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The phloem unloading pathway remains unclear in fleshy fruits accumulating a high level of soluble sugars. A structural investigation in apple fruit (Malus domestica Borkh. cv Golden Delicious) showed that the sieve element-companion cell complex of the sepal bundles feeding the fruit flesh is symplasmically isolated over fruit development. 14 C-autoradiography indicated that the phloem of the sepal bundles was functional for unloading. Confocal laser scanning microscopy imaging of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands of the sepal bundles from the basal to the apical region of the fruit. A 52-kD putative monosaccharide transporter was immunolocalized predominantly in the plasma membrane of both the sieve elements and parenchyma cells and its amount increased during fruit development. A 90-kD plasma membrane H 1 -ATPase was also localized in the plasma membrane of the sieve element-companion cell complex. Studies of [ 14 C]sorbitol unloading suggested that an energy-driven monosaccharide transporter may be functional in phloem unloading. These data provide clear evidence for an apoplasmic phloem unloading pathway in apple fruit and give information on the structural and molecular features involved in this process.The partitioning of sugars in economically important sink organs such as fruits or seeds is governed by several complex physiological processes, including photosynthetic rate, phloem loading in the source leaf, long-distance translocation in the phloem, phloem unloading in sink organs, postphloem transport, and metabolism of imported sugars in sink cells (Oparka, 1990;Patrick, 1997). It is now well accepted that phloem unloading plays a key role in the partitioning of photoassimilate (Fisher and Oparka, 1996;Patrick, 1997;Viola et al., 2001). The process of phloem unloading has been studied extensively over the last 20 years (for review, see Oparka, 1990;Patrick, 1997;Schulz, 1998) but still remains poorly understood. Elucidation of the cellular pathway of phloem unloading is central to this process, because, to a large extent, the unloading path determines the key transport events responsible for assimilate movement from the sieve elements (SEs) to the recipient sink cells (Fisher and Oparka, 1996;Patrick, 1997). A symplasmic phloem unloading pathway predominates in most sink tissues such as vegetative apices (Oparka et al., 1995;Patrick, 1997;Imlau et al., 1999), sink leaves (Roberts et al., 1997;Imlau et al., 1999;Haupt et al., 2001), and potato tubers that represent a typical terminal vegetative storage sink (Oparka and Santa Cruz, 2000;Viola et al., 2001). Symplasmic unloading is also efficient in the maternal tissues of developing seeds that represent a class of terminal reproductive storage sinks (Ellis et al., 1992;Patrick et al., 1995;Wang et al., 1995a;Patrick, 1997), although transfer of solutes to the apoplasm may occur at some point along the postphloem pathway (Patrick, 1997). In some cases, symplasmic unloading also occurs in elongating z...

SUMMARYSeveral genes encoding transcription factors have been shown to be essential for male fertility in plants, suggesting that transcriptional regulation is a major mechanism controlling anther development in Arabidopsis. DYSFUNCTIONAL TAPETUM 1 (DYT1), a putative bHLH transcription factor, plays a critical role in regulating tapetum function and pollen development. Here, we compare the transcriptomes of young anthers of wild-type and the dyt1 mutant, demonstrating that DYT1 is upstream of at least 22 genes encoding transcription factors and regulates the expression of a large number of genes, including genes involved in specific metabolic pathways. We also show that DYT1 can bind to DNA in a sequence-specific manner in vitro, and induction of DYT1 activity in vivo activated the expression of the downstream transcription factor genes MYB35 and MS1. We generated DYT1-SRDX transgenic plants whose fertility was dramatically reduced, implying that DYT1 probably acts as a transcriptional activator. Furthermore, we used yeast two-hybrid assays to show that DYT1 forms homodimers and heterodimers with other bHLH transcription factors. Our results demonstrate the important role of DYT1 in regulating anther transcriptome and function, and supporting normal pollen development.

Developmental analyses reveal early arrests of the spore-bearing parts of reproductive organs in unisexual flowers of cucumber (Cucumis sativus L.)Abstract To understand the regulatory mechanisms governing unisexual flower development in cucumber, we conducted a systematic morphogenetic analysis of male and female flower development, examined the dynamic changes in expression of the C-class floral organ identity gene CUM1, and assessed the extent of DNA damage in inappropriate carpels of male flowers. Accordingly, based on the occurrence of distinct morphological events, we divided the floral development into 12 stages ranging from floral meristem initiation to anthesis. As a result of our investigation we found that the arrest of stamen development in female flowers, which occurs just after the differentiation between the anther and filament, is mainly restricted to the primordial anther, and that it is coincident with downregulation of CUM1 gene expression. In contrast, the arrest of carpel development in the male flowers occurs prior to the differentiation between the stigma and ovary, given that no indication of ovary differentiation was observed even though CUM1 gene expression remained detectable throughout the development of the stigma-like structures. Although the male and female reproductive organs have distinctive characteristics in terms of organ differentiation, there are two common features regarding organ arrest. The first is that the arrest of the inappropriate organ does not affect the entirety of the organ uniformly but occurs only in portions of the organs. The second feature is that all the arrested portions in both reproductive organs are spore-bearing parts.