MAX2 affects multiple hormones to promote photomorphogenesis

Ubiquitin-26S proteasome system (UPS) has been shown to play central roles in light and hormone-regulated plant growth and development. Previously, we have shown that MAX2, an F-box protein, positively regulates facets of photomorphogenic development in response to light. However, how MAX2 controls these responses is still unknown. Here, we show that MAX2 oppositely regulates GA and ABA biosynthesis to optimize seed germination in response to light. Dose-response curves showed that max2 seeds are hyposensitive to GA and hypersensitive to ABA in seed germination responses. RT-PCR assays demonstrated that the expression of GA biosynthetic genes is down-regulated, while the expression of GA catabolic genes is up-regulated in the max2 seeds compared to wild-type. Interestingly, expression of both ABA biosynthetic and catabolic genes is up-regulated in the max2 seeds compared to wild-type. Treatment with an auxin transport inhibitor, NPA, showed that increased auxin transport in max2 seedlings contributes to the long hypocotyl phenotype under light. Moreover, light-signaling phenotypes are restricted to max2, as the biosynthetic mutants in the strigolactone pathway, max1, max3, and max4, did not display any defects in seed germination and seedling de-etiolation compared to wild-type. Taken together, these data suggest that MAX2 modulates multiple hormone pathways to affect photomorphogenesis.     

Different Phototransduction Kinetics of Phytochrome A and Phytochrome B in Arabidopsis thaliana

The kinetics of phototransduction of phytochrome A (phyA) and phytochrome B (phyB) were compared in etiolated Arabidopsis thaliana seedlings. The responses of hypocotyl growth, cotyledon unfolding, and expression of a light-harvesting chlorophyll a/b-binding protein of the photosystem II gene promoter fused to the coding region of β-glucuronidase (used as a reporter enzyme) were mediated by phyA under continuous far-red light (FR) and by phyB under continuous red light (R). The seedlings were exposed hourly either to n min of FR followed by 60 minus n min in darkness or to n min of R, 3 min of FR (to back-convert phyB to its inactive form), and 57 minus n min of darkness. For the three processes investigated here, the kinetics of phototransduction of phyB were faster than that of phyA. For instance, 15 min R h⁻¹ (terminated with a FR pulse) were almost as effective as continuous R, whereas 15 min of FR h⁻¹ caused less than 30% of the effect of continuous FR. This difference is interpreted in terms of divergence of signal transduction pathways downstream from phyA and phyB.     

Photoresponses of transgenic Arabidopsis seedlings expressing introduced phytochrome B-encoding cDNAs: evidence that phytochrome A and phytochrome B have distinct photoregulatory functions

The photocontrol of hypocotyl elongation has been studied in two transgenic lines of Arabidopsis thaliana which contain elevated levels of phytochrome B encoded by either an introduced rice- or Arabidopsis -derived cDNA driven by the 35S CaMV promoter. Inhibition of hypocotyl growth in etiolated seedlings of the phyB -transformed lines was saturated at photon fluence rates of continuous red light (R) which were markedly lower than those required for inhibition of growth in seedlings of the isogenic wild-type (WT). Inhibition of hypocotyl growth in etiolated seedlings of the phyB -transgenic lines under continuous far-red irradiation (FR), however, showed the same relationship with fluence rate as WT. Light-grown seedlings of the phyB -transgenic lines responded to end-of-day FR by an acceleration of growth, in a manner comparable with WT. This response was unaltered when the end-of-day FR was extended from a 15 min pulse to 14 h of continuous irradiation. The response of light-grown, phyB -transformed seedlings to decreasing R:FR ratio was also qualitatively similar to WT, i.e. increased elongation growth of the hypocotyl and petioles occurred under low R:FR quantum ratio. However, absolute elongation growth was markedly less in the transgenic seedlings at all R:FR ratios tested than in WT. Together, these data indicate that seedlings over-expressing phytochrome B are more responsive to R than are WT, but are unaltered in their responsiveness to FR. By contrast, seedlings overexpressing phytochrome A are more responsive than WT to both R and FR; whereas the phytochrome B-deficient mutant hy3 is unresponsive to R while retaining WT-like responsiveness to FR. These data indicate that in WT etiolated seedlings phytochrome A mediates the effects of continuous FR, and phytochrome B the effects of continuous R. The evidence thus supports the conclusion that these two molecular species of the photoreceptor have differential regulatory roles in the plant.     

