The C-terminal sequence from common bugle leaf galactan:galactan galactosyltransferase is a non-sequence-specific vacuolar sorting determinant

Characteristics and expression patterns of six α-galactosidases in cucumber (Cucumis sativus L.)

Six putative α-galactosidase genes (α-Gals), three acid forms (CsGAL1, CsGAL2, CsGAL3) and three alkaline forms (CsAGA1, CsAGA2, CsAGAL3), were found in the cucumber genome. It is interesting to know the expression pattern and possible function of these α-Gals in the cucumber plant since it is a stachyose-translocating species. In this study, full-length cDNAs of six α-Gals were cloned and heterologously expressed. The result showed that all recombinant proteins revealed acid or alkaline α-Gal activities with different substrate specificities and pH or temperature responding curves, indicating their distinct roles in cucumber plants. Phylogenetic analysis of collected α-Gal amino acid sequences from different plants indicated that the ancestor of both acid and alkaline α-Gals existed before monocots and dicots separated. Generally, six α-Gal genes are universally expressed in different cucumber organs. CsGAL2 highly expressed in fasting-growing leaves, fruits and germinating seeds; CsGAL3 mainly distributes in vacuoles and significantly expressed in cucumber fruits, senescent leaves and seeds during late stage germination; The expression of CsAGA1 increased from leaf 1 to leaf 3 (sink leaves) and then declined from leaf 4 to leaf 7 (source leaves), and this isoform also highly expressed in male flowers and germinating seeds at early stage; CsAGA2 significantly expressed in cucumber leaves and female flowers; CsAGA3 is localized in plastids and also actively expressed in senescent leaves and germinating seeds; The role of CsGAL1 in cucumber plants is now unclear since its expression was relatively low in all organs. According to their expression patterns, subcellular localizations and previously reported functions of these isoforms in other plants, combining the data of soluble sugars contents in different tissues, the possible functions of these α-Gals were discussed.

Abiotic stress regulates expression of galactinol synthase genes post-transcriptionally through intron retention in rice

Expression of the Galactinol synthase genes in rice is regulated through post-transcriptional intron retention in response to abiotic stress and may be linked to Raffinose Family Oligosaccharide synthesis in osmotic perturbation. Galactinol synthase (GolS) is the first committed enzyme in raffinose family oligosaccharide (RFO) synthesis pathway and synthesizes galactinol from UDP-galactose and inositol. Expression of GolS genes has long been implicated in abiotic stress, especially drought and salinity. A non-canonical regulation mechanism controlling the splicing and maturation of rice GolS genes was identified in rice photosynthetic tissue. We found that the two isoforms of Oryza sativa GolS (OsGolS) gene, located in chromosomes 3(OsGolS1) and 7(OsGolS2) are interspersed by conserved introns harboring characteristic premature termination codons (PTC). During abiotic stress, the premature and mature transcripts of both isoforms were found to accumulate in a rhythmic manner for very small time-windows interrupted by phases of complete absence. Reporter gene assay using GolS promoters under abiotic stress does not reflect this accumulation profile, suggesting that this regulation occurs post-transcriptionally. We suggest that this may be due to a surveillance mechanism triggering the degradation of the premature transcript preventing its accumulation in the cell. The suggested mechanism fits the paradigm of PTC-induced Nonsense-Mediated Decay (NMD). In support of our hypothesis, when we pharmacologically blocked NMD, the full-length pre-mRNAs were increasingly accumulated in cell. To this end, our work suggests that a combined transcriptional and post transcriptional control exists in rice to regulate GolS expression under stress. Concurrent detection and processing of prematurely terminating transcripts coupled to repressed splicing can be described as a form of Regulated Unproductive Splicing and Translation (RUST) and may be linked to the stress adaptation of the plant, which is an interesting future research possibility.

Metabolic process of raffinose family oligosaccharides during cold stress and recovery in cucumber leaves

Raffinose family oligosaccharides (RFOs) accumulate under stress conditions in many plants and have been suggested to act as stress protectants. To elucidate the metabolic process of RFOs under cold stress, levels of RFOs, and related carbohydrates, the expression and activities of main metabolic enzymes and their subcellular compartments were investigated during low-temperature treatment and during the recovery period in cucumber leaves. Cold stress induced the accumulation of stachyose in vacuoles, galactinol in vacuoles and cytosol, and sucrose and raffinose in vacuoles, cytosol, and chloroplasts. After cold stress removal, levels of these sugars decreased gradually in the respective compartments. Among four galactinol synthase genes (CsGS), CsGS1 was not affected by cold stress, while the other three CsGSs were up-regulated by low temperature. RNA levels of acid-α-galactosidase (GAL) 3 and alkaline-α-galactosidase (AGA) 2 and 3, and the activities of GAL and AGA, were up-regulated after cold stress removal. GAL3 protein and GAL activity were exclusively located in vacuoles, whereas AGA2 and AGA 3 proteins were found in cytosol and chloroplasts, respectively. The results indicate that RFOs, which accumulated during cold stress in different subcellular compartments in cucumber leaves, could be catabolized in situ by different galactosidases after stress removal.

