P PROTEIN IN THE PHLOEM OF CUCURBITA

Abstract

During maturation of sieve elements in Cucurbita maxima Duchesne, the P-protein bodies (slime bodies) usually disperse in the tonoplast-free cell. In some sieve elements the P-protein bodies fail to disperse. The occurrence of dispersal or nondispersal of P-protein bodies can be related to the position of the sieve elements in the stem or petiole. In the sieve elements within the vascular bundle the bodies normally disperse; in the extrafascicular sieve elements the bodies often fail to disperse. Extrafascicular sieve elements showing partial dispersal also occur. The appearance of the sieve plate in fixed material is related to the degree of dispersal or nondispersal of the P-protein bodies. In sieve elements in which complete dispersal occurs the sieve plate usually has a substantial deposit of callose, and the sieve-plate pores are filled with P protein. In sieve elements containing nondispersing P-protein bodies the sieve plate bears little or no callose, and its pores usually are essentially "open." The dispersed P-protein components may aggregate into loosely organized "strands," which sometimes extend vertically through the cell and continue through the sieve-plate pores; but they may be oriented otherwise in the cell, even transversely.

The Origin and Composition of Cucurbit “Phloem” Exudate

“…However, from the figures presented by Crafts (1932) and Kempers et al (1993), it is reasonable to assume that no more than 200 longitudinally oriented extrafascicular sieve elements are severed when the petiole is cut. Since the diameter of a single extrafascicular sieve element is approximately 20 mm 2 (measured in micrographs from Crafts [1932] and Cronshaw and Esau [1968]), the total cross-sectional area of extrafascicular sieve elements in the petiole is approximately 6.3 3 10 4 mm 2 , 3.4 times the area of the fascicular sieve elements. Exudation of 59.4 mL, therefore, would drain the volume of extrafascicular sieve elements in 0.9 m of phloem, again an excessive figure.…”

Sieve element occlusion(SEO) genes encode structural phloem proteins involved in wound sealing of the phloem

“…The synthesis of P-proteins begins in immature, nucleated sieve elements (5), resulting in electron-dense proteinaceous structures. Their ultrastructural characteristics have been described variously as granular, fibrillar, or tubular, even within the same cell (6)(7)(8)(9)(10)(11)(12)(13)(14), and today these structures are thought to represent the various different stages of P-protein differentiation (3). In young sieve elements, subunits accumulate within the cytoplasm, forming large P-protein bodies (9).…”

Micromorfología de los foliolos de pejibaye Bactris gasipaes (Arecaceae) var. Diamantes-10

“…Se visualizaron células compañeras con plasmodesmatas ramificados unidos a un elemento de los tubos cribosos, el cual en su interior presenta filamentos electrón-densos que corresponden a cuerpos de proteína (P-proteína) (Fig. 7B) (Cronshaw & Esau, 1968).…”

“…En este sentido, haces vasculares compuestos por un vaso agrandado de metaxilema con vasos menores de protoxilema, acompañados de floema y rodeado de parénquima con un anillo de células esclerenquimatosas, es la disposición típica de un haz vascular de una planta C 3 (Lambers, Pons, & Chapin, 2008). Los cuerpos de P-proteína observada en los elementos de los tubos cribosos es una proteína característica de este tipo de células, en este caso se observó el tipo fibrilar y no el tubular (Cronshaw & Esau, 1968;Algan & Bakar, 2002).…”