Photoresponses of Light-Grown phyA Mutants of Arabidopsis (Phytochrome A Is Required for the Perception of Daylength Extensions)

Several aspects of the photophysiology of wild-type Arabidopsis thaliana seedlings were compared with those of a phytochrome A null mutant, phyA-1, and a mutant, fhy1, that is putatively involved in the transduction of light signals from phytochrome A. Although phyA seedlings display a near wild-type phenotype when grown in white light (W), they nevertheless display several photomorphogenic abnormalities. Thus, whereas the germination of wild-type and fhy1 seeds is almost fully promoted by a pulse of red light (R) or by continuous far-red light (FR), phyA seed germination is responsive only to R. Following growth under day/night cycles, but not under continuous W, the hypocotyls of light-grown phyA and fhy1 seedlings are more elongated than those of wild-type seedlings. For seedlings grown under low red/far-red (R/FR) ratio light conditions, phyA and fhy1 seedlings display a more marked promotion of hypocotyl elongation than wild-type seedlings. Similarly, seedlings that are doubly null for phytochrome A and phytochrome B(phyA phyB) also have more elongated hypocotyls under low R/FR ratio conditions than phyB seedlings. This indicates that phytochrome A action in light-grown seedlings is antagonistic to the action of phytochrome B. Although wild-type, fhy1, and phyA seedlings flower at essentially the same time under both short-day and long-day conditions, an obvious consequence of phytochrome A deficiency is a pronounced late flowering under conditions where a short day of 8 h of fluorescent W is extended by 8 h of low-fluence-rate incandescent light. The evidence thus indicates that phytochrome A plays a role in seed germination, in the control of elongation growth of light-grown seedlings, and in the perception of daylength.     

Overexpression of oat phytochrome A gene differentially affects stem growth responses to red/far-red ratio signals characteristic of sparse or dense canopies

The effects of overexpression of oat phytochrome A on neighbour detection and on stem-growth responses to changes in red light (R), far-red light (FR) and blue light (B) simulating neighbours were investigated in transgenic tobacco seedlings grown under natural radiation. In wild-type (WT) seedlings, stem extension growth was promoted: (1) by lowering the R:FR by means of daytime supplementary FR, end-of-day FR, neighbours reflecting FR, or selective light filters placed around the base of the shoot to reduce R without affecting FR; and (2) by lowering phytochrome-absorbable radiation (R+FR) reaching the stem. Transgenic seedlings only responded to reductions in R:FR involving no significant changes in FR irra-diance, i.e. end-of-day FR and filters placed around the stem to reduce R. Neither daytime supplementary R nor selective filters placed around the stem to reduce B affected stem growth in any genotype. In growing canopies, WT seedlings responded to the reduction of R:FR caused by FR reflected in neighbour plants. Transgenic seedlings responded to plant density about a week later, when mutual plant shading reduced R and (to a lesser extent) FR below sunlight levels. Overexpression of phytochrome A impaired early neighbour detection.     

Light-grown plants of transgenic tobacco expressing an introduced oat phytochrome A gene under the control of a constitutive viral promoter exhibit persistent growth inhibition by far-red light

