sucrose, glucose, and fructose), the raffinose family oligosaccharides [RFOs (such as stachyose and raffinose)], and polyols (such as sorbitol, pinitol, and mannitol). For instance, simple sugars not only act as osmolytes but can also stabilize membranes
W. Roland Leatherwood, D. Mason Pharr, Lisa O. Dean, and John D. Williamson
Paul H. Jennings and Cecil Stushnoff
Various carbohydrates have been shown to be associated with stress tolerance in some plant species. Specifically, the content of soluble sugars have been correlated with desiccation tolerance and winter hardiness. We have previously demonstrated that radicles of cucumber seed become progressively more sensitive to chilling injury during the early stages of germination and that cultivar differences exist. Sucrose, raffinose, and stachyose contents of `Poinsett 76' and `Ashley' seed were determined in dry seed during imbibition and at three stages of radicle emergence. The more chilling-tolerant cultivar (Ashley) contained lower raffinose and higher stachyose contents than the less chilling-tolerant `Poinsett 76'. In both cultivars, the contents of raffinose and stachyose declined dramatically between the 1-mm and 5- to 7-mm stage of radicle emergence. At the 1-mm stage, when cultivar chilling-tolerance differences are most pronounced, `Ashley' appears to have a higher content of stachyose and lower raffinose content.
Bruce W. Wood
[retention time (RT) = 7.0 min, fructose (RT = 7.6 min), sorbitol (RT = 8.4 min), glucose (RT = 9.0 min), sucrose (RT = 10.5 min), raffinose (RT = 13.7 min), and stachyose (RT = 16.6 min)] based on the same retention times exhibited by both pure and mixed
Gayle M. Volk, Edith E. Haritatos, and Robert Turgeon
Raffinose family oligosaccharides (RFOs) perform several physiological functions in plants. In addition to accumulating during seed formation, raffinose and stachyose are translocated in the phloem and may accumulate in response to low temperatures, drought, or salt stress. Although the synthesis of galactinol, as mediated by galactinol synthase (GAS), is the first committed step in RFO formation, its expression patterns are poorly understood in most species. We have cloned and characterized the expression of two galactinol synthase gene family members in melon (Cucumis melo L. Cantalupensis Group). Both CmGAS1 and CmGAS2 are highly expressed in mature leaves. Galactinol synthase transcription in leaves was not upregulated by either water or low temperature stresses. Transcripts of CmGAS1 were present in developing melon seeds at a time coincident with the formation of raffinose and stachyose. Based on the GAS expression and RFO accumulation patterns, we propose that RFOs in melon function in carbon translocation and seed desiccation tolerance.
Virgil Esensce and Daniel I. Leskovar
Spinach (Spinacia oleracea, L. cv. `Ark88-354'. `Fall Green', `Cascade') seeds of varying sensitivities to high temperatures during imbibition and germination were subjected to constant 18, 30 and 36°C for 96 hours during imbibition. Those cultivars less sensitive to high temperatures (`Ark88-354' and `Fall Green') imbibed water more rapidly at higher temperatures and had greater initial levels of raffinose and sucrose than the sensitive cultivar `Cascade'. Glucose levels were initially zero in all cultivars and increased slightly with time. Germination was more rapid at 18°C and 30°C in `Ark88-354' and `Fall Green' than with `Cascade'; the latter also failed 10 germinate at the higher temperature. Raffinose and sucrose have been implicated in membrane stabilization during desiccation and extreme low temperatures. They may serve a similar role during imbibition and germination of spinach at high temperatures, reducing secondary thermodormancy.
Joyce C. Pennycooke, Ramarao Vepachedu, Cecil Stushnoff, and Michelle L. Jones
Previous studies of plant tolerance to low temperature have focused primarily on the cold acclimation response, the process by which plants increase their tolerance to freezing in response to low nonfreezing temperatures, while studies on the deacclimation process have been largely neglected. In some plants, cold acclimation is accompanied by an increase in raffinose family oligosaccharides (RFO). The enzyme α-galactosidase (EC 126.96.36.199) breaks down RFO during deacclimation by hydrolyzing the terminal galactose moieties. Here we describe the isolation of PhGAL, an α-galactosidase cDNA clone from Petunia (Petunia ×hybrida `Mitchell'). The putative α-galactosidase cDNA has high nucleotide sequence homology (>80%) to other known plant α-galactosidases. PhGAL expression increased in response to increased temperature and there was no evidence of developmental regulation or tissue specific expression. Increases in α-galactosidase transcript 1 hour into deacclimation corresponded with increases in α-galactosidase activity and a concomitant decrease in raffinose content, suggesting that warm temperature may regulate RFO catabolism by increasing the transcription of the α-galactosidase gene. This information has potential practical applications whereby α-galactosidase may be targeted to modify endogenous raffinose accumulation in tissues needed for freezing stress tolerance.
