Interactions Between Light and Plant Hormones During De-etiolation

The transition from a dark-grown (etiolated) to a light-grown (de-etiolated) morphology is marked by a number of dramatic phenotypic changes such as a significant reduction in the rate of shoot elongation, opening of the apical hook, expansion of true leaves and the development of mature chloroplasts. Many of these developmental processes are also known to be regulated by plant hormones. In this review we discuss the interactions between light and plant hormones and their role in mediating phenotypic change during de-etiolation. Clear evidence exists for a light-mediated reduction in gibberellin A, GA levels and response in pea, which is thought to be responsible, at least in part, for the reduction of shoot elongation during de-etiolation. Indirect evidence from a number of species has been used to suggest that the reduction in shoot elongation could also be mediated by a reduction in brassinosteroid (BR) levels. However, direct evidence recently obtained from pea and rice demonstrates that de-etiolation is not mediated, or even accompanied, by a reduction in BR levels. Ethylene is known to play an integral role in apical hook formation and maintenance in plants. However, the physiological significance of light-induced changes in IAA and ABA levels found in some species is not clear. Recent molecular data provide evidence of interactions between light-and IAA/CK-signalling pathways. Potential mechanisms for these interactions are discussed.     

Involvement of Plant Hormones and Plant Growth Regulators on in vitro Somatic Embryogenesis

In spite of the importance attained by somatic embryogenesis and of the many studies that have been conducted on this developmental process, there are still many aspects that are not fully understood. Among those features, the involvement of plant hormones and plant growth regulators on deTermining the conversion of somatic onto embryogenic tissues, and on allowing progression and maturation of somatic embryos, are far away from being completely comprehended. Part of these difficulties relies on the frequent appearance of contradictory results when studying the effect of a particular stimulus over a specific stage in somatic embryogenesis. Recent progress achieved on understanding the interaction between exogenously added plant growth regulators over the concentration of endogenous hormones, together with the involvement of sensitivity of the tissues to particular hormone groups, might help clarifying the occurrence of divergent patterns in somatic embryogenesis, and in tissue culture in general. The aspects described above, emphasizing on the effect of the concentration of plant hormones and of the addition of plant growth regulators during the different phases of somatic embryogenesis, will be reviewed in this paper. Citations will be limited to review articles as much as possible and to individual articles only in those cases in which very specific or recent information is presented.     

Endogenous levels of free and conjugated forms of auxin, cytokinins and abscisic acid during seed development in Douglas fir

Endogenous levels of free and conjugated forms of three classes of planthormones were quantified at various stages of megagametophyte development inDouglas fir. Megagametophytes were excised weekly from 8–16 weeks pastpollination (WPP), a period encompassing the central cell to the earlymaturation stage of seed development. The hormones indole-3 acetic acid (IAA),indole-3-aspartate (IAAsp), zeatin (Z), zeatin riboside (ZR), isopentenyladenine(iP), isopentenyladenosine (iPA), abscisic acid (ABA) and abscisic acid glucoseester (ABA-GE) were extracted, purified, fractionated by high- performanceliquid chromatography (HPLC), and then quantified using an enzyme-linkedimmunosorbent assay (ELISA) method. Z levels ranged from 0–25ng/g dry weight (DW) and were highest in megagametophytes at thecentral cell stage (8 WPP). During embryogenesis, Z levels peakedduring week 13. In contrast, the ZR conjugate was not detected over the periodstudied. The iP content of megagametophytes increased at 10 and 13WPP, while the iPA concentration increased at 13 WPP.Prior to fertilisation, IAA was highest in megagametophytes at 9WPP. During embryogenesis, the major IAA accumulations occurred at11, 13 and 15 WPP, the concentration ranging from 0–0.43g/g DW. IAAsp concentrations reached their highest level duringembryogenesis at 14 WPP. ABA content increased at 11 and 13WPP, with a concentration range of 0.1–13 g/gDW. In contrast, ABA-GE levels were relatively constant over the 9-weekperiod analyzed. The endogenous levels of plant hormones varied withmegagametophyte development and were associated with morphological changes.     

