The phloem performs essential systemic functions in tracheophytes, yet little is known about its molecular genetic specification. In the Arabidopsis primary root meristem, the protophloem is the earliest tissue to differentiate and its continuity is essential for post-embryonic meristem growth and thus, root growth. Delicate quantitative interplay between two opposing signaling pathways determines cellular commitment to protophloem sieve element fate, with a recently described phloem-specific protein acting as a positive, quantitative master regulator. Interference with the specification of protophloem disrupts its continuity through stochastic suppression of protophloem sieve element precursor differentiation and is associated with systemic effects that propagate throughout the meristem, such as globally reduced auxin response. Incidentally, this abolishes the formative sieve element precursor cell division that creates the protophloem and incipient metaphloem cell files. This division apparently depends on, and is particularly sensitive to local auxin activity. Our data thus support a scenario in which the protophloem serves as the principal route for the delivery of essential growth cues that potentiate auxin response in the root meristem, possibly auxin itself.
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