The cytokinins

Transcriptional profile analysis of E3 ligase and hormone-related genes expressed during wheat grain development

BACKGROUND

Wheat grains are an important source of food, stock feed and raw materials for industry, but current production levels cannot meet world needs. Elucidation of the molecular mechanisms underlying wheat grain development will contribute valuable information to improving wheat cultivation. One of the most important mechanisms implicated in plant developmental processes is the ubiquitin-proteasome system (UPS). Among the different roles of the UPS, it is clear that it is essential to hormone signaling. In particular, E3 ubiquitin ligases of the UPS have been shown to play critical roles in hormone perception and signal transduction.

RESULTS

A NimbleGen microarray containing 39,179 UniGenes was used to study the kinetics of gene expression during wheat grain development from the early stages of cell division to the mid-grain filling stage. By comparing 11 consecutive time-points, 9284 differentially expressed genes were identified and annotated during this study. A comparison of the temporal profiles of these genes revealed dynamic transcript accumulation profiles with major reprogramming events that occurred during the time intervals of 80-120 and 220-240°Cdays. The list of the genes expressed differentially during these transitions were identified and annotated. Emphasis was placed on E3 ligase and hormone-related genes. In total, 173 E3 ligase coding genes and 126 hormone-related genes were differentially expressed during the cell division and grain filling stages, with each family displaying a different expression profile.

CONCLUSIONS

The differential expression of genes involved in the UPS and plant hormone pathways suggests that phytohormones and UPS crosstalk might play a critical role in the wheat grain developmental process. Some E3 ligase and hormone-related genes seem to be up- or down-regulated during the early and late stages of the grain development.

Paclobutrazol treatment as a potential strategy for higher seed and oil yield in field-grown Camelina sativa L. Crantz

Background

Camelina (Camelina sativa L. Crantz) is a non-food oilseed crop which holds promise as an alternative biofuel energy resource. Its ability to grow in a variety of climatic and soil conditions and minimal requirements of agronomical inputs than other oilseed crops makes it economically viable for advanced biofuel production. We designed a study to investigate the effect of paclobutrazol [2RS, 3RS)-1-(4-Chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl)pentan-3-ol] (PBZ), a popular plant growth regulator, on the seed and oil yield of Camelina sativa (cv. Celine).

Results

A field-based micro-trial setup was established in a randomized block design and the study was performed twice within a span of five months (October 2010 to February 2011) and five different PBZ treatments (Control: T₀; 25 mg l^-1: T₁; 50 mg l^-1: T₂; 75 mg l^-1: T₃; 100 mg l^-1: T₄; 125 mg l^-1: T₅) were applied (soil application) at the time of initiation of flowering. PBZ at 100 mg l^-1 concentration (T₄) resulted in highest seed and oil yield by 80% and 15%, respectively. The seed yield increment was mainly due to enhanced number of siliques per plant when compared to control. The PBZ - treated plants displayed better photosynthetic leaf gas exchange characteristics, higher chlorophyll contents and possessed dark green leaves which were photosynthetically active for a longer period and facilitated higher photoassimilation.

Conclusion

We report for the first time that application of optimized PBZ dose can be a potential strategy to achieve higher seed and oil yield from Camelina sativa that holds great promise as a biofuel crop in future.

The involvement of cytokinins in a host–parasite relationship between the tomato (Lycopersicon esculentum) and a nematode (Meloidogyne javanica)

The influence of Meloidogyne javanica on cytokinins in the host Lycopersicon esculentum has been studied at different stages of the nematode's life-cycle. Marked differences were detected in cytokinin content of root homogenates between infected and control plants, particularly at the 3rd (32 day), 4th (39 day) and 5th (55 day) harvests. Most of the cytokinin detected appeared to be associated with root homogenates in which the nematode was in the rapid post-moult growth stage. The influence of these nematodes on cytokinins in the host's xylem exudate was not nearly so pronounced. The freshly hatched 2nd-stage infective larvae of M. javanica were themselves capable of exuding cytokinin-like substances.