Localization of RNS2 ribonuclease to the vacuole is required for its role in cellular homeostasis

Localization of the RNase RNS2 to the vacuole via a C-terminal targeting signal is essential for its function in rRNA degradation and homeostasis. RNase T2 ribonucleases are highly conserved enzymes present in the genomes of nearly all eukaryotes and many microorganisms. Their constitutive expression in different tissues and cell types of many organisms suggests a housekeeping role in RNA homeostasis. The Arabidopsis thaliana class II RNase T2, RNS2, is encoded by a single gene and functions in rRNA degradation. Loss of RNS2 results in RNA accumulation and constitutive activation of autophagy, possibly as a compensatory mechanism. While the majority of RNase T2 enzymes is secreted, RNS2 is located within the vacuole and in the endoplasmic reticulum (ER), possibly within ER bodies. As RNS2 has a neutral pH optimum, and the endomembrane organelles are connected by vesicle transport, the site within the endomembrane system at which RNS2 functions is unclear. Here we demonstrate that localization to the vacuole is essential for the physiological function of RNS2. A mutant allele of RNS2, rns2-1, results in production of an active RNS2 RNase but with a mutation that removes a putative C-terminal vacuolar targeting signal. The mutant protein is, therefore, secreted from the cell. This results in a constitutive autophagy phenotype similar to that observed in rns2 null mutants. These findings illustrate that the intracellular retention of RNS2 and localization within the vacuole are critical for its cellular function.

Significance of galactinol and raffinose family oligosaccharide synthesis in plants

Abiotic stress induces differential expression of genes responsible for the synthesis of raffinose family of oligosaccharides (RFOs) in plants. RFOs are described as the most widespread D-galactose containing oligosaccharides in higher plants. Biosynthesis of RFOs begin with the activity of galactinol synthase (GolS; EC 2.4.1.123), a GT8 family glycosyltransferase that galactosylates myo-inositol to produce galactinol. Raffinose and the subsequent higher molecular weight RFOs (Stachyose, Verbascose, and Ajugose) are synthesized from sucrose by the subsequent addition of activated galactose moieties donated by Galactinol. Interestingly, GolS, the key enzyme of this pathway is functional only in the flowering plants. It is thus assumed that RFO synthesis is a specialized metabolic event in higher plants; although it is not known whether lower plant groups synthesize any galactinol or RFOs. In higher plants, several functional importance of RFOs have been reported, e.g., RFOs protect the embryo from maturation associated desiccation, are predominant transport carbohydrates in some plant families, act as signaling molecule following pathogen attack and wounding and accumulate in vegetative tissues in response to a range of abiotic stresses. However, the loss-of-function mutants reported so far fail to show any perturbation in those biological functions. The role of RFOs in biotic and abiotic stress is therefore still in debate and their specificity and related components remains to be demonstrated. The present review discusses the biology and stress-linked regulation of this less studied extension of inositol metabolic pathway.

Frost tolerance in excised leaves of the common bugle (Ajuga reptans L.) correlates positively with the concentrations of raffinose family oligosaccharides (RFOs)

Mass increases in raffinose family oligosaccharides (RFOs, alpha1,6-galactosyl extensions of sucrose) are well documented in the generative tissues of many plants upon cold acclimation, and they (i.e. mainly the two shortest RFO members, raffinose and stachyose) have been suggested as frost stress protectants. Our focus here was on the longer RFO members as they commonly occur in the frost-hardy evergreen labiate Ajuga reptans in its natural habitat, and accumulate to their highest concentrations in winter when the plant is faced with sub-zero temperatures. We examined the effects of RFO concentration and chain length on frost tolerance using excised leaves which accumulate long-chain RFOs under both cold and warm conditions, thereby uncoupling the acclimation temperature from RFO production. We demonstrated that frost tolerance in excised A. reptans leaves correlates positively with long-chain RFO accumulation under both acclimation temperatures. After 24 d post-excision in the warm, the leaves had increased their RFO concentrations (mainly long-chain RFOs) 22-fold to 78 mg g(-1) fresh weight, and decreased their EL(50) values (temperature at which 50% leakage occurred) from -10.5 to -24.5 degrees C, suggesting a protective role for these oligosaccharides in the natural frost tolerance of A. reptans.