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Several regulators of phloem unloading have been identified up to now.

Phloem unloading is mostly active in growing sink tissues such as the root apex, and the restriction of phloem unloading impairs primary root growth. The process by which solutes exit the phloem, referred to as phloem unloading, regulates the molecular flux into sink organs to accommodate their high energy and signaling demands. Phloem-mediated translocation of molecules including nutrients, proteins, RNAs, and hormones is critical for plant growth and development. Our studies indicate the involvement of callose in the interaction between root meristem growth and heat stress and show that CalS8 negatively regulates the thermotolerance of Arabidopsis roots. Furthermore, we identify the loss-of-function of CALLOSE SYNTHASE 8 ( CalS8), which is expressed specifically in the phloem pole pericycle, decreasing the plasmodesmal callose deposition at the interface between the sieve element and phloem pole pericycle and alleviating the suppression at root meristem size by heat stress. Here, we show that heat stress reduces the root meristem size and inhibits phloem unloading by inducing callose accumulation at plasmodesmata that connect the sieve element and phloem pole pericycle. However, the mechanisms of plasmodesmata modification and molecules involved in the phloem unloading process under stress are still not well understood. Various stresses are known to affect the cell-to-cell molecular trafficking modulated by plasmodesmal permeability. The intercellular transport of sugars, nutrients, and small molecules is essential for plant growth, development, and adaptation to environmental changes.