Long-Distance Signaling via the Phloem

In addition to nutrients, the phloem transports hormones, proteins, RNA, lipids, and signals involved in defense responses, gene silencing, flowering, and nodulation.  The list of molecules identified in phloem extracts is growing, as is the list of roles that those molecules play in whole-plant growth and development.

The phloem's role in long-distance signaling was dramatically demonstrated by specifically expressing a gene in the smallest veins (known as the minor veins) of leaves, and observing morphological changes in recipient tissues in the apical meristem.  Specifically, CONSTANS was expressed in the minor veins from the galactinol synthase promoter and an accelerated change from vegetative to reproductive growth (i.e., flowering) was observed.  These experiments demonstrate that CONSTANS may function directly in generating florigen, the elusive floral stimulant described in the classic literature.

Ayre BG, Turgeon R (2004) Graft transmission of a floral stimulant derived from CONSTANS.  Plant Physiology; 135: 2271-2278. [Abstract] [Full Text] [PDF]

The following poster on CONSTANS signaling is compressed to fit on letter size paper.  To facilitate reading, print or magnify to >200%.  CONSTANS poster, ASPB, 2004 (pdf)