Promoted Elongation Growth Research Articles

An exotic allele of barley EARLY FLOWERING 3 contributes to developmental plasticity at elevated temperatures.

Increase in ambient temperatures caused by climate change affects various morphological and developmental traits of plants, threatening crop yield stability. In the model plant Arabidopsis thaliana, EARLY FLOWERING 3 (ELF3) plays prominent roles in temperature sensing and thermomorphogenesis signal transduction. However, how crop species respond to elevated temperatures is poorly understood. Here, we show that the barley ortholog of AtELF3 interacts with high temperature to control growth and development. We used heterogeneous inbred family (HIF) pairs generated from a segregating mapping population and systematically studied the role of exotic ELF3 variants in barley temperature responses. An exotic ELF3 allele of Syrian origin promoted elongation growth in barley at elevated temperatures, whereas plant area and estimated biomass were drastically reduced, resulting in an open canopy architecture. The same allele accelerated inflorescence development at high temperature, which correlated with early transcriptional induction of MADS-box floral identity genes BM3 and BM8. Consequently, barley plants carrying the exotic ELF3 allele displayed stable total grain number at elevated temperatures. Our findings therefore demonstrate that exotic ELF3 variants can contribute to phenotypic and developmental acclimation to elevated temperatures, providing a stimulus for breeding of climate-resilient crops.

Overexpression of a small GTP‐binding protein Ran1 in Arabidopsis leads to promoted elongation growth and enhanced disease resistance against P. syringae DC3000

Plants resist infection through an innate immune response, which is usually associated with slowing of growth. The molecular mechanisms underlying the trade-off between plant growth and defense remain unclear. The present study reveals that growth/defense trade-offs mediated by gibberellin (GA) and salicylic acid (SA) signaling pathways are uncoupled during constitutive overexpression of transgenic AtRAN1 and AtRAN1Q72L (active, GTP-locked form) Arabidopsis plants. It is well known that the small GTP-binding protein Ran (a Ras-related nuclear protein) functions in the nucleus-cytoplasmic transport of proteins. Although there is considerable evidence indicating that nuclear-cytoplasmic partitioning of specific proteins can participate in hormone signaling, the role of Ran-dependent nuclear transport in hormone signaling is not yet fully understood. In this report, we used a combination of genetic and molecular methods to reveal whether AtRAN1 is involved in both GA and SA signaling pathways. Constitutively overexpressed AtRAN1 promoted both elongation growth and the disease resistance response, whereas overexpression of AtRAN1Q72L in the atran2atran3 double mutant background clearly inhibited elongation growth and the defense response. Furthermore, we found that AtRAN1 coordinated plant growth and defense by promoting the stability of the DELLA protein RGA in the nucleus and by modulating NPR1 nuclear localization. Interestingly, genetically modified rice (Oryza sativa) overexpressing AtRAN1 exhibited increased plant height and yield per plant. Altogether, the ability to achieve growth/defense trade-offs through AtRAN1 overexpression provides an approach to maximizing crop yield to meet rising global food demands.

Growth and morphology responses to narrow-band blue light and its co-action with low-level UVB or green light: A comparison with red light in four microgreen species

To explore the action mode of blue light on plant elongation growth, the growth and morphology traits of arugula (Brassica eruca L., ‘Rocket’), cabbage (Brassica oleracea, unknown variety name), mustard (Brassica juncea, ‘Ruby Streaks’), and kale (Brassica napus, ‘Red Russian’) microgreens were compared under four light quality treatments: (1) R, narrow-band red light (660 nm); (2) B, narrow-band blue light (455 nm); (3) BU, broad-band blue light created by mixing B with a low level (≈ 0.7 μmol m−2 s−1) of UVB (310 nm); and (4) BG, broad-band blue light created by mixing B with a low level (≈ 6 %) of green light (530 nm). Continuous (24 -h) light-emitting diode lighting with 100 μmol m−2 s−1 photosynthetic photon flux density was used in the above treatments at ≈ 22 °C. After 7–8 days of light treatment, B, compared to R, had promoted elongation growth (i.e., longer hypocotyls or petioles, faster stem extension), and induced some other typical shade-avoidance responses (e.g., smaller cotyledons, lighter plant color, or greater biomass allocation to stem), which was associated with longer cells in the hypocotyls and lower cell density in the cotyledons except for mustard. Compared with B, the elongation growth was inhibited under BU for arugula and kale, but was promoted under BG for all the species except arugula. If considering all the plant traits together, BU and BG effect was similar to B, but different from R. Therefore, narrow-band blue vs. red light can promote elongation growth as a shade-avoidance response at both plant and cell levels, and adding low-level UVB or green light can only slightly change the narrow-band-blue-light-promoted shade-avoidance response, despite varying sensitivity among species.