Mechanisms of hormonal regulation of the peripheral circadian clock in the colon

Colonic function is controlled by an endogenous clock that allows the colon to optimize its function on the daytime basis. For the first time, this study provided evidence that the clock is synchronized by rhythmic hormonal signals. In rat colon, adrenalectomy decreased and repeated applications of dexamethasone selectively rescued circadian rhythm in the expression of the clock gene Per1. Dexamethasone entrained the colonic clock in explants from mPer2^Luc mice in vitro. In contrast, pinealectomy had no effect on the rat colonic clock, and repeated melatonin injections were not able to rescue the clock in animals maintained in constant light. Additionally, melatonin did not entrain the clock in colonic explants from mPer2^Luc mice in vitro. However, melatonin affected rhythmic regulation of Nr1d1 gene expression in vivo. The findings provide novel insight into possible beneficial effects of glucocorticoids in the treatment of digestive tract-related diseases, greatly exceeding their anti-inflammatory action.     

Involvement of indole-3-acetic acid in the circadian growth of the first internode of Arabidopsis

The extension rate of the first inflorescence node of Arabidopsis was measured during light/dark or continuous light exposure and was found to exhibit oscillations which showed a circadian rhythmicity. Decapitation induced a strong inhibition of stem extension. Subsequent application of IAA restored growth and the associated extension–rate oscillations. In addition, IAA treatments, after decapitation, re-established the circadian rhythmicity visible in the intact plants during free run. This indicates that the upper zone of the inflorescence has a major influence on the extension rate of floral stems and implies a role for auxin. Application of N-(1-naphthyl)phthalamic acid, an IAA transport inhibitor, to an intact floral stem inhibited growth and the rhythmicity in the extension rate oscillations, indicating that IAA polar transport may play a role in the dynamics of stem elongation. Furthermore, IAA-aspartate application, after decapitation, did not restore growth and rhythmicity. Nevertheless, biochemical analysis of IAA and IAA-aspartate demonstrated circadian fluctuations of the endogenous levels of both compounds. These observations suggest that IAA metabolism is an essential factor in the regulation of the circadian growth rhythm of Arabidopsis floral stems. Received: 21 September 1998 / Accepted: 23 January 1999     

Plant hormone homeostasis and the control of avocado fruit size

Control of plant hormone homeostasis is crucial for normal organdevelopment in plants. To elucidate the contribution of plant hormonehomeostasis to fruit growth, tissue distribution and activity of xanthinedehydrogenase (XDH), abscisic aldehyde (AB-ald)- and indole acetaldehyde(IA-ald) oxidase, and cytokinin oxidase (CKOX) were determined in seed, seedcoat and mesocarp of normal 'Hass avocado and its small-fruitphenotype during the linear phase of growth. Activity of these enzymes wasrelated to the tissue content of indole-3-acetic acid (IAA) and abscisic acid(ABA). IA-ald oxidase was present only in seed tissue whereas AB-ald oxidase andXDH activity was found in seed and mesocarp tissue. Seed of the small'Hass fruit had increased XDH and AB-ald oxidase activity and highendogenous ABA, but reduced IA-ald oxidase activity and adenine. There was nodifference in seed, seed coat and mesocarp CKOX activity between normal andsmall fruit. Inhibition of XDH activity in whole fruit by treatment withallopurinol decreased IAA and increased ABA of seed tissue. In mesocarp ofripening fruit allopurinol increased ABA and IAA but had no effect on levels ofiP. Results indicate that activity of IA-ald and AB-ald oxidases in avocadofruit contribute to maintenance of the IAA/ABA ratio in seed and mesocarp tissueand that increased AB-ald oxidase, or reduced IA-ald oxidase, may be part of thesyndrome associated with the appearance of a small-fruit phenotype.     

