Complexity untwined: The structure and function of cucumber (Cucumis sativus L.) shoot phloem

Abstract

Cucurbit phloem is complex, with large sieve tubes on both sides of the xylem (bicollateral phloem), and extrafascicular elements that form an intricate web linking the rest of the vasculature. Little is known of the physical interconnections between these networks or their functional specialization, largely because the extrafascicular phloem strands branch and turn at irregular angles. Here, export in the phloem from specific regions of the lamina of cucumber (Cucumis sativus L.) was mapped using carboxyfluorescein and 14 C as mobile tracers. We also mapped vascular architecture by conventional microscopy and X-ray computed tomography using optimized whole-tissue staining procedures. Differential gene expression in the internal (IP) and external phloem (EP) was analyzed by laser-capture microdissection followed by RNA-sequencing. The vascular bundles of the lamina form a nexus at the petiole junction, emerging in a predictable pattern, each bundle conducting photoassimilate from a specific region of the blade. The vascular bundles of the stem interconnect at the node, facilitating lateral transport around the stem. Elements of the extrafascicular phloem traverse the stem and petiole obliquely, joining the IP and EP of adjacent bundles. Using pairwise comparisons and weighted gene coexpression network analysis, we found differences in gene expression patterns between the petiole and stem and between IP and EP, and we identified hub genes of tissue-specific modules. Genes related to transport were expressed primarily in the EP while those involved in cell differentiation and development as well as amino acid transport and metabolism were expressed mainly in the IP.

Disruption of the amino acid transporter CsAAP2 inhibits auxin‐mediated root development in cucumber

“…In our previous researches, the CsAAP3 (Sui et al ., 2018), which was mainly expressed in the internal phloem of fruit vascular bundles, and CsAAP6 (Sui et al ., 2021), which was most likely involved in amino acid transport in the external phloem of petiole, have been identified using transcriptome data. To examine tissue‐specific expression of cucumber CsAAPs , absolute qRT‐PCR was performed in various organs (Fig.…”

“…Although relatively extensive progress has been made in identification and functional characterization of amino acid transporters in Arabidopsis, rice, and other plants, few transporters were functionally characterized in cucumber. In previous works, we reported on the identification of CsAAP3 and CsAAP6 , which are associated with amino acid transport in vasculature of fruit and petiole respectively (Sui et al ., 2018, 2021). In this study, the CsAAP2 was found to be highly expressed in cucumber pericycle cells and closely related to root development.…”

CsSWEET2, a Hexose Transporter from Cucumber (Cucumis sativus L.), Affects Sugar Metabolism and Improves Cold Tolerance in Arabidopsis

“…In line with the rapid growth of SWEETs research in Arabidopsis [ 2 , 13 , 14 , 28 , 29 ], rice [ 11 ], and maize [ 30 , 31 , 32 ], studies on SWEET homologues in other plant species such as tomato [ 33 ], watermelon [ 25 ], and the tea plant [ 15 , 16 ] have been reported, and findings have been updated. Although SWEETs are important versatile regulators and affect yield potential, only two cucumber CsSWEET genes have been reported in detail to date [ 21 , 22 , 23 ]. In this study, we characterized the CsSWEET2 gene in cucumber and found that overexpression of CsSWEET2 in Arabidopsis improves tolerance to cold stress.…”

“…Owing to its tropical origin and heat-loving habit, cucumber is extremely sensitive to cold stress. In previous studies, 17 CsSWEET genes were identified in the cucumber genome by bioinformatic analysis [ 19 , 20 ], among which are CsSWEET7a , responsible for sugar phloem unloading in cucumber fruits, receptacles, and nectaries [ 21 , 22 ], and CsSWEET12c , which is involved in promoting plant growth and flowering [ 23 ]. However, the exact roles of the other 15 CsSWEET genes in cucumber have not yet been established.…”

Functional Characterization of CsSWEET5a, a Cucumber Hexose Transporter That Mediates the Hexose Supply for Pollen Development and Rescues Male Fertility in Arabidopsis

“…Among the only sugar transporters that can mediate the efflux of sugars across cell membranes, the 17 CsSWEETs identified in the cucumber genome [22] were the best candidates for genes that export sugars from anther tissues into anther locules. However, out of the seventeen CsSWEET genes, only three have been characterized in detail; CsSWEET2 enhances cold tolerance in plants [33]; CsSWEET7a is involved in hexose unloading in cucumber receptacles, nectaries, and fruits [26,27]; and CsSWEET12c promotes plant growth and blooming [39]. None of these three genes are related to pollen development.…”