Phototropin is partly involved in blue-light-mediated stem elongation, flower initiation, and leaf expansion: A comparison of phenotypic responses between wild Arabidopsis and its phototropin mutants

Our previous study on bedding plants indicated that blue light in association with low phytochrome activity, relative to red light, can promote elongation growth as one of the shade-avoidance responses. We hypothesized that phototropin is involved in the blue light promotion effects. To test the hypothesis, plant growth and morphology were examined in wild Arabidopsis (Col-0), and its three phototropin-deficient mutants (phot1, phot2, and phot1phot2) under four light quality treatments: (1) R, pure red light (660 nm); (2) B, pure blue light (455 nm); (3) BR, unpure blue light created by mixing B with low-level (6 %) R; and (4) BRF, unpure blue light created by adding low-level far-red light (735 nm) to BR with red/far-red ≈ 1. Continuous (24-h) light-emitting diode lighting with 100 μmol m−2·s−1 photosynthetic photon flux density, and an air temperature of ≈ 23 ℃ were used with the above treatments. The calculated phytochrome photoequilibrium, an indicator of phytochrome activity, was 0.89, 0.69, 0.50, and 0.60 for R, BR, B, and BRF, respectively. After 20 days of light treatments, for the wild-type plants, B or BRF, compared to R or BR, promoted stem elongation, plant flowering, and leaf expansion, showing typical shade-avoidance responses. However, for the phototropin mutants, the promotion effects of B or BRF, relative to R or BR, on the above plant traits were reduced. In the absence of the phototropin(s), plants had reduced stem elongation and delayed flower initiation under B or BRF rather than R or BR, or reduced leaf expansion to a larger degree under B or BRF than R or BR. Our results suggest that phototropin is partly involved in the promotion of stem elongation, plant flowering, and leaf expansion mediated by blue light in association with low phytochrome activity.

Blue versus Red Light Can Promote Elongation Growth Independent of Photoperiod: A Study in Four Brassica Microgreens Species

An elongated stem has beneficial effects on microgreen production. Previous studies indicate that under 24-hour light-emitting diode (LED) lighting, monochromatic blue light, compared with red light, can promote plant elongation for some species. The objective of this study was to investigate whether shortened photoperiod can change blue vs. red light effects on elongation growth. The growth and morphology traits of arugula (Brassica eruca, ‘Rocket’), cabbage (Brassica oleracea, unknown variety name), mustard (Brassica juncea, ‘Ruby Streaks’), and kale (Brassica napus, ‘Red Russian’) seedlings were compared during the stage from seeding to cotyledon unfolding under two light quality × two photoperiod treatments: 1) R, monochromatic red light (665 nm) and 2) B, monochromatic blue light (440 nm) using continuous (24-hour light/0-hour dark) or periodic (16-hour light/8-hour dark) LED lighting. A photosynthetic photon flux density of ≈100 μmol·m−2·s−1 and an air temperature of ≈22 °C was used for the preceding treatments. After 7 to 8 days of lighting treatment, regardless of photoperiod, B promoted elongation growth compared with R, as demonstrated by a greater stem extension rate, hypocotyl length, or petiole length in the tested microgreen species, except for mustard. The promotion effects on elongation were greater under 24- vs. 16-hour lighting in many cases. Among the tested species, mustard showed the lowest sensitivity in elongation response to B vs. R, which was independent of photoperiod. This suggests that the blue-light-promoted elongation is not specifically from 24-hour lighting, despite the varying promotion degree under different photoperiods or for different species. The elongation growth promoted by blue LED light under a photoperiod of either 24 hours or 16 hours can potentially benefit indoor production of microgreens.