The effect of plant growth regulator treatments on the levels of ethylene emanating from excised turgid and wilted wheat leaves

Abscisic acid (ABA) inhibits the production of ethylene induced by water stress in excised wheat leaves and counteracts the stimulatory effect of 6-benzyladenine (BA) on this process. The stimulatory effect of BA and the inhibitory effect of ABA were equally pronounced whether external or endogenous ethylene levels were determined. When leaves were sprayed or floated on solutions of BA, indole-3-acetic acid (IAA), gibberellic acid (GA₃), or ABA, the relative activities of these growth regulators on stress-induced ethylene at 10^-4 mol l^-1 were BA>IAA >GA₃>controls>ABA. In non-stressed leaves, however, where the levels of ethylene produced were 2–20 times smaller, the relative activities were IAA >BA>GA₃>controls>ABA. The effects of BA and ABA spray treatment on water stress induced ethylene were closely similar whether the solutions were applied 2 or 18 h prior to the initiation of water stress. The relationships between the levels of endogenous growth regulators in the plant and ethylene release induced by water stress are discussed.Abbreviations BA 6-benzyladenine - IAA indole-3-acetic acid - GA₃ gibberellic acid - ABA abscisic acid - GLC gas-liquid chromatography - _leaf leaf water potential     

Light quality regulation of endogenous levels of auxin,abscisic acid and ethylene production in petioles and leaves of wild type and ACC deaminase transgenic <Emphasis Type="Italic">Brassica napus</Emphasis> seedlings

Brassica napus L. seedlings responded to low red to far-red (R/FR) ratio by elongating petioles and decreasing leaf expansion. These typical shade avoidance traits were correlated with significantly decreased endogenous indole-3-acetic acid (IAA) levels and significantly increased endogenous abscisic acid (ABA) levels and ethylene production. The transgenic (T) B. napus line bearing the bacterial ACC deaminase gene, did not respond to low R/FR ratio with altered petiole and leaf growth and less ethylene (especially by petioles) was produced. As with WT seedlings, T seedlings had significantly lower IAA levels in both petioles and leaves under low R/FR ratio. However, ABA levels of low R/FR ratio-grown T seedlings either increased (petioles) or were unaltered (leaves). Our results further suggest that low R/FR ratio regulates endogenous IAA levels independently of ethylene, but there may be an interaction between ABA and ethylene in leaf development.     

The effect of plant-hormone pretreatments on ethylene production and synthesis of 1-aminocyclopropane-1-carboxylic acid in water-stressed wheat leaves

Excised wheat (Triticum aestivum L.) leaves, when subjected to drought stress, increased ethylene production as a result of an increased synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC) and an increased activity of the ethyleneforming enzyme (EFE), which catalyzes the conversion of ACC to ethylene. The rise in EFE activity was maximal within 2 h after the stress period, while rehydration to relieve water stress reduced EFE activity within 3 h to levels similar to those in nonstressed tissue. Pretreatment of the leaves with benzyladenine or indole-3-acetic acid prior to water stress caused further increase in ethylene production and in endogenous ACC level. Conversely, pretreatment of wheat leaves with abscisic acid reduced ethylene production to levels produced by nonstressed leaves; this reduction in ethylene production was accompanied by a decrease in ACC content. However, none of these hormone pretreatments significantly affected the EFE level in stressed or nonstressed leaves. These data indicate that the plant hormones participate in regulation of water-stress ethylene production primarily by modulating the level of ACC.Abbreviations ABA abscisic acid - ACC 1-aminocyclopropane-1-carboxylic acid - BA N⁶-benzyladenine - EFE ethylene-forming enzyme - IAA indole-3-acetic acid     

Hormonal Interactions and Stomatal Responses

Both environmental and hormonal factors and their interactions affect stomatal behavior. Methodologies for identifying hormonal interactions affecting stomatal function are reviewed. Although there is abundant evidence that abscisic acid (ABA) closes stomata, evidence that the other classical plant hormones (auxins, cytokinins, ethylene, gibberellins) in isolation alter stomatal response often comes from exogenous applications to detached epidermes and leaves, rather than correlation of endogenous concentrations with stomatal conductance (g_s). Evidence for hormonal interactions comes from isolated tissues with exogenous hormones supplied at nonphysiological concentrations, or from variation in stomatal response to xylem ABA concentration in planta. The roles of hormonal changes in causing stomatal closure following changes in soil environment are considered. Although soil drying induces multiple changes in xylem sap composition, analysis of stomatal responses suggests a dominant role for increased endogenous ABA concentrations and relatively little evidence of roles for other hormones. A similar picture emerges from studies of soil compaction. Although soil flooding decreases ABA export from the root system, there is some evidence that apoplastic ABA accumulation elicits stomatal closure. Stomatal closure following nitrogen deprivation does not appear to involve ABA and may provide a suitable experimental system to investigate roles for other hormones. The availability of mutant or transgenic lines with altered hormone homeostasis or sensitivity provides opportunities to screen for altered stomatal behavior in response to different environments, and may provide new evidence that hormonal interactions are important in the control of stomatal behavior.     