A comparison of the photoregulation of development has been made for etiolated and light-grown plants of wild-type (WT) tobacco (Nicotiana tabacun L.) and an isogenic transgenic line which expresses an introduced oat phytochrome gene (phyA) under the control of a constitutive viral promoter. Etiolated seedlings of both the WT and transgenic line showed irradiance-dependent inhibition of hypocotyl growth under continuous far-red (FR) light; transgenic seedlings showed a greater level of inhibition under a given fluence rate and this is considered to be the result of the heterologous phytochrome protein (PhyA) functioning in a compatible manner with the native etiolated phytochrome. Deetiolation of WT seedlings resulted in a loss of responsiveness to prolonged FR. Light-grown transgenic seedlings, however, continued to respond in an irradiance-dependent manner to prolonged FR and it is proposed that this is a specific function of the constitutive PhyA. Mature green plants of the WT and transgenic lines showed a qualitatively similar growth promotion to a brief end-of-day FR-treatment but this response was abolished in the transgenic plants under prolonged irradiation by this same FR source. Growth inhibition (McCormac et al. 1991, Planta 185, 162–170) and enhanced levels of nitrate-reductase activity under irradiance of low red:far-red ratio, as achieved by the FR-supplementation of white light, emphasised that the introduced PhyA was eliciting an aberrant mode of photoresponse compared with the normal phytochrome population of light-grown plants. Total levels of the oat-encoded phytochrome in the etiolated transgenic tobacco were shown to be influenced by the wavelength of continuous irradiation in a manner which was qualitatively similar to that seen for the native, etiolated tobacco phytochrome, and distinct from that seen in etiolated oat tissues. These results are discussed in terms of the proposal that the constitutive oat-PhyA pool in the transgenic plants leads to a persistence of a mode of response normally restricted to the situation in etiolated plants.Abbreviations FR far-red light - R red light - WL white light - WL + FR white light supplemented with FR - HIR high-irradiance response - PAR photosynthetically active radiation - Pr, Pfr R- and FR-absorbing forms of phytochrome - Ptot total phytochrome - phyA (PhyA) gene (encoded protein) for phytochrome - WT wild type This work was supported by an Agricultural and Food Research Council research grant to H.S. and A.M.; J.R. Cherry and R.D. Vierstra, (Department of Horticulture, University of Wisconsin-Madison, USA) are thanked for the provision of the transgenic tobacco line.     

Selected Components of the Shade-Avoidance Syndrome Are Displayed in a Normal Manner in Mutants of Arabidopsis thaliana and Brassica rapa Deficient in Phytochrome B

Several growth parameters associated with the phytochrome-mediated shade avoidance syndrome have been measured in seedlings and mature plants of a wild-type and a hy3 mutant of Arabidopsis thaliana deficient in phytochrome B. Growth parameters were compared in plants grown in either white light (high red:far-red [R:FR] ratio) or white light plus added far-red (FR) light (low R:FR ratio). Wild-type Arabidopsis exhibited increased hypocotyl and petiole extension under a low, compared with a high, R:FR ratio. The hy3 mutant did not respond to low R:FR ratio by increase in hypocotyl or petiole length. Extension growth of wild-type plants was stimulated by brief end-of-day FR pulses, but similar treatment had no effect on extension growth of hy3 mutant plants. However, some responses to low R:FR ratio seen in the wild-type plants were also evident in the hy3 mutants. The number of days to bolting, the developmental stage at bolting, the leaf area, and the specific stem weight (weight per unit of length) all decreased in the wild-type and hy3 seedlings in response to low R:FR ratio. Low R:FR ratio caused a larger decrease in leaf area and specific stem weight in the mutant seedlings than in wild-type seedlings. The effects of low R:FR ratio on leaf area and specific stem weight were opposite to those of the hy3 lesion, which resulted in increased leaf area and specific stem weight in comparison with the wild type. Both leaf area and specific stem weight responses to low R:FR ratio also were unchanged in the ein mutant of Brassica rapa, known to be deficient in phytochrome B. These responses represent components of the shade-avoidance syndrome, and, consequently, the results indicate that phytochrome B cannot be solely responsible for the perception of R:FR ratio and the induction of shade-avoidance responses. The hypothesis is proposed that different phytochromes may be responsible for the regulation of extension growth and the regulation of lateral or radial expansion.     

