Coaction of light and cytokinin in photomorphogenesis

The interaction of light quality and irradiance with gibberellins, cytokinins and auxin in regulating growth of Helianthus annuus hypocotyls

A reduced red to far-red (R/FR) light ratio and low photosynthetically active radiation (PAR) irradiance are both strong signals for inducing etiolation growth of plant stems. Under natural field conditions, plants can be exposed to either a reduced R/FR ratio or lower PAR, or to a combination of both. We used Helianthus annuus L., the sunflower, to study the effect of reduced R/FR ratio, low PAR or their combination on hypocotyl elongation. To accomplish this, we attempted to uncouple light quality from light irradiance as factors controlling hypocotyl elongation. We measured alterations in the levels of endogenous gibberellins (GAs), cytokinins (CKs) and the auxin indole-3-acetic acid (IAA), and the effect of exogenous hormones on hypocotyl growth. As expected, both reduced R/FR ratio and lower PAR can significantly promote sunflower hypocotyl elongation when given separately. However, providing the reduced R/FR ratio at a low PAR resulted in the greatest hypocotyl growth, and this was accompanied by significantly higher levels of endogenous IAA, GA1, GA8, GA20 and of a wide range of CKs. Providing a reduced R/FR ratio under normal PAR also significantly increased growth and again gave significantly higher levels of endogenous IAA, GAs and CKs. However, only under the de-etiolating influence of a normal R/FR ratio did lowering PAR significantly increase levels of GA1, GA8 and GA20. We thus conclude that light quality (e.g. the R/FR ratio) is the most important component of shade for controlling hypocotyl growth and elevated growth hormone content.

Down-regulation of phytochrome mRNA abundance by red light and benzyladenine in etiolated cucumber cotyledons

Northern blot analysis revealed that a single 4.2 kb phytochrome mRNA species was detectable in cotyledons excised from five-day-old etiolated cucumber seedlings. Intact etiolated five-day-old cucumber seedlings were given a red light or benzyladenine treatment, and cotyledons were harvested at various times following treatment. The abundance of phytochrome mRNA in the cotyledons was quantitated using 32P-labeled RNA probes and slot blot analysis. By 2 h after irradiation the phytochrome mRNA level was reduced to 40% of the initial abundance and reaccumulation began by 3 h after irradiation. Reaccumulation of phytochrome mRNA to the time-zero dark control level was achieved by 10 h after treatment. A decrease in phytochrome mRNA abundance was evident by 2 h after benzyladenine treatment, and a maximal reduction to 45% of the time-zero dark control was attained by 4 h after treatment. No recovery of the phytochrome mRNA level was evident by 8 h after benzyladenine treatment. The abundance of actin mRNA was unaffected by benzyladenine treatment.

Benzyladenine modulation of the expression of two genes for nuclear-encoded chloroplast proteins in Lemna gibba: Apparent post-transcriptional regulation

Cytokinins and phytochrome have both been reported to promote chloroplast development, and possible interactions between the two have been suggested. We have examined the effects of red light (R) and a cytokinin, benzyladenine (N(6)-benzylaminopurine; BA), on the levels of four mRNAs coding for chloroplast proteins in Lemna gibba L. The amounts of hybridizable RNA coding for both the major chlorophyll a/b-binding protein and for the small subunit of ribulose-1,5-bisphosphate (RuBP) carboxylase decrease to a low level when white-light-grown L. gibba plants are placed in the dark. We have previously shown that a subsequent R treatment causes a several-fold increase in the levels of these two messages, and this increase is phytochrome-mediated. We have now found that addition of submicromolar concentrations of BA to plants kept in total darkness also results in an increase in levels of these two mRNAs. Furthermore, BA treatment magnifies the extent of the response to R treatment. However, the levels of mRNAs encoding the large subunit of RuBP carboxylase and the 32-kDa herbicide-binding protein, which are both chloroplastsynthesized messages, are not significantly altered by either R or BA treatment during the same time period. The relative amount of β-actin mRNA, a nuclear-encoded message for a cytoplasmic protein, is also not altered either by R or BA treatment. Thus, BA treatment does not simply alter the proportion of mRNA to total RNA. This conclusion is also supported by the observation that levels of mRNA hybridizing to a sequence abundant in dark-treated plants are not altered by BA treatment. The amplification by BA of the R-induced increase in the level of chlorophyll a/b-binding protein mRNA, consistently seen in total RNA, is not observed in RNA isolated from nuclei from plants receiving the same treatments. We therefore suggest that cytokinin is regulating expression of this message at a post-transcriptional level, possibly by affecting the stability of the RNA.

Gene expression in cytokinin-and light-mediated plastogenesis of Cucurbita cotyledons: ribulose-1,5-bisphosphate carboxylase/oxygenase

The effects of a cytokinin (N(6)-benzyladenine, BA) and light on plastogenesis have been studied in detached Cucurbita cotyledons using the key enzyme of photosynthetic CO2 fixation, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCase), as an example of a coordinated program of plastid and nucleo-cytoplasmic gene expression. Treatment of etiolated cotyledons with either BA in darkness or in light or light alone results in a marked and correlated stimulation of enzyme activity, quantity and biosynthesis (in-vivo [(14)C]leucine incorporation into immunoprecipitated enzyme protein), indicating an increase of de-novo synthesis under the influence of the two effectors. Cell-free translation of non-polyadenylated (poly(A)(-))RNA in an Escherichia coli system and total RNA in a wheat-germ system likewise demonstrate a light and hormone-dependent increase in the amounts of translatable mRNAs for the large (LS) and small subunits (SS) of RuBPCase (among other polypeptides). Hybridisation of poly(A)(-)RNA with a nick-translated LS gene of spinach RuBPCase reveals also two-to three-fold BA-or light-induced enhancement of LS mRNA content. Indications for stimulation of SS mRNA transcription are derived from inhibitor experiments with cordycepin. We conclude that the observed stimulation of de-novo RuBPCase synthesis by cytokinin and by light can be referred to the level of mRNA transcription, but it remains open whether this is a primary action. Furthermore, our results indicate that the coaction of the two exogenous factors is additive at different steps of RuBPCase formation, indicating independent actions in the causal chain between effector-signal transduction and RuBPCase gene expression.