The influence of growth regulators absorbed by the root on sex expression in hemp plants

Application, through the root system, of growth regulators to hemp (Cannabis sativa L.) plants having 2–3 pairs of visible leaves caused pronounced shifts of sex expression in the adult individuals. Treatment with gibberellic acid (25 mg/l) resulted in more than 80% of the plants being male, i.e. having staminate flowers (controls, ca. 30%). Treatment with 6-benzylaminopurine and with indole-3-acetic acid (in either case, 15 mg/l) resulted in all plants being either female (pistillate flowers) or intersexes (bisexual flowers); treatment with abscisic acid (10 mg/l) had a similar but somewhat less pronounced effect.     

大豆花荚败育期间的植物激素变化

大豆开花结荚期,不同发育阶段的幼蕾与花荚的脱落率不同,其中以花后５ｄ内的幼荚脱落最严重。与败育花荚相比,正常花荚中的干物质积累量均较高。细胞分裂素（ＤＨＺＲｓ,ＺＲｓ,ｉＰＡ）含量也较高,花后３～５ｄ的幼荚中表现更明显。脱落酸（ＡＢＡ）则是以败育幼蕾及花后３～５ｄ的幼荚中含量较高。不同发育阶段的大豆生殖器官中,正常开放花中的玉米赤霉烯酮（ＺＥＮ）含量最高     

植物激素ABA调控植物根系生长的研究进展

脱落酸（abscisic acid,ABA）是一种重要的植物激素,在调控种子发育、种子休眠与萌发、抑制生长、促进落花落果、参与植物应对外界环境胁迫等过程中发挥着重要的生理功能。ABA还能与其他植物激素（如生长素、乙烯等）互作进而精细调控植物根系的生长。本文以模式植物拟南芥（Arabidopsis thaliana（L.）Heynh）为主要对象,对近年来国内外在ABA调控植物根系生长方面的研究成果、ABA与其他植物激素（如GA等）互作调控根系生长及调控非生物逆境下根系发育的机理等进行综述,并对其未来的研究方向进行了展望。     

Interactions between abscisic acid, ethylene and gibberellin in internodal elongation in floating rice: the promotive effect of abscisic acid at low humidity

The enhancement of internodal elongation in floating or deepwater rice (Oryza sativa L. cv. Habiganj Aman II) by treatment with ethylene or gibberellic acid (GA₃) at high relative humidity (RH) is inhibited by abscisic acid (ABA). Here, we examined the interactive effects of ethylene, gibberellin (GA) and ABA at low RH on internodal elongation of deepwater rice stem segments. Although ethylene alone hardly promoted internodal elongation of stem sections at 30% RH, it enhanced the internodal elongation induced by GA₃. Application of ABA alone to stem segments had no effect on internodal elongation. However, in the presence of ethylene and GA₃ at 30% RH, ABA further promoted internodal elongation. This promotive effect of ABA was not found in the internodes of stem segments treated either with ethylene or with GA₃ at 30% RH or in the internodes of stem segments treated with ethylene and/or GA₃ at 100% RH.     

Cytokinin-induced hypocotyl elongation in light-grown<Emphasis Type="Italic"> Arabidopsis</Emphasis> plants with inhibited ethylene action or indole-3-acetic acid transport

