Increasing evidence suggests that proteins present in the angiosperm sieve tube system play an important role in the long distance signaling system of plants. To identify the nature of these putatively non-cell-autonomous proteins, we adopted a large scale proteomics approach to analyze pumpkin phloem exudates. Phloem proteins were fractionated by fast protein liquid chromatography using both anion and cation exchange columns and then either in-solution or in-gel digested following further separation by SDS-PAGE. A total of 345 LC-MS/MS data sets were analyzed using a combination of Mascot and X!Tandem against the NCBI non-redundant green plant database and an extensive Cucurbit maxima expressed sequence tag database. In this analysis, 1,209 different consensi were obtained of which 1,121 could be annotated from GenBank TM and BLAST search analyses against three plant species, Arabidopsis thaliana, rice (Oryza sativa), and poplar (Populus trichocarpa). Gene ontology (GO) enrichment analyses identified sets of phloem proteins that function in RNA binding, mRNA translation, ubiquitinmediated proteolysis, and macromolecular and vesicle trafficking. Our findings indicate that protein synthesis and turnover, processes that were thought to be absent in enucleate sieve elements, likely occur within the angiosperm phloem translocation stream. In addition, our GO analysis identified a set of phloem proteins that are associated with the GO term "embryonic development ending in seed dormancy"; this finding raises the intriguing question as to whether the phloem may exert some level of control over seed development. The universal significance of the phloem proteome was highlighted by conservation of the phloem proteome in species as diverse as monocots (rice), eudicots (Arabidopsis and pumpkin), and trees (poplar). These results are discussed from the perspective of the role played by the phloem proteome as an integral component of the whole plant communication system.

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A, the RSA should be 0.9 if labeling is the same as that expected from averufin (Lin et al., 1973). In our experiments the quite low RSA for aflatoxin made from acetate (0.007) compared to the relatively high RSA (0.475) for aflatoxin made from the labeled versicolorin A pigment offers additional strong evidence that this pigment is incorporated essentially intact and is not broken down into acetate units before incorporation into aflatoxin . report 45-58% conversion of sterigmatocystin to aflatoxin and conclude that sterigmatocystin or a closely related metabolite is an intermediate in the biosynthesis of aflatoxins. The 46% conversion of versicolorin A to aflatoxin found in our experiments suggests that this pigment is as efficiently converted to aflatoxin as is sterigmatocystin and offers experimental proof to the theory hypothesized by Heathcote et al. (1973) in which they propose versicolorin A as a precursor to aflatoxin .

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