Elongation growth mediated by blue light varies with light intensities and plant species: A comparison with red light in arugula and mustard seedlings

Previous studies have indicated that at a photosynthetic photon flux density (PPFD) of 50 or 100 μmol m−2·s−1, pure blue light, compared to red light, can promote plant elongation of de-etiolated seedlings for arugula, but not for mustard. The objective of this study was to investigate whether the promotion effect of blue light can also occur under wider range of PPFD levels. The growth and morphology traits of arugula (Brassica eruca, ‘Rocket’) and mustard (Brassica juncea, ‘Ruby Streaks’) seedlings were compared during the stage from seeding to cotyledon unfolding under two light quality × six light intensity treatments: (1) R, “pure” red light (665 nm); and (2) B, “pure” blue light (440 nm) using light-emitting diode lighting at PPFD levels of 20, 70, 120, 250, 450 and 650 μmol m−2·s−1. Continuous (24-h light/0-h dark) lighting and an air temperature of approximately 22 °C was used for the above treatments. After 7 to 8 days of lighting treatment, regardless of species, B promoted hypocotyl and petiole elongation compared to R at certain PPFD levels, associated with some shade-avoidance responses such as reduced biomass partitioning to cotyledons, cotyledon size, and plant coloring, which varied between species. The blue light promotion effect on hypocotyl elongation was detected at all the tested PPFD levels for arugula, but at a narrower range of PPFD levels (i.e., 250–650 μmol m−2·s−1) for mustard. Also, for arugula, the hypocotyl promotion by blue light decreased with increased PPFD levels. Generally, the promotion effects of blue vs. red light were more obvious for arugula than mustard. This suggests that blue vs. red light can promote elongation growth of de-etiolated seedlings for arugula and mustard, but the promotion effects vary with light intensities and plant species.

Pure blue light effects on growth and morphology are slightly changed by adding low-level UVA or far-red light: A comparison with red light in four microgreen species

To explore the action mode of blue light on elongation growth of microgreens, the growth and morphology traits of arugula (Brassica eruca L., ‘Rocket’), cabbage (Brassica oleracea, unknown variety name), mustard (Brassica juncea, ‘Ruby Streaks’), and kale (Brassica napus, ‘Red Russian’) seedlings were compared under four light quality treatments: (1) R, “pure” red light (660 nm); (2) B, “pure” blue light (450 nm); (3) BU, “unpure” blue light created by mixing B with a low level of UVA (≈7.5% photosynthetic photon flux); and (4) BF, “unpure” blue light created by mixing B with a low level of far-red light (≈ 10% photosynthetic photon flux). Continuous (24-hour) light-emitting diode lighting with either 100 or 50 μmol m−2·s−1 photosynthetic photon flux density at ≈ 23 ℃ was used with the above treatments. After 11 to 14 days of light treatment, B vs. R promoted elongation growth as demonstrated by a greater stem extension rate, hypocotyl length, or petiole length in the tested microgreen species, except for mustard. The promotion effects on elongation were stronger under lower vs. higher light for cabbage and kale, and cotyledon size was also reduced by B vs. R light in most cases, which suggested that stem elongation under pure blue light appeared to be one of the shade-avoidance responses in de-etiolated seedlings. BU vs. B inhibited elongation growth for some species at 100 μmol m−2·s−1 in most cases, but BU vs. R did not, except for mustard at 50 μmol m−2·s−1. BF vs. B increased stem extension rate, and hypocotyl length for arugula and mustard, and petiole length for arugula, but not in other cases. If considering all plant traits together using principle component analysis, BU and BF effects were similar to B, but different from R. This suggests that after adding UVA or far-red light, the pure blue light effects on growth and morphology are slightly changed, with varying sensitivity among species.

Blue light associated with low phytochrome activity can promote elongation growth as shade-avoidance response: A comparison with red light in four bedding plant species