A low content in zeatin type cytokinins is not restrictive for the occurrence of G1/S transition in tobacco BY-2 cells

Theories on the importance of cytokinins in G1/S transition control are manifold and contradictory. By establishing a double A(phi-PZ block, maximal synchronization of a BY-2 suspension culture was obtained to investigate the effect of cytokinin depletion on G1/S transition. Lovastatin was used as a specific inhibitor of cytokinin biosynthesis. Flow cytometry showed that the G1/S transition occurred regardless of the cytokinin drop. This observation indicates an extremely low dose requiry for that stage of the cell cycle. It is very likely that precisely the downregulation of zeatin type cytokinins matters for the G1/S transition to occur, since cytokinin addition at early G1 blocked the cycle at G1/S.

INCREASED RATES OF PHOTOSYNTHESIS IN LOCALIZED REGIONS OF A BARLEY LEAF INFECTED WITH BROWN RUST

The rate of photosynthesis was examined in whole leaves of barley infected with brown rust (Puccinia hordei Otth.) and within localized regions of the infected leaf, within brown rust pustules and in regions between them, from flecking to green island formation. In addition, oxygen evolution was measured from localized regions of a rusted leaf after feeding with inorganic phosphate (Pi) to investigate the hypothesis that fungal sequestration of Pi from the host cytoplasm may, totally or partially, be responsible for the decline in photosynthesis. The rate of net photosynthesis and the quantum yield of oxygen evolution declined in whole infected leaves as the disease progressed. However, the rate of gross photosynthesis (per unit chlorophyll) was increased in rusted leaves. Autoradiographic and oxygen evolution studies showed that the decline in net photosynthesis could be attributed largely to a reduction in the rate of photosynthesis in the regions between pustules. Within areas of the leaf invaded by the fungus, gross photosynthesis was increased in comparison to control tissue. Feeding leaf tissue with Pi did not significantly increase the rate of photosynthesis in pustules or regions between pustules, implying that Pi was not previously limiting the rate of photosynthesis. Possible mechanisms underlying the increase and decrease in photosynthesis in different regions of an infected leaf are discussed.

Cytokinin inhibition of respiration by cells and mitochondria of soybean, Glycine max (L.) Merrill

Cells of a soybean tissue strain, suspended in an aerated liquid medium, caused disappearance of p-coumaric acid from the medium and oxidation of guaiacol, benzidine, pyrogallol, L-dihydroxyphenylalanine and L-epinephrine. Both the disappearance and the oxidations were inhibited by 6-benzylaminopurine (BAP) at a concentration of 0.5 mM. BAP at other concentrations either promoted or inhibited oxidation of epinephrine in precisely the pattern reported earlier for the disappearance of coumarate; therefore, the disappearance of coumarate probably involves its oxidation. The effectiveness of other cytokinins in inhibiting the oxidation was studied.At 0.5 mM, and perhaps even at 0.5 μM, some of the several cytokinins tested inhibited oxygen consumption by the soybean cells. This inhibition, which did not require any of the above metabolizable compounds, was especially marked in the presence of cyanide, azide or Antimycin A, and was detectable in 10 min or less. Either Antimycin A or salicylhydroxamic acid alone promoted O2 consumption but together they were quite inhibitory. The soybean cells apparently have an alternate respiratory pathway and cytokinins may influence its operation.Several cytokinins at 0.5 mM, and perhaps at 0.5 μM, also inhibited oxygen consumption by mitochondrial preparations from the soybean cells, the inhibition being evident in about 20 s. The consumption required a substrate such as malate, succinate or NADH. Cytokinins and related compounds varied in effectiveness as follows: BAP and 6-isopentenyla-minopurine ≥ 9-tetrahydropyranyl-BAP > kinetin, ribosyl-isopentenylaminopurine, 9-methyl-BAP and 9-methoxymethyl-BAP > 6,6-dimethylaminopurine and zeatin (slight activity) > 6-methylaminopurine, nicotinamide and adenine (ineffective). To a great extent this order parallels the order of effectiveness of the compounds in causing cell division. Mitochondria, therefore, may contain a site for an important cytokinin action.

