Department of Plant Biology, Cornell University, Ithaca, New York 14853 USA.
Am J Bot. 2001 Aug;88(8):1331-9.
Phylogenetic analysis provides a rational basis for comparative studies of phloem structure and phloem loading. Although several types of minor vein companion cell have been identified, and progress has been made in correlating structural features of these cells with loading mechanisms, little is known about the phylogenetic relationships of the different types. To add to the available data on companion cells, we analyzed the ultrastructure of minor veins in Euonymus fortunei and Celastrus orbiculatis (Celastraceae) leaves and determined that in these species they are specialized as intermediary cells. This cell type has been implicated in symplastic phloem loading. The data were added to published data sets on minor vein phloem characteristics, which were then mapped to a well-supported molecular tree. The analysis indicates that extensive plasmodesmatal continuity between minor vein phloem and surrounding cells is ancestral in the angiosperms. Reduction in plasmodesmatal frequency at this interface is a general evolutionary trend, punctuated by instances of the reverse. This is especially true in the case of intermediary cells that have many plasmodesmata, but other distinguishing characteristics as well, and have arisen independently at least four, and probably six, times in derived lineages. The character of highly reduced plasmodesmatal frequency in minor vein phloem, common in crop plants, has several points of origin in the tree. Thus, caution should be exercised in generalizing results on apoplastic phloem loading obtained from model species. Transfer cells have many independent points of origin, not always from lineages with reduced plasmodesmatal frequency.
系统发生分析为韧皮部结构和韧皮部装载的比较研究提供了合理的依据。虽然已经鉴定出几种类型的小叶脉伴胞，并且在将这些细胞的结构特征与装载机制相关联方面取得了进展，但对于不同类型的系统发生关系却知之甚少。为了增加关于伴胞的可用数据，我们分析了卫矛属和南蛇藤属（卫矛科）叶片中小脉的超微结构，确定在这些物种中，它们是专门的中间细胞。这种细胞类型被认为与共质体韧皮部装载有关。将这些数据添加到已发表的关于小叶脉韧皮部特征的数据集上，然后将其映射到一个支持良好的分子树上。分析表明，在被子植物中，小叶脉韧皮部与周围细胞之间广泛的胞间连丝连续性是祖先的。在这个界面上胞间连丝频率的减少是一个普遍的进化趋势，偶尔会出现相反的情况。这在中间细胞中尤其如此，中间细胞有许多胞间连丝，但也有其他区别特征，并且至少在四个，可能是六个，衍生谱系中独立出现。在作物植物中常见的小叶脉韧皮部中高度降低的胞间连丝频率的特征在树中有几个起源点。因此，在从模式物种获得的质外体韧皮部装载的结果上进行概括时应谨慎。转移细胞有许多独立的起源点，并不总是来自胞间连丝频率降低的谱系。