Shahidul Islam, Carmen Rafaela, and James Garner
Twenty-five cultivars were initially screened for germination at 10 °C, 30 °C, and 40 °C. Four cultivars were chosen for further study for physiological and biochemical characteristics—namely, `Texas Cream 40' (TC-40), which showed ability to germinate at very high (40 °C) and low (10 °C) temperatures; `Black Crowder' (BC), which had acceptably high germination at 40 °C, but reduced germination at 10 °C; and `Mississippi Purple' (MP), which exhibited lower germination at all temperatures tested. The main sugars present in cowpea seed were sucrose, raffinose, and stachyose. Sugar contents were affected by cultivar, type of tissue, and temperature. Sucrose contents were higher in embryo tissue of cultivars with a lower germination percentage, and reduced in the cultivar with a higher germination percentage, suggesting the use of sucrose for germination. Sucrose decreased greatly at 30 °C and increased again at 40 °C. Sucrose “de novo” synthesis was higher at higher temperature. An accumulation of sucrose was evident in embryo tissues of cultivars with reduced ability to germinate at low temperature. Raffinose and stachyose contents were higher in ungerminated seed. In germinated seed, raffinose and stachyose contents were found only in cotyledon tissues at 10 °C. The peroxidase activity was affected by cultivars, type of tissue, and temperature. The highest peroxidase activity was found at low temperature (10 °C) in embryo tissue of the cultivar with the highest germination. The result also suggests that high peroxidase activity was related to ability of seed to germinate at low temperature.
C.L. Haynes, O.M. Lindstrom, and M.A. Dirr
Decreasing photoperiods and decreasing temperatures induce cold acclimation and the accumulation of soluble sugars in many plants. Two cultivars of southern magnolia differing in cold hardiness and acclimation patterns, were monitored to determine photoperiod × temperature interaction on cold hardiness and soluble sugar content. Cold hardiness increased with low temperatures and short photoperiods. Total soluble sugars, sucrose, and raffinose consistently increased in the leaves and stems of both cultivars in response primarily to low temperature. `Little Gem' was less responsive to photoperiod than `Claudia Wannamaker'
Canes of three field-grown cultivars of red raspberry (Rubus idaeus L. `Maurin Makea', `Ottawa', and `Muskoka') were sampled from October to April. Carbohydrate contents of canes and flower buds were analyzed, and cold hardiness (LT50) was determined by controlled freezing. Starch, sucrose, glucose, fructose, and minor amounts of raffinose and stachyose were present in both cane and bud tissues. Glucose and fructose were the predominant sugars in buds. In canes, the proportion of sucrose of all sugars was greater than in buds. Seasonal changes in carbohydrates were related to changes in cold hardiness and mean air temperature during a 5-day period preceding sampling. Starch decreased during fall and was barely detectable in midwinter. Soluble carbohydrates accumulated to 73 to 89 mg·g-1 dry weight in canes and 113 to 131 mg·g-1 dry weight in buds in midwinter. The most striking increase occurred in the concentration of sucrose, but glucose, fructose, raffinose, and stachyose also accumulated. There was a positive correlation between LT50 and the amount of starch, but a negative correlation between LT50 and the amounts of total soluble carbohydrates, sucrose, glucose, and fructose. High levels of sucrose, total soluble carbohydrates, and a high ratio of sucrose to glucose plus fructose were characteristic of a hardy cultivar. Results are evidence of the importance of carbohydrate reserves, especially sucrose, on winter survival of red raspberry.
Cecil Stushnoff, Richard L. Remmele Jr., and V. Esensee
Aqueous fractions in dormant buds of Amelanchier alnifolia Nutt. `Smoky', may exist either as liquid, ice or glass phases depending on the temperature history and the water content of the tissue. Phase diagrams for these states were constructed from differential scanning calorimetry (DSC) freezing and warming scans. The diagrams show that glass transition temperatures shift to warmer temperatures as cold hardening increases and as the water content is lowered by controlled desiccation. Glass transitions were detected from -60 to -20° C, during slow freezing scans in the DSC, suggesting that survival of this extremely cold hardy tissue is based upon a potential to undergo glass transitions in the dormant state. Endogenous raffinose family oligosaccharides (RFO) increase during cold hardening, and decrease as hardiness diminishes with the onset of growth.