Phytochrome B and at Least One Other Phytochrome Mediate the Accelerated Flowering Response of Arabidopsis thaliana L. to Low Red/Far-Red Ratio

We have investigated the involvement of phytochrome B in the early-flowering response of Arabidopsis thaliana L. seedlings to low red:far-red (R/FR) ratio light conditions. The phytochrome B-deficient hy3 (phyB) mutant is early flowering, and in this regard it resembles the shade-avoidance phenotype of its isogenic wild type. Seedlings carrying the hy2 mutation, resulting in a deficiency of phytochrome chromophore and hence of active phytochromes, also flower earlier than wild-type plants. Whereas hy3 or hy2 seedlings show only a slight acceleration of flowering in response to low R/FR ratio, seedlings that are doubly homozygous for both mutations flower earlier than seedlings carrying either phytochrome-related mutation alone. This additive effect clearly indicates the involvement of one or more phytochrome species in addition to phytochrome B in the flowering response as well as indicating the presence of some functional phytochrome B in hy2 seedlings. Seedlings that are homozygous for the hy3 mutation and one of the fca, fwa, or co late-flowering mutations display a pronounced early-flowering response to low R/FR ratio. A similar response to low R/FR ratio is displayed by seedlings doubly homozygous for the hy2 mutation and any one of the late-flowering mutations. Thus, placing the hy3 or hy2 mutations into a late-flowering background has the effect of uncovering a flowering response to low R/FR ratio. Seedlings that are triply homozygous for the hy3, hy2 mutations and a late-flowering mutation flower earlier than the double mutants and do not respond to low R/FR ratio. Thus, the observed flowering responses to low R/FR ratio in phytochrome B-deficient mutants can be attributed to the action of at least one other phytochrome species.     

Impaired stem-growth responses to blue-light irradiance in light-grown transgenic tobacco seedlings overexpressing Avena phytochrome A

The effects of blue light (B) on stem extension-growth were compared in light-grown seedlings, of tobacco overexpressing Avena phytochrome A and its isogenic wild type (WT). Under natural radiation, lowering the levels of B reaching the whole shoot promoted stem extension growth in WT but not in transgenic seedlings. Under controlled conditions, the seedlings were exposed to white light (WL) or WL minus B, each one provided at two different irradiances. In WT seedlings stem extension growth was promoted by lowering B at both irradiance levels. In transgenic seedlings a reduction of B was promotive only at low irradiance levels. The seedlings were also grown under WL, WL minus B, WL minus red light (R) and far-red light (FR) or WL minus R, FR and B. In the WT, lowering B promoted stem extension growth irrespective of R+FR levels. In the transgenics, B was effective only at very low levels of R+FR (i.e. at low phytochrome cycling rates). Lowering the Pfr levels at the end of the day promoted extension growth in wild type and transgenic seedlings. Responses to B were not observed in transgenic seedlings having low Pfr levels at the end of the day. The results suggest that the overexpressed phytochrome A acts mainly via irradiance-dependent reactions. When these reactions are highly expressed, B responses are not observed.     

Phytochrome A is an irradiance-dependent red light sensor

Plants perceive red (R) and far-red (FR) light signals using the phytochrome family of photoreceptors. In Arabidopsis thaliana, five phytochromes (phyA-phyE) have been identified and characterized. Unlike other family members, phyA is subject to rapid light-induced proteolytic degradation and so accumulates to relatively high levels in dark-grown seedlings. The insensitivity of phyA mutant seedlings to prolonged FR and wild-type appearance in R has led to suggestions that phyA functions predominantly as an FR sensor during the early stages of seedling establishment. The majority of published photomorphogenesis experiments have, however, used <50 micromol m(-2) sec(-1) of R when characterizing phytochrome functions. Here we reveal considerable phyA activity in R at higher (>160 micromol m(-2) sec(-1)) photon irradiances. Under these conditions, plant architecture was observed to be largely regulated by the redundant actions of phytochromes A, B and D. Moreover, quadruple phyBphyCphyDphyE mutants containing only functional phyA displayed R-mediated de-etiolation and survived to flowering. The enhanced activity of phyA in continuous R (Rc) of high photon irradiance correlates with retarded degradation of the endogenous protein in wild-type plants and prolonged epifluorescence of nuclear-localized phyA:YFP in transgenic lines. Such observations suggest irradiance-dependent 'photoprotection' of nuclear phyA in R, providing a possible explanation for the increased activity observed. The discovery that phyA can function as an effective irradiance sensor, even in light environments that establish a high Pfr concentration, raises the possibility that phyA may contribute significantly to the regulation of growth and development in daylight-grown plants.     

