolerance of seedlings and 9-cis-epoxycarotenoid dioxygenase (NCED) is considered to be the rate-limiting enzyme involved in ABA biosynthesis. However, the genes encoding NCED in M. hupehensis Rehd. have not been reported.

Methods

In this study, a gene encoding NCED, MhNCED3, was isolated from the roots of M. hupehensis Rehd. Its functions were investigated in M. hupehensis Rehd. seedlings and transgenic Arabidopsis lines under various abiotic stresses.

Results

The expression of MhNCED3 in M. hupehensis Rehd. roots was differentially induced by dehydration, chilling, salt and cadmium stresses and ABA biosynthesis was highly correlated with MhNCED3 expression. Ectopic expression of MhNCED3 successfully complemented the phenotypic defects of the 129B08/nced3 mutant. Furthermore, overexpression of MhNCED3, when it was transformed into the wild type (WT) seedling resulted in enhanced tolerance to osmotic and cadmium stresses compared to the normal WT seedling. The transgenic lines displayed higher rates of seed germination, improved growth and developmental status, reduced water loss/oxidative damage, lowered apoptosis rates and increased ABA accumulation. Furthermore, the higher antioxidant enzyme activities detected in the transgenic lines were probably responsible for the decrease in oxidative damage and apoptosis rates.

Conclusions

Overall, MhNCED3 played a significant role in enhancing plant tolerance to abiotic stresses through the regulation of endogenous ABA biosynthesis.     

Anthocyanin accumulation in the hypocotyl of an ABA-over producing male-sterile tomato (Lycopersicon esculentum) mutant

Anthocyanin accumulation is known to be regulated by light and plant hormones but its occurrence varies with plant species and/or organ and tissue, and it has been negatively correlated with male sterility. In this study, we have examined the light responsive changes in anthocyanin in an abscisic acid (ABA) over-producer, male-sterile 7B-1 mutant and wild-type (WT) tomato hypocotyls. The results show that light-induced anthocyanin accumulation in the hypocotyl was more in WT compared with the 7B-1 mutant and more so under white light (W) compared with blue light (B) or red light (R). In contrast, the chlorophyll content was greater in the mutant than in WT. Exogenous ABA caused a transitory increase in anthocyanin content in WT but a reduction in 7B-1 , both in W and B. The high level of anthocyanin in WT under light conditions was not correlated with increased mRNA levels of phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS) and dihydroflavonol 4-reductase (DFR), some of the anthocyanin biosynthetic genes. However, the activity of PAL (EC 4.3.1.5) was higher in the WT than in 7B-1 hypocotyls, and exogenous ABA caused an increase in PAL activity in the WT but a reduction in the mutant. The results presented show that high ABA negatively affects anthocyanin accumulation and that in the 7B-1 mutant it is related, in part, to reduced PAL activity. The results also support the view that the 7B-1 mutant has a defect in light perception and ABA sensitivity.     

The blue light receptor Phototropin 1 suppresses lateral root growth by controlling cell elongation

We investigated the relationship between the blue light receptor phototropin 1 (phot1) and lateral root growth in Arabidopsis thaliana seedlings. Fluorescence and confocal microscopy images, as well as PHOT1 mRNA expression studies provide evidence that it is highly expressed in the elongation zone of lateral roots where auxin is accumulating. However, treatment with the auxin transport inhibitor N‐1‐naphthylphthalamic acid significantly reduced PHOT1 expression in this zone. In addition, PHOT1 expression was higher in darkness than in light. The total number of lateral roots was higher in the phot1 mutant than in wild‐type Arabidopsis. Cells in the elongation zone of lateral roots of the phot1 mutant were longer than those of wild‐type lateral roots. These findings suggest that PHOT1 plays a role(s) in elongation of lateral roots through the control of an auxin‐related signalling pathway.     

