EXPRESSION OF THE PHLOEM LECTIN IS DEVELOPMENTALLY LINKED TO VASCULARDIFFERENTIATION IN CUCURBITS

The conducting elements of phloem in angiosperms are a complex of two cell types, sieve elements and companion cells, that form a single dev elopmental and functional unit. During ontogeny of the sieve element/c ompanion cell complex, specific proteins accumulate forming unique str uctures within sieve elements. Synthesis of these proteins coincides w ith vascular development and was studied in Cucurbita seedlings by fol lowing accumulation of the phloem lectin (PP2) and its mRNA by RNA blo t analysis, enzyme-linked immunosorbent assay, immunocytochemistry and in situ hybridization. Genes encoding PP2 were developmentally regula ted during vascular differentiation in hypocotyls of Cucurbita maxima Duch. Accumulation of PP2 mRNA and protein paralleled one another duri ng hypocotyl elongation, after which mRNA levels decreased, while the protein appeared to be stable. Both PP2 and its mRNA were initially de tected during metaphloem differentiation. However, PP2 mRNA was detect ed in companion cells of both bundle and extrafascicular phloem, but n ever in differentiating sieve elements. At later stages of development , PP2 mRNA was most often observed in extrafascicular phloem. In devel oping stems of Cucurbita moschata L., PP2 was immunolocalized in compa nion cells but not to filamentous phloem protein (P-protein) bodies th at characterize immature sieve elements of bundle phloem. In contrast, PP2 was immunolocalized to persistent P-protein bodies in sieve eleme nts of the extrafascicular phloem. Immunolocalization of PP2 in mature wound sieve elements was similar to that in bundle phloem. It appears that PP2 is synthesized in companion cells, then transported into dif ferentiated sieve elements where it is a component of P-protein filame nts in bundle phloem and persistent P-protein bodies in extrafascicula r phloem. This differential accumulation in bundle and extrafascicular elements may result from different functional roles of the two types of phloem.