Figure 3. Phloem-mobile signals in root development.

Nodulation (red arrows): In unmodulated roots, shoot-derived miR2111 translocates to the roots, where it inhibits TML, thereby promoting susceptibility to nodulation. Following sufficient nodulation, a root-derived CK-dependent signal represses miR2111 production, which restricts further nodule formation. Nitrogen acquisition (blue arrows): Under nitrogen-starved conditions, CEP translocates in the xylem from roots to shoots, where it is sensed by its receptor, CEPR. The CEP–CEPR module promotes the production of CEPD1 and 2 proteins, and, together with CK, the expression of CEPDL2. These proteins are transported in the phloem to the roots to regulate the expression of the nitrogen transporters NRT2.1, thus influencing nitrogen uptake and transport. HY5 integrates carbon fixation and photosynthetic status with nutrient acquisition by translocating from leaves to roots, where it auto-activates its expression and promotes NRT2.1-mediated nitrogen uptake. During phosphate starvation (orange arrows), miR399 is expressed in the leaves and translocates in the SE to the roots, where it suppresses expression of PHO2, allowing an increase in phosphate uptake and loading. Architecture and root morphology (pink arrows): CYP1 and IAA 18 and 28 transcripts translocate from shoot to root, where they coordinate light and photosynthetic status in the leaves with root development and nutrient acquisition in the roots. Abbreviations, plants species and references for all compounds are detailed in Table 1.