Arabidopsis Casein Kinase1 Proteins CK1.3 and CK1.4 Phosphorylate Cryptochrome2 to Regulate Blue Light Signaling

Casein kinase1 (CK1) plays crucial roles in regulating growth and development via phosphorylating various substrates throughout the eukaryote kingdom. Blue light is crucial for normal growth of both plants and animals, and blue light receptor cryptochrome2 (CRY2) undergoes blue light–dependent phosphorylation and degradation in planta. To study the function of plant CK1s, systematic genetic analysis showed that deficiency of two paralogous Arabidopsis thaliana CK1s, CK1.3 and CK1.4, caused shortened hypocotyls, especially under blue light, while overexpression of either CK1.3 or CK1.4 resulted in the insensitive response to blue light and delayed flowering under long-day conditions. CK1.3 or CK1.4 act dependently on CRY2, and overexpression of CK1.3 or CK1.4 significantly suppresses the hypersensitive response to blue light by CRY2 overexpression. Biochemical studies showed that CK1.3 and CK1.4 directly phosphorylate CRY2 at Ser-587 and Thr-603 in vitro and negatively regulate CRY2 stability in planta, which are stimulated by blue light, further confirming the crucial roles of CK1.3 and CK1.4 in blue light responses through phosphorylating CRY2. Interestingly, expression of CK1.3 and CK1.4 is stimulated by blue light and feedback regulated by CRY2-mediated signaling. These results provide direct evidence for CRY2 phosphorylation and informative clues on the mechanisms of CRY2-mediated light responses.     

Analyzing photosynthetic activity and growth of Solanum lycopersicum seedlings exposed to different light qualities

To investigate how light quality influences tomato (Solanum lycopersicum L) seedlings, we examined changes in plant growth, chloroplast ultrastructure, photosynthetic parameters and some photosynthesis-related genes expression levels. For this, tomato plants were grown under different light qualities with the same photosynthetic photon flux density: red (R), blue (B), yellow (Y), green (G) and white (W) lights. Our results revealed that, compared with plants grown under W light, the growth of plants grown under monochromatic lights was inhibited with the growth reduction being more significant in the plants grown under Y and G lights. However, the monochromatic lights had their own effects on the growth and photosynthetic function of tomato seedlings. The plant height was reduced under blue light, but expression of rbcS, rbcL, psbA, psbB genes was up-regulated, and the ΦPSII and electron transport rate (ETR) values were enhanced. More starch grains were accumulated in chloroplasts. The root elongation, net photosynthetic rate (Pn), NPQ and rbcS and psbA genes expression were promoted under red light. Yellow light- and green light-illuminated plants grew badly with their lower Rubisco content and Pn value observed, and less starch grains accumulated in chloroplast. However, less influence was noted of light quality on chloroplast structure. Compared with yellow light, the values of ΦPSII, ETR, qP and NPQ of plants exposed to green light were significantly increased, suggesting that green light was beneficial to both the development of photosynthetic apparatus to some extent.     

The Effect of Epibrassinolide on Arabidopsis Seedling Morphogenesis and Hormonal Balance under Green Light

We studied the effect of 24-epibrassinolide (EB) on the levels of endogenous hormones and photomorphogenesis of Arabidopsis thaliana (L.) Heynh wild-type (Ler) and mutant (hy4) seedlings. This mutant is deficient in the cryptochrome 1 (CRY1) synthesis. CRY1, which is a product of the HY4 gene, is a blue light photoreceptor in wild-type plants, but is sensitive to green light as well. In dark-grown seven-day-old mutant seedlings, the ABA/zeatin ratio differed from this ratio in wild-type seedlings. Thehy4 mutant exhibited a lower zeatin and higher free-ABA contents, which could retard its hypocotyl growth in darkness. EB retarded the growth of hypocotyls in etiolated hy4 seedlings and enlarged their cotyledons more efficiently than in wild-type seedlings. Green light (GL) did not affect the growth of hypocotyls but enlarged cotyledons of hy4 seedlings, which might be associated with some increase in the level of free IAA and a considerable decrease in free ABA and also with a decrease in the cytokinin level in seedlings. The hy4 cotyledon response to GL depended evidently on photoreceptors other than CRY1. GL enhanced the effects of EB on the morphogenesis of both Ler and hy4 seedlings, which was coupled with changes in the balance of endogenous IAA, ABA, and cytokinins. We may suppose that EB is involved in the control of photomorphogenesis by interaction with endogenous hormones, which are involved in the transduction of a light signal absorbed by the GL photoreceptors.