To explore the action mode of blue light on elongation growth of bedding plants, the plant growth and morphology traits of petunia (Petunia × hybrida, ‘Duvet Red’), calibrachoa (Calibrachoa × hybrida, ‘Kabloom Deep Blue’), geranium (Pelargonium × hortorum, ‘Pinto Premium Salmon’), and marigold (Tagetes erecta, ‘Antigua Orange’) were compared under four light quality treatments: (1) R, “pure” red light (660 nm); (2) B, “pure” blue light (450 nm); (3) BR, “unpure” blue light created by mixing B with a low level of R to provide B/R ≈ 9; (4) BRF, “unpure” blue light created by adding a low level of far red light to BR with red/far red ≈ 1. Continuous (24-h) light-emitting diode lighting with either 100 or 50 μmol m−2 s−1 photosynthetic photon flux density at ≈ 23℃ was used with the above treatments. After 14–20 day of lighting treatment, B promoted elongation growth compared to R, as demonstrated by a greater canopy height, main stem length, internode length, and daily main stem extension rate. However, BR showed similar or inhibitory effects on these traits relative to R, while BRF exhibited similar promotion effects as B. The calculated phytochrome photoequilibrium, an indication of phytochrome activity, was higher for R (0.89) and BR (0.74) than for B (0.49) and BRF (0.63). Adding red (or far red) light reversed the effects of B (or BR) on elongation growth and the phytochrome photoequilibrium, suggesting that blue light promotion of elongation growth is related to the lower phytochrome activity. Also, B and BRF, when compared to R or BR, promoted elongation growth to a greater degree at 50 than 100 μmol m−2 s−1 for petunia and calibrachoa. In addition to the promoted elongation growth, B and BRF reduced side branch number, biomass allocation to side branches, leaf epinasty, leaf angle, and/or leaf chlorophyll content relative to R or BR, but increased individual leaf area, petiole length, and/or biomass allocation to main stem, which varied with different species. It suggests that the promoted elongation growth by blue light associated with lower phytochrome activity is one of shade-avoidance responses with varying sensitivity among species.

Constitutive Expression of Arabidopsis SMALL AUXIN UP RNA19 (SAUR19) in Tomato Confers Auxin-Independent Hypocotyl Elongation.

The plant hormone indole-3-acetic acid (IAA or auxin) mediates the elongation growth of shoot tissues by promoting cell expansion. According to the acid growth theory proposed in the 1970s, auxin activates plasma membrane H+-ATPases (PM H+-ATPases) to facilitate cell expansion by both loosening the cell wall through acidification and promoting solute uptake. Mechanistically, however, this process is poorly understood. Recent findings in Arabidopsis (Arabidopsis thaliana) have demonstrated that auxin-induced SMALL AUXIN UP RNA (SAUR) genes promote elongation growth and play a key role in PM H+-ATPase activation by inhibiting PP2C.D family protein phosphatases. Here, we extend these findings by demonstrating that SAUR proteins also inhibit tomato PP2C.D family phosphatases and that AtSAUR19 overexpression in tomato (Solanum lycopersicum) confers the same suite of phenotypes as previously reported for Arabidopsis. Furthermore, we employ a custom image-based method for measuring hypocotyl segment elongation with high resolution and a method for measuring cell wall mechanical properties, to add mechanistic details to the emerging description of auxin-mediated cell expansion. We find that constitutive expression of GFP-AtSAUR19 bypasses the normal requirement of auxin for elongation growth by increasing the mechanical extensibility of excised hypocotyl segments. In contrast, hypocotyl segments overexpressing a PP2C.D phosphatase are specifically impaired in auxin-mediated elongation. The time courses of auxin-induced SAUR expression and auxin-dependent elongation growth were closely correlated. These findings indicate that induction of SAUR expression is sufficient to elicit auxin-mediated expansion growth by activating PM H+-ATPases to facilitate apoplast acidification and mechanical wall loosening.

Modified light spectral conditions prior to cryopreservation alter growth characteristics and cryopreservation success of potato (Solanum tuberosum L.) shoot tips in vitro

Light is one of the most important factors affecting growth and morphogenesis of plants. Light intensity, photoperiod and spectral composition greatly affect morphogenetic responses of in vitro plants. Modification of light spectra during recovery after cryopreservation improves survival and regeneration, but the effect of modified light conditions prior to cryopreservation are not known. Therefore, the aim of the present study was to follow the photomorphogenetic response of potato plants (Solanum tuberosum L.) under different light qualities i.e. cool white fluorescent (CW) used as control, warm white (HQI), white LEDs (W), blue LEDs (B), red LEDs (R) and a combination of red with 10 % of blue LEDs (RB) prior to cryopreservation, affecting recovery of cultivars Agrie Dzeltenie, Bintje, Maret, Anti and Desiree in vitro. Light spectral quality had a significant effect on growth characteristics of potato plants in vitro. Red light (R) promoted elongation growth but biomass accumulation remained low under monochromatic light treatments. Some of the pre-cryopreservation light treatments significantly affected post-cryopreservation success. Under blue LEDs, high early recovery was observed for all cultivars tested, whereas under red (R) or (HQI), lowest survival percentages were obtained 2–4 weeks after thawing. Specifically, during early recovery, blue light increased survival from 26 to 66 %, 4 to 31 % and 16 to 48 % for cultivars Agrie Dzeltenie, Anti, and Desiree, compared to illumination by red LEDs. Therefore, light spectral quality prior to cryopreservation can significantly affect the cryopreservation success of potato shoot tips.

