ABSTRACT
An arabinogalactan-protein (AGP) was purified from differentiating xylem of loblolly pine (Pinus taeda L.) and the N-terminal sequence used to identify a cDNA clone. The protein, PtaAGP3, was not coded for by any previously identified AGP-like genes. Moreover, PtaAGP3 was abundantly and preferentially expressed in differentiating xylem. The encoded protein contains four domains, a signal peptide, a cleaved hydrophilic region, a region rich in serine, alanine, and proline/hydroxyproline, and a hydrophobic C-terminus. It is postulated to contain a GPI (glycosylphosphatidylinositol) anchor site. If the protein is cleaved at the putative GPI anchor site, as has been observed in other classical AGPs, all but the Ser-Ala-Pro/Hyp-rich domain may be missing from the mature protein. Xylem-specific AGPs are hypothesized to be involved in xylem development.
Subject(s)
Mucoproteins/isolation & purification , Plant Proteins/isolation & purification , Amino Acid Sequence , Blotting, Northern , Cloning, Molecular , Electrophoresis, Polyacrylamide Gel , Molecular Sequence Data , Mucoproteins/chemistry , Mucoproteins/metabolism , Pinus taeda , Plant Proteins/chemistry , Plant Proteins/metabolism , RNA, Plant/analysisABSTRACT
The amino acid, proline (PRO), may have an important role in plant adaptation to drought stress. To study the effect of drought stress on PRO content of pine tissues, we measured free PRO by high pressure liquid chromatography in control ( -0.4 MPa) and drought-stressed ( -0.8, -1.0, -1.3, -1.6 MPa) callus tissue of Pinus taeda L. after eight weeks of growth. Drought stress was induced by adding polyethylene glycol (PEG) to the nutrient media. The relation between PRO accumulation and water potential was influenced by composition of the medium and temperature. Callus growing in media with water potentials of -0.8 MPa showed no increase in PRO compared to control callus in media at -0.4 MPa. When callus tissue was subjected to low water potentials (-1.6 MPa), endogenous PRO concentration increased 40-fold, while callus fresh weight decreased by more than 90%. When exogenous PRO was added to the nutrient media, endogenous PRO was directly proportional to the exogenous PRO concentration rather than reduction in callus growth. Thus low water potential in callus results in endogenous PRO accumulation and large reductions in callus fresh weight growth. Proline accumulation in pine cells appears to be related to mechanisms associated with tolerance to desiccation.
ABSTRACT
Eight cultivars Sorghum bicolor (L.) Moench were grown as callus cultures under induced, prolonged water stress (8 weeks), with polyethylene glycol in the medium. Concentrations of soluble carbohydrates and organic acids in callus were measured at the end of the growth period to determine differences in response to prolonged water stress. Sucrose, glucose, fructose, and malate were the predominant solutes detected in all callus at all water potentials. All cultivars had high levels of solutes in the absence of water stress and low levels in the presence of prolonged water stress. However, at low water potentials, low levels of solutes were observed in drought-tolerant cultivar callus and high solute levels were observed in drought-susceptible cultivar callus. Estimated sucrose concentrations were significantly higher in water-stressed, susceptible cultivar callus. Large solute concentrations in susceptible cultivar callus were attributed to osmotic adjustment and/or reduced growth during water stress.
