Identification of immunologically related proteins in sieve-tube exudate collected from monocotyledonous and dicotyledonous plants

Abstract

The mature, functional sieve-tube system in higher plants is dependent upon protein import from the companion cells to maintain a functional long-distance transport system. Soluble proteins present within the sieve-tube lumen were investigated by analysis of sievetube exudates which revealed the presence of distinct sets of polypeptides in seven monocotyledonous and dicotyledonous plant species. Antibodies directed against sieve-tube exudate proteins from Ricinus communis L. demonstrated the presence of shared antigens in the phloem sap collected from Triticum aestivum L., Oryza sativa L., Yucca ®lamentosa L., Cucurbita maxima Duch., Robinia pseudoacacia L. and Tilia platyphyllos L. Speci®c antibodies were employed to identify major polypeptides. Molecular chaperones related to Rubiscosubunit-binding protein and cyclophilin, as well as ubiquitin and the redox proteins, thioredoxin h and glutaredoxin, were detected in the sieve-tube exudate of all species examined. Actin and pro®lin, a modulator of actin polymerization, were also present in all analyzed phloem exudates. However, some proteins were highly species-speci®c, e.g. cystatin, a protease-inhibitor was present in R. communis but was not detected in exudates from other species, and orthologs of the well-known squash phloem lectin, phloem protein 2, were only identi®ed in the sieve-tube exudate of R. communis and R. pseudoacacia. These ®ndings are discussed in terms of the likely roles played by phloem proteins in the maintenance and function of the enucleate sieve-tube system of higher plants.

Lost in Transit: Long-Distance Trafficking and Phloem Unloading of Protein Signals in Arabidopsis Homografts

“…We think it is very unlikely that the large number of diverse proteins present in phloem exudate each contain a signal that interacts with the pore-plasmodesmata between CC and SE, but rather that the organelle-targeting sequences for proteins expressed in CCs are insufficiently strong to prevent their entry into the translocation stream. In previous studies, it has been suggested that exudate proteins with strong organelle-targeting sequences may be artifacts of sample preparation, emanating from nonphloem tissues near the cut ends of stems or petioles (Schobert et al, 1998;Lin et al, 2009) or resulting from sudden pressure release of the phloem during wounding (Oparka and Turgeon, 1999). Similarly, it has been argued that some of the proteins detected by aphid stylectomy might be artifactual as they have no obvious signaling or protein turnover functions within SEs (Atkins et al, 2011).…”

Forisomes, a novel type of Ca2+‐dependent contractile protein motor

“…Sieve tubes consist of sieve elements, highly specialized cells that lose their nucleus, dictyosomes, central vacuole, and microtubules during differentiation [Sjolund, 1997]. Microfilaments also vanish during sieve element maturation [Parthasarathy and Pesacreta, 1980], while, intriguingly, actin and profilin seem present in sieve tube exudates [Schobert et al, 1998]. Abutting sieve elements develop numerous pores in the cell walls between them, giving rise to the sieve plates that are characteristic of this tissue.…”

A Novel Type of Thioredoxin Dedicated to Symbiosis in Legumes

“…The plant Trx family is divided into six different types according to the primary structure and localization of its members: the f, m, x, and y types are in the chloroplasts (Buchanan, 1991;Mestres-Ortega and Meyer, 1999;Lemaire et al, 2003b), the o type is in the mitochondria (Laloi et al, 2001), and the h type (Johnson et al, 1987;Florencio et al, 1988;RiveraMadrid et al, 1995) is mainly found in the cytosol and the phloem sap (Ishiwatari et al, 1995;Schobert et al, 1998), although members of this type have also recently been demonstrated to be targeted to mitochondria or the extracellular matrix (Gelhaye et al, 2004b;Juarez-Diaz et al, 2006). The h type, which has been further divided in three groups (Gelhaye et al, 2004a), is the largest type with 11 isoforms in Arabidopsis.…”