Phytochrome interacting factors 4 and 5 control seedling growth in changing light conditions by directly controlling auxin signaling

Plant growth is strongly influenced by the presence of neighbors that compete for light resources. In response to vegetational shading shade-intolerant plants such as Arabidopsis display a suite of developmental responses known as the shade-avoidance syndrome (SAS). The phytochrome B (phyB) photoreceptor is the major light sensor to mediate this adaptive response. Control of the SAS occurs in part with phyB, which controls protein abundance of phytochrome-interacting factors 4 and 5 (PIF4 and PIF5) directly. The shade-avoidance response also requires rapid biosynthesis of auxin and its transport to promote elongation growth. The identification of genome-wide PIF5-binding sites during shade avoidance revealed that this bHLH transcription factor regulates the expression of a subset of previously identified SAS genes. Moreover our study suggests that PIF4 and PIF5 regulate elongation growth by controlling directly the expression of genes that code for auxin biosynthesis and auxin signaling components.

PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) regulates auxin biosynthesis at high temperature

At high ambient temperature, plants display dramatic stem elongation in an adaptive response to heat. This response is mediated by elevated levels of the phytohormone auxin and requires auxin biosynthesis, signaling, and transport pathways. The mechanisms by which higher temperature results in greater auxin accumulation are unknown, however. A basic helix-loop-helix transcription factor, PHYTOCHROME-INTERACTING FACTOR 4 (PIF4), is also required for hypocotyl elongation in response to high temperature. PIF4 also acts redundantly with its homolog, PIF5, to regulate diurnal growth rhythms and elongation responses to the threat of vegetative shade. PIF4 activity is reportedly limited in part by binding to both the basic helix-loop-helix protein LONG HYPOCOTYL IN FAR RED 1 and the DELLA family of growth-repressing proteins. Despite the importance of PIF4 in integrating multiple environmental signals, the mechanisms by which PIF4 controls growth are unknown. Here we demonstrate that PIF4 regulates levels of auxin and the expression of key auxin biosynthesis genes at high temperature. We also identify a family of SMALL AUXIN UP RNA (SAUR) genes that are expressed at high temperature in a PIF4-dependent manner and promote elongation growth. Taken together, our results demonstrate direct molecular links among PIF4, auxin, and elongation growth at high temperature.

Inhibition of the shade avoidance response by formation of non-DNA binding bHLH heterodimers

In shade-intolerant plants such as Arabidopsis, a reduction in the red/far-red (R/FR) ratio, indicative of competition from other plants, triggers a suite of responses known as the shade avoidance syndrome (SAS). The phytochrome photoreceptors measure the R/FR ratio and control the SAS. The phytochrome-interacting factors 4 and 5 (PIF4 and PIF5) are stabilized in the shade and are required for a full SAS, whereas the related bHLH factor HFR1 (long hypocotyl in FR light) is transcriptionally induced by shade and inhibits this response. Here we show that HFR1 interacts with PIF4 and PIF5 and limits their capacity to induce the expression of shade marker genes and to promote elongation growth. HFR1 directly inhibits these PIFs by forming non-DNA-binding heterodimers with PIF4 and PIF5. Our data indicate that PIF4 and PIF5 promote SAS by directly binding to G-boxes present in the promoter of shade marker genes, but their action is limited later in the shade when HFR1 accumulates and forms non-DNA-binding heterodimers. This negative feedback loop is important to limit the response of plants to shade.

Flower stalk segments of Arabidopsis thaliana ecotype Columbia lack the capacity to grow in response to exogenously applied auxin.

