Phloem Development and Function in Plants: Implications for Flowering, Nitrogen Fixation, and Crop Yield

In tracheophyte plants, vascular bundles transport water, nutrients, and signalling molecules, including photoassimilates, amino acids, phytohormones, and inorganic ions. The vascular bundle contains xylem and phloem, which primarily transport water and nutrients, respectively. Recently, there has been remarkable progress in understanding the function of phloem in plant development and physiology. In this review, we describe the roles of genes and proteins that determine phloem cell fates and functions in the development of Arabidopsis (Arabidopsis thaliana) and several important crop plants. In particular, we expound upon phloem's role in sink organ development and determination of flowering time, highlighting two critical phloem-mediated processes: the allocation of sugars to sink tissues and the transport of FLOWERING LOCUS T (FT) to promote flowering. We also discuss the regulatory mechanisms governing FT expression and movement in Arabidopsis. Moreover, we elaborate on the pivotal role of sugar distribution in nitrogen fixation within root nodules, a specialized sink organ in leguminous plants. Clarifying the roles of phloem in these processes will illuminate fundamental principles underlying plant growth and development, thereby providing pivotal insights for improving crop yield and quality.