Coupling of phytochrome B to the control of hypocotyl growth in Arabidopsis

Etiolated seedlings of the wild-type (WT) and of the phyB-1 mutant of Arabidopsis thaliana (L.) Heynh. were exposed to red-light (R) and far-red light (FR) treatments to characterize the action of phytochrome B on hypocotyl extension growth. A single R or FR pulse had no detectable effects on hypocotyl growth. After 24-h pre-treatment with continuous FR (FRc) a single R, compared to FR pulse inhibited (more than 70%) subsequent hypocotyl growth in the WT but not in the phyB-1 mutant. This effect of FRc was fluence-rate dependent and more efficient than continuous R (Rc) or hourly FR pulses of equal total fluence. Hypocotyl growth inhibition by Rc was larger in WT than phyB-1 seedlings when chlorophyll screening was reduced either by using broadband Rc (maximum emission 610 nm) or by using narrow-band Rc (658 nm) over short periods (24 h) or with seedlings bleached with Norflurazon. Hourly R or R + FR pulses had similar effects in WT and phyB-1 mutant etiolated seedlings. It is concluded that phytochrome B is not the only photoreceptor of Rc and that the action of phytochrome B is enhanced by a FRc high-irradiance reaction. Complementary experiments with the phyA-201 mutant indicate that this promotion of a phytochrome B-mediated response occurs via co-action with phytochrome A.Abbreviations D darkness - FR far-red light - FRc continuous FR - Pfr FR-absorbing form of phytochrome - HIR high-irradiance reaction - Pfr/P proportion of phytochrome as Pfr - phyA phytochrome A - phyB phytochrome B - R red light - Rc continuous R - WT wild-type I thank Professors R.E. Kendrick and M. Koornneef (Wageningen Agricultural University, The Netherlands) and Professor J. Chory (Salk Institute, Calif., USA) for their kind provision of the original WT and phyB-1 and phyA-201 seed, respectively. This work was financially supported by grants PID and PID-BID from CONICET, AG 040 from Universidad de Buenos Aires and A 12830/1-000019 from Fundación Antorchas.     

Phytochrome A enhances the promotion of hypocotyl growth caused by reductions in levels of phytochrome B in its far-red-light-absorbing form in light-grown Arabidopsis thaliana.

We sought to determine if phytochrome B (phyB)-mediated responses to the red light (R)/far-red light (FR) ratio are affected by phytochrome A (phyA) activity in light-grown seedlings of Arabidopsis thaliana. Pulses of FR delayed into the dark period were less effective than end-of-day (EOD) FR in promoting hypocotyl growth over a given period in darkness. White light minus blue light interposed instead of darkness between the end of the white-light photoperiod and the FR pulse was sufficient to maintain responsivity to the decrease in phyB in FR-light-absorbing form in wild-type (WT) seedlings, but not in the phyA mutant. Compared with EOD R, hourly R+FR pulses provided throughout the night caused a stronger promotion of stem growth than a single EOD R+FR pulse in WT Arabidopsis, cucumber, mustard, sunflower, tobacco, and tomato, but not in phyA Arabidopsis or in the aurea mutant of tomato. WT seedlings of Arabidopsis responded to a range of high EOD R/FR ratios, whereas the phyA mutant required stronger reductions in the EOD R/FR ratio. In sunlight, phyA seedlings of Arabidopsis showed no response to the "early warning" signals of neighboring vegetation, and hypocotyl-growth promotion occurred at higher plant densities than in the WT. Thus, under a series of light conditions, the sensitivity or responsivity to reductions in the R/FR ratio were larger in WT than in phyA seedlings. A product of phyA is therefore proposed to enhance the hypocotyl-growth response to decreases in phyB in FR-light-absorbing form in light grown seedlings.     