Exogenously applied IAA stimulated cell elongation of segments excised from flower stalks of Arabidopsis thaliana ecotype Landsberg erecta (Ler) by increasing the cell wall extensibility, but it did not affect that of ecotype Columbia (Col). Treatment with a low pH buffer solution (pH 4.0) or fusicoccin (FC), a reagent activating H(+)-ATPases, significantly increased the cell wall extensibility and promoted elongation growth of flower stalk segments of both ecotypes, indicating that the flower stalk segments of Col possess the capacity to grow under acidic pH conditions. IAA promoted the proton excretion in segments of Ler but not of Col. On the other hand, FC increased the proton excretion in segments of Col as much as that of Ler. These results suggest that IAA activates the plasma membrane H(+)-ATPases in the segments of Ler but not those of Col, while FC activates them in both ecotypes. Flower stalks of Col may lack the mechanisms of activation by IAA of the plasma membrane H(+)-ATPases.

Auxin-induced elongation growth and expressions of cell wall-bound exo- and endo-beta-glucanases in barley coleoptiles.

When auxin stimulates rapid cell elongation growth of cereal coleoptiles, it causes a degradation of 1,3:1,4-beta-glucan in hemicellulosic polysaccharides. We examined gene expressions of endo-1,3:1,4-beta-glucanase (EI) and exo-beta-glucanase (ExoII), of which optimum pH are about 5, and molecular distribution of hemicellulosic polysaccharides in barley (Hordeum vulgare L.) coleoptile segments treated with or without IAA. IAA (10(-5) M) stimulated the gene expression of EI, while it did not affect that of ExoII. IAA induced gene expression of EI after 4 h and increased wall-bound glucanase activity after 8 h. The molecular weight distribution of hemicellulosic polysaccharides from coleoptile cell walls was shifted to lower molecular weight region by 2 h of IAA treatment. Fusicoccin (10(-6) M) mimicked IAA-induced elongation growth and the decrease in molecular weight of hemicellulosic 1,3:1,4-beta-glucan of coleoptiles in the first 4 h, but it did not promote elongation growth thereafter. These facts suggest that acidification of barley cell walls by IAA action enhances pre-existing cell wall-bound glucanase activity in the early first phase of IAA-induced growth and the late second phase involves the gene expression of EI by IAA.

Inhibition of IAA-induced Ethylene Production and Proline Accumulation in Wheat Coleoptiles by Fusicoccin

Summary The influence of FC on ethylene production and proline accumulation in comparison to its influence on elongation growth was studied in wheat coleoptiles. FC promoted elongation growth in isolated coleoptile segments, but proline accumulation induced by acetic acid and IAA as well as IAA-induced ethylene production were inhibited by 10 −6 M FC. FC did not influence the proline content or ethylene production in control plants (not treated with acetic acid or IAA). The in vivo conversion of ACC to ethylene was also not affected by FC. FC was effective only in reversing the effect of acetic acid- and IAA-induced proline accumulation and of IAA-induced ethylene production. These results are discussed in relation to changes of the p Hc under the influence of these treatments. The obtained results are considered as a further support of the idea that p Hc is involved in stress-induced metabolic reactions, e.g. proline accumulation and ethylene production, as well as the mechanism of IAA action ({scGoring} and {scZoglauer} 1979; {scBrummer} and {scParish} 1983; {scGoring} 1984; {scGoring} and {scPlescher} 1986).

Trehalose Toxicity in Cuscuta reflexa

alpha,alpha-Trehalose induced a rapid blackening of the terminal 2.5-centimeter region of excised Cuscuta reflexa Roxb. vine. The incorporation of radioactivity from [(14)C]glucose into alkali-insoluble fraction of shoot tip was markedly inhibited by 12 hours of trehalose feeding to an excised vine. This inhibition was confined to the apical segment of the vine in which cell elongation occurred. The rate of blackening of shoot tip explants was hastened by the addition of gibberellic acid A(3), which promoted elongation growth of isolated Cuscuta shoot tips. The symptom of trehalose toxicity was duplicated by 2-deoxyglucose, which has been shown to be a potent inhibitor of cell wall synthesis in yeast. The observations suggest that trehalose interferes with the synthesis of cell wall polysaccharides, the chief component of which was presumed to be cellulose.

Gibberellin and Growth in Isolated Wheat Embryos

Gibberellic acid promoted elongation growth in the coleoptiles and leaves of embryos from gamma-irradiated wheat grains whether the embryos were isolated or attached to the endosperm. Thus gibberellic acid affected this growth directly rather than indirectly through an effect upon endosperm. However, root growth was promoted by gibberellic acid only on embryos attached to endosperm, suggesting an indirect effect of gibberellic acid upon growth.