In view of recent findings, it is still a matter of debate whether the composition of the phloem sap of higher plants is specific and based on a plasmodesmal selectivity filter for macromolecular transport, or whether simply related to size, abundance and half-life of the macromolecules within the phloem sap. A range of reports indicates specific function of phloem-mobile signaling molecules such as the florigen making it indispensable to discriminate specific macromolecules entering the phloem from others which cannot cross this selectivity filter. Nevertheless, several findings have discussed for a non-selective transport via plasmodesmata, or contamination of the phloem sap by degradation products coming from immature still developing young sieve elements undergoing differentiation. Here, we discuss several possibilities, and raise the question how selectivity of the phloem sap composition could be achieved thereby focusing on mobility and dynamics of sucrose transporter mRNA and proteins.

中文翻译：

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什么决定韧皮部汁液的组成？进入韧皮部筛子元件的大分子有选择性过滤器吗？

鉴于最近的发现，高等植物韧皮部液汁的组成是否特定且基于大分子运输的胞质选择性滤膜还是仅与大麦的大小，丰度和半衰期有关，尚有争议。韧皮部汁液中的大分子。一系列报道表明韧皮部移动信号分子（例如荧光素）的特定功能使得区分进入韧皮部的特定大分子与无法穿过该选择性过滤器的其他大分子是必不可少的。然而，已经讨论了一些发现，这些信息涉及通过纤毛虫的非选择性转运，或韧皮部汁液被未成熟但仍在发育的年轻筛分元件经历分化的降解产物污染。在这里，我们讨论几种可能性，