Wiring a plant: genetic networks for phloem formation in Arabidopsis thaliana roots

In plants, phloem conduits form a specialized vascular network mediating the exchange of nutrients and signaling molecules between distantly separated organs. To become effective transport elements, protophloem cells undergo a rather unique, differentiation program that involves nucleus degradation, organelle rearrangement and cell wall thickening. Yet, protophloem sieve elements remain alive because their essential metabolic functions are supported by their neighboring companion cells. In spite of the importance of the phloem, the molecular mechanisms orchestrating protophloem specification and differentiation remain still poorly understood. In this review, I provide a summary of recent discoveries regarding morphogenetic events that determine phloem formation, and also a discussion of the systemic effects on root architecture derived from impaired protophloem differentiation programs.