Comparative analysis of the action of cytokinin and light on the growth of rye leaves

In etiolated rye seedlings (Secale cereale L.) the cytokinin supply controls leaf growth. Dry weight and total amino-nitrogen, protein, total nucleic acid, DNA, and rRNA levels were similarly lowered in leaves depleted of their endogenous cytokinin supply by early excision of the seedling roots and increased by 70-100% after the addition of kinetin. The proportion of cytoplasmic ribosomes bound as polyribosomes was only slightly increased from 40% in cytokinin-deficient to 50% in kinetin-treated darkgrown leaves. White light increased the polyribosome proportion to 61%. In cytokinin-supplied leaves uptake and accumulation of L-[(3)H]leucine were greater than in cytokinin-deficient leaves. Under all conditions of cytokinin supply the same proportion of the total amino nitrogen content (80%) was present as protein nitrogen and virtually, the same percentage (60%) of the total uptake of L-[(3)H]leucine was incorporated into protein in dark-grown leaves. In light, significantly higher proportions of the amino nitrogen content and of the L-[(3)H]leucine uptake were incorporated into protein. The results suggest that the accumulation of substrate is a main cytokinin-controlled step determining the growth of the leaves.

Endogenous cytokinins in vernalized winter wheat grains

The soybean callus assay was used to study the effect of vernalization on the endogenous cytokinin levels of the grains of winter wheat (Triticum vulgare L.), cv. Ferto and Fema. Vernalization of the grains was carried out at 3° C in the dark for 0, 9, 18, 27, 36 and 45 days. Cytokinin activities could be detected in both the aqueous and butanol extracts of vernalized grains of both cultivars. It is suggested that flowering of winter wheat may involve changes in the levels of endogenous cytokinins during vernalization.

Cytokinin translocation: Changes in zeatin and zeatin-riboside levels in the root exudate of tomato plants during their development

The zeatin and zeatin riboside content of tomato (Lycopersicon esculentum Mill.) root exudates were determined at different stages of development. Zeatin riboside was found to be the major translocational form of cytokinin in the xylem during early vegetative growth. During flower bud formation this cytokinin decreased markedly in concentration so that, at anthesis, there was no appreciable difference in the zeatin and zeatin riboside concentration in the root exudate.

Biosynthesis and cytokinin activity of 8-hydroxy and 2,8-dihydroxy derivatives of zeatin and N-6(increment-2-isopentenyl)adenine

8-Hydroxy and 2,8-dihydroxy derivatives of the cytokinins, 6-(4-hydroxy-3-methyl-trans-2-butenylamino)purine and N-6-(increment -2-isopentenyl)adenine, have been biosynthesized by xanthine oxidase oxidation. 8-Hydroxy derivatives have been shown to be the major intermdeiates. These compounds were tested for cytokinin activity in the tobacco bioassay. The results suggest that substitution of the 8 position with a hydroxyl group causes less decrease of cytokinin activity than substitution of both the 2 and 8 positions with hydroxyl groups.

Effects of N(6)-benzyladenine on the rate of turnover of [(3)H]GA 20 by shoots of dwarf Pisum sativum

Application of the cytokinin, N(6)-benzyladenine, to light-grown shoots of dwarf "Meteor" pea seedlings (Pisum sativum L.) markedly increased the rate of turnover of GA20, a native pea gibberellin. It is suggested that endogenous cytokinins may affect gibberellin metabolism in plants by controlling rates of gibberellin turnover.

Cytokinins in Populus x robusta Schneid: A complex in leaves

At least seven cytokinins have been detected in mature leaves of Populus x robusta Schneid after chromatography on Sephadex LH-20. Two of these have similar elution volumes to zeatin and zeatin riboside. A third appears to be a cytokinin glucoside. A fourth is a new, unidentified cytokinin, susceptible to mild oxidation, and yielding two cytokinin active products after acid hydrolysis. This cytokinin complex has been found in fully expanded leaves, a tissue in which cell division is completed.