Phloem Parenchyma

Abstract

Phloem parenchyma is one of the key cell types constituting the phloem of angiosperms. Phloem parenchyma cells play essential roles in the transfer of diverse compounds, such as sucrose and amino acids, into the sieve element‐companion cell complex (SECCC). A fraction of the phloem parenchyma cells trans ‐differentiate into phloem parenchyma transfer cells (PPTC) by developing plasma membrane amplification associated with cell wall ingrowth. The increased membrane surface area in PPTCs may create additional space for the transporters to localise thereby enhancing phloem loading efficiency. Recent advances in single‐cell sequencing and imaging techniques have provided new information on the potential roles of the phloem parenchyma, not only in the transport of nutrients but also in plant metabolism and defence mechanisms. Key Concepts The phloem consists of sieve elements, companion cells and phloem parenchyma cell types. The phloem parenchyma cells play pivotal roles during phloem loading. Certain phloem parenchyma cells undergo trans ‐differentiation and develop cell wall ingrowth. The entry of sucrose into the SECCC follows at least one of the two paths – the apoplasmic and/or the symplasmic pathway. In Arabidopsis , SWEET11, SWEET12 and SWEET13, sucrose transporters are localised to the phloem parenchyma and efflux sucrose to the apoplasm. Sucrose in the apoplasm is actively taken up into the SECCC complex by SUT1/SUC2 proton‐coupled sucrose symporters for long‐distance transport. Metabolic activities in the phloem parenchyma and the adjacent companion cells likely contribute to reshaping the metabolomes of the phloem and xylem. The ontogenetic development of phloem parenchyma remains elusive, requiring future research activities.