A possible tyrosine phosphorylation of phytochrome

Maize protein phosphatase gene family: identification and molecular characterization

BACKGROUND

Protein phosphatases (PPs) play critical roles in various cellular processes through the reversible protein phosphorylation that dictates many signal transduction pathways among organisms. Recently, PPs in Arabidopsis and rice have been identified, while the whole complement of PPs in maize is yet to be reported.

RESULTS

In this study, we have identified 159 PP-encoding genes in the maize genome. Phylogenetic analyses categorized the ZmPP gene family into 3 classes (PP2C, PTP, and PP2A) with considerable conservation among classes. Similar intron/exon structural patterns were observed in the same classes. Moreover, detailed gene structures and duplicative events were then researched. The expression profiles of ZmPPs under different developmental stages and abiotic stresses (including salt, drought, and cold) were analyzed using microarray and RNA-seq data. A total of 152 members were detected in 18 different tissues representing distinct stages of maize plant developments. Under salt stress, one gene was significantly up-expressed in seed root (SR) and one gene was down-expressed in primary root (PR) and crown root (CR), respectively. As for drought stress condition, 13 genes were found to be differentially expressed in leaf, out of which 10 were up-regulated and 3 exhibited down-regulation. Additionally, 13 up-regulated and 3 down-regulated genes were found in cold-tolerant line ETH-DH7. Furthermore, real-time PCR was used to confirm the expression patterns of ZmPPs.

CONCLUSIONS

Our results provide new insights into the phylogenetic relationships and characteristic functions of maize PPs and will be useful in studies aimed at revealing the global regulatory network in maize abiotic stress responses, thereby contributing to the maize molecular breeding with enhanced quality traits.

Tyrosine phosphorylation regulates the activity of phytochrome photoreceptors

Phytochromes are red/far-red light receptors that function in photomorphogenesis of plants. Photoisomerization of phytochrome by red light leads to its translocation to the nucleus, where it regulates gene expression. We examined whether phytochrome is phosphorylated in response to light, and we report that phytochrome B (phyB)'s N terminus contains a region with a number of phosphoserines, threonines, and tyrosines. The light-dependent phosphorylation of tyrosine 104 (Y104) appears to play a negative role in phyB's activity, because a phosphomimic mutant, phyBY104E, is unable to complement any phyB-related phenotype, is defective in binding to its signaling partner PIF3, and fails to form stable nuclear bodies even though it retains normal photochemistry in vitro. In contrast, plants stably expressing a nonphosphorylatable mutant, phyBY104F, are hypersensitive to light. The proper response to changes in the light environment is crucial for plant survival, and our study brings tyrosine phosphorylation to the forefront of light-signaling mechanisms.

Structural characterization, expression analysis and evolution of the red/far-red sensing photoreceptor gene, phytochrome C (PHYC), localized on the 'B' genome of hexaploid wheat (Triticum aestivum L.)

Phytochromes are a family of red/far-red light perceiving photoreceptors. The monocot phytochrome family is represented by three members, PHYA, PHYB and PHYC. We have isolated and characterized the first PHY gene member (TaPHYC) from common wheat, Triticum aestivum var. CPAN1676. It codes for a species of the photoreceptor, phyC, which is known to be light-stable in all plants analyzed so far. A sequence of 7.2 kb has been determined, which includes 3.42 kb of coding region. This is the second full-length PHYC gene sequenced from a monocot (first was from rice). TaPHYC gene shares structural similarities with the rice PHYC containing four exons and three introns in the coding region. The 5' UTR is 1.0-kb-long and harbors an upstream open reading frame (URF) encoding 28 aa. Southern blot analysis of TaPHYC indicates that it represents single locus in the wheat genome, although the possibility of additional loci cannot be completely ruled out. Chromosomal localization using nullisomic-tetrasomic lines of Triticum aestivum var. Chinese Spring places TaPHYC on chromosome 4B. PHYC represents a constitutively expressed gene in all the organs tested and under light/dark conditions. However, PHYC was found to be developmentally regulated showing maximal expression in 3-day-old dark-grown seedlings, which declined thereafter. In silico analysis has also been done to compare TaPHYC gene with the partial sequences known from other wheat species and cultivars. The presence of a topoisomerase gene immediately downstream of the PHYC gene, both in rice and wheat genomes, presents yet another example of synteny in cereals and its possible significance has been discussed.

