Mechanisms of phloem unloading and post-phloem transport of photoassimilates during the rapid elongation of young Dendrocalamus brandisii shoots.

Phloem unloading plays a key role in the transport and allocation of photoassimilates. Elucidating the unloading process and metabolic dynamics of photoassimilates in rapidly elongating internodes is crucial for understanding the rapid growth mechanism of bamboo. We systematically investigated the phloem unloading and the post-unloading transport mechanism in internodes of young Dendrocalamus brandisii culms through integrated anatomical observations, transcriptomic analysis, and gene functional verification. Phloem unloading shifts from coexisting apoplasmic and symplasmic pathways in immature internodes to an exclusively symplasmic mechanism in mature internodes. DbSWEET1a and DbPMT1 were highly expressed in rapidly elongating internodes. Exogenous sucrose application further confirmed their role as key genes by upregulating genes involved in apoplasmic phloem unloading and post-unloading transport in immature internodes. Plasma membrane proteins DbSWEET1a and DbPMT1 mainly localized in phloem companion cells and adjacent parenchyma cells. Yeast complementation assays verified that DbSWEET1a transports sucrose and fructose, while DbPMT1 transports glucose and fructose. SWEET1a and PMT1 proteins participate in apoplasmic phloem unloading and transport in immature internodes, whereas in mature internodes, they facilitate the retrieval of apoplasmic sugars for reintegration into the symplasmic unloading pathway.