Cytokinins inhibit hypocotyl elongation in darkness but have no obvious effect on hypocotyl length in the light. However, we found that cytokinins do promote hypocotyl elongation in the light when ethylene action is blocked. A 50% increase in Arabidopsis thaliana (L.) Heynh. hypocotyl length was observed in response to N⁶-benzyladenine (BA) treatment in the presence of Ag⁺. The level of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid was strongly increased, indicating that ethylene biosynthesis was up-regulated by treatment with cytokinin. Furthermore, the effects of cytokinins on hypocotyl elongation were also tested using a series of mutants in the cascade of the ethylene-signal pathway. In the ethylene-insensitive mutants etr1-3 and ein2-1, cytokinin treatment resulted in hypocotyl lengths comparable to those of wild-type seedlings treated with both Ag⁺ and BA. A similar phenotypical response to cytokinin was observed when auxin transport was blocked by -naphthylphthalamic acid (NPA). Applied cytokinin largely restored cell elongation in the basal and middle parts of the hypocotyls of NPA-treated seedlings and at the same time abolished the NPA-induced decrease in indole-3-acetic acid levels. Our data support the hypothesis that, in the light, cytokinins interact with the ethylene-signalling pathway and conditionally up-regulate ethylene and auxin synthesis.     

Molecular interactions between light and hormone signaling to control plant growth

As sessile organisms, plants modulate their growth rate and development according to the continuous variation in the conditions of their surrounding environment, an ability referred to as plasticity. This ability relies on a web of interactions between signaling pathways triggered by endogenous and environmental cues. How changes in environmental factors are interpreted by the plant in terms of developmental or growth cues or, in other words, how they contribute to plant plasticity is a current, major question in plant biology. Light stands out among the environmental factors that shape plant development. Plants have evolved systems that allow them to monitor both quantitative and qualitative differences in the light that they perceive, that render important changes in their growth habit. In this review we focus on recent findings about how information from this environmental cue is integrated during de-etiolation and in the shade-avoidance syndrome, and modulated by several hormone pathways—the endogenous cues. In some cases the interaction between a hormone and the light signaling pathways is reciprocal, as is the case of the gibberellin pathway, whereas in other cases hormone pathways act downstream of the environmental cue to regulate growth. Moreover, the circadian clock adds an additional layer of regulation, which has been proposed to integrate the information provided by light with that provided by hormone pathways, to regulate daily growth.     

Hormone interactions at the root apical meristem

Plants exhibit an amazing developmental flexibility. Plant embryogenesis results in the establishment of a simple apical-basal axis represented by apical shoot and basal root meristems. Later, during postembryonic growth, shaping of the plant body continues by the formation and activation of numerous adjacent meristems that give rise to lateral shoot branches, leaves, flowers, or lateral roots. This developmental plasticity reflects an important feature of the plant's life strategy based on the rapid reaction to different environmental stimuli, such as temperature fluctuations, availability of nutrients, light or water and response resulting in modulation of developmental programs. Plant hormones are important endogenous factors for the integration of these environmental inputs and regulation of plant development. After a period of studies focused primarily on single hormonal pathways that enabled us to understand the hormone perception and signal transduction mechanisms, it became obvious that the developmental output mediated by a single hormonal pathway is largely modified through a whole network of interactions with other hormonal pathways. In this review, we will summarize recent knowledge on hormonal networks that regulate the development and growth of root with focus on the hormonal interactions that shape the root apical meristem.     

Interactions between ethylene, gibberellin and abscisic acid regulate emergence and growth rate of adventitious roots in deepwater rice

Growth of adventitious roots is induced in deepwater rice (Oryza sativa L.) when plants become submerged. Ethylene which accumulates in flooded plant parts is responsible for root growth induction. Gibberellin (GA) is ineffective on its own but acts in a synergistic manner together with ethylene to promote the number of penetrating roots and the growth rate of emerged roots. Studies with the GA biosynthesis inhibitor paclobutrazol revealed that root emergence was dependent on GA activity. Abscisic acid (ABA) acted as a competitive inhibitor of GA activity. Root growth rate on the other hand was dependent on GA concentration and ABA acted as a potent inhibitor possibly of GA but also of ethylene signaling. The results indicated that root emergence and elongation are distinct phases of adventitious root growth that are regulated through different networking between ethylene, GA and ABA signaling pathways. Adventitious root emergence must be coordinated with programmed death of epidermal cells which cover root primordia. Epidermal cell death is also controlled by ethylene, GA and ABA albeit with cell-type specific cross-talk. Different interactions between the same hormones may be a means to ensure proper timing of cell death and root emergence and to adjust the growth rate of emerged adventitious roots.