Inter-domain crosstalk in the phytochrome molecules

Phytochromes are bifunctional photoreceptors with a two-domain structure, consisting of the N-terminal photosensory domain and the C-terminal regulatory domain. The photo-induced Pr <--> Pfr phototransformation accompanies subtle conformational changes, primarily triggered by the apoprotein-chromophore interactions in the N-terminal domain. The conformational signals are subsequently transmitted to the C-terminal domain through various inter-domain crosstalks, resulting in the interaction of the activated C-terminal domain with phytochrome interacting factors. Thus the inter-domain crosstalks play critical roles in the photoactivation of the phytochromes. Protein phosphorylation, such as that of Ser-598, is implicated in this process by inducing conformational changes and by modulating inter-domain signaling.

Elementary processes of photoperception by phytochrome A for high-irradiance response of hypocotyl elongation in Arabidopsis

Elementary processes of photoperception by phytochrome A (PhyA) for the high-irradiance response (HIR) of hypocotyl elongation in Arabidopsis were examined using a newly designed irradiator with LED. The effect of continuous irradiation with far-red (FR) light could be replaced by intermittent irradiation with FR light pulses if given at intervals of 3 min or less for 24 h. In this response, the Bunsen-Roscoe law of reciprocity held in each FR light pulse. Therefore, we determined the action spectrum for the response by intermittent irradiation using phyB and phyAphyB double mutants. The resultant action spectrum correlated well with the absorption spectrum of PhyA in far-red-absorbing phytochrome (Pfr). Intermittent irradiation with 550 to 667 nm of light alone had no significant effect on the response. In contrast, intermittent irradiation with red light immediately after each FR light pulse completely reversed the effect of FR light in each cycle. The results indicate that neither red-absorbing phytochrome synthesized in darkness nor photoconverted Pfr are physiologically active, and that a short-lived signal is induced during photoconversion from Pfr to red-absorbing phytochrome. The mode of photoperception by PhyA for HIR is essentially different from that by PhyA for very-low-fluence responses and phytochrome B for low-fluence responses.

A protein tyrosine kinase of chloroplast thylakoid membranes phosphorylates light harvesting complex II proteins

Phosphorylation of chloroplast thylakoid proteins, in particular light harvesting complex II (LHC II), is believed to play an important role in regulating photosynthetic electron transfer. Evidence supporting the involvement of multiple protein kinases in this system is mounting. We have re-examined pea thylakoid membranes and found evidence for a membrane-associated protein tyrosine kinase (PTK). Phosphorylation of many thylakoid proteins, including LHC II, is sensitive to treatment with the tyrosine kinase inhibitor genistein. Anti-phosphotyrosine antibodies react specifically with nine thylakoid proteins, two of which have been identified as components of LHC II. The phosphate associated with these two proteins is also resistant to strong base and acid treatment, further substantiating the assignment of phosphotyrosine. Potential interactions between this novel chloroplast PTK activity and the well-documented threonine kinase activities are discussed and the presence of a cascade of thylakoid protein kinases is proposed.

A model for signal transduction during gamete release in the fucoid alga pelvetia compressa

Fucoid algae release gametes into seawater following an inductive light period (potentiation), and gamete expulsion from potentiated receptacles of Pelvetia compressa began about 2 min after a light-to-dark transition. Agitation of the medium reversed potentiation, with an exponential time course completed in about 3 h. Light regulated two signaling pathways during potentiation and gamete expulsion: a photosynthetic pathway and a photosynthesis-independent pathway in which red light was active but blue light was not. Uptake of K+ appears to have an important role in potentiation, because a 50% inhibition of potentiation occurred in the presence of the tetraethylammonium ion, a K+-channel blocker. A central role of anion channels in the maintenance of potentiation is suggested by the premature release of gametes in the light when receptacles were incubated with inhibitors of slow-type anion channels. An inhibitor of tyrosine kinases, tyrphostin A63, also inhibited potentiation. A model for gamete release from P. compressa is presented that proposes that illumination results in the accumulation of ions (e.g. K+) throughout the cells of the receptacle during potentiation, which then move into the extracellular matrix during gamete expulsion to generate osmomechanical force, resulting in gamete release.