Stem extension-growth responses to blue light require Pfr in tomato seedlings but are not reduced by the low phytochrome levels of the aurea mutant

The effects of blue light (B) on stem extension growth were investigated in wild-type (WT) and aurea (au ) mutant seedlings of tomato. The au mutant has reduced phytochrome levels. Etiolated seedlings were grown under background red light (R) or far-red light (FR) with or without B. Hypocotyl growth was inhibited by B added to R but not by B added to FR, both in WT and au seedlings. The levels of B and/or R reaching the stem of fully de-etiolated seedlings grown in a glasshouse were reduced by means of collars around it. Both in WT and au -mutant seedlings the responses to B were larger at high than at low R/FR quantum ratios. In etiolated and light-grown au seedlings, changing the levels of phytochrome-absorbable radiation did not cause the same effect as changing B levels, indicating the action of specific BL/UV-A photoreceptor(s) (BAP). The responses to B are reduced by the low calculated levels of Pfr established by light treatments but not by the low levels of phytochrome present in the au mutant. The au mutant appears to be deficient in a phytochrome pool that is not essential for the interdependent co-action observed between phytochrome and BAP in the control of stem extension growth in tomato.     

Elimination of the Far-Red-Induced Lag Phase of Chlorophyll a Accumulation by a Brief Light in Pharbitis nil Cotyledons

In dark-grown Pharbitis nil seedlings, far-red light (FR) irradiationof 48 h or less promotes Chl a accumulation in the first 2-hof a subsequent white light (WL) period, without a lag phaseof Chl a accumulation. However, continuous FR irradiation of72 h or more, causes the so-called FR-induced lag phase. A 5-minWL given 4 h before the onset of the continuous WL promotesChl a accumulation irrespective of the length of the precedingFR irradiation period, if a 4-h dark period is inserted betweenthe 5-min WL and continuous WL. This suggests that the effectof the brief WL is independent of and additive to the effectof the preceding FR irradiation, although the effect of theFR irradiation changes from promotive to inhibitory with anincrease in the irradiation period. Red light (R) is more active than blue light (B) in this brieflight effect. The R effect is reversed by subsequent exposureto FR when the period of the preceding FR irradiation is 24h, but not when it is 72 h. The relative effectiveness of Bto R increases after prolonged FR irradiation. (Received August 6, 1986; Accepted March 12, 1987)     

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.     

不同光质的LED对蝴蝶兰组织培养增殖及生根的影响

以蝴蝶兰(Phalaenopsis ssp.)栽培种绿熊(Green Bear)和大辣椒(Big Chilli)为培养材料, 研究不同光质的发光二极管(LED)组合对组织培养过程中增殖及生根的影响。结果表明, 红光更有利于蝴蝶兰单芽增殖、干重、鲜重以及株高的增加, 但不利于叶片叶绿素的积累; 蓝光有利于叶片叶绿素的积累, 并能提高根系活力; 远红光则对根长和根系活力的增加作用更显著。增殖扩繁阶段的最适LED为暖白, 2种蝴蝶兰芽增殖系数分别比白色荧光灯(对照)高出53.17%和46.37%。生根诱导阶段的最适LED组合为红:蓝:远红=3:6:1, 根长及根系活力均较对照显著增加。该研究结果为LED光源在蝴蝶兰组织培养中的应用奠定了基础。     

The rosette habit of Arabidopsis thaliana is dependent upon phytochrome action: novel phytochromes control internode elongation and flowering time

A major function of phytochromes in light-grown plants involves the perception of changes in the relative amounts of red and far-red light (R:FR ratio) and the initiation of the shade-avoidance response. In Arabidopsis thaliana, this response is typified by increased elongation growth of petioles and accelerated flowering and can be fully induced by end-of-day far-red light (EOD FR) treatments. Phytochrome B-deficient (phyB) mutants, which have a constitutive elongated-petiole and early-flowering phenotype, do not display a petiole elongation growth response to EOD FR, but they do respond to EOD FR by earlier flowering. Seedlings deficient in both phytochrome A and phytochrome B (phyA phyB), have a greatly reduced stature compared with wild-type or either monogenic mutant. The phyA phyB double null mutants also respond to EOD FR treatments by flowering early, suggesting the operation of novel phytochromes. Contrary to the behaviour of wild-type or monogenic phyA or phyB seedlings, petiole elongation in phyA phyB seedlings is reduced in response to EOD FR treatments. This reduction in petiole elongation is accompanied by the appearance of elongated internodes such that under these conditions the plants no longer display a rosette habit.