A quantitative gibberellin signaling biosensor reveals a role for gibberellins in internode specification at the shoot apical meristem.

Growth at the shoot apical meristem (SAM) is essential for shoot architecture construction. The phytohormones gibberellins (GA) play a pivotal role in coordinating plant growth, but their role in the SAM remains mostly unknown. Here, we developed a ratiometric GA signaling biosensor by engineering one of the DELLA proteins, to suppress its master regulatory function in GA transcriptional responses while preserving its degradation upon GA sensing. We demonstrate that this degradation-based biosensor accurately reports on cellular changes in GA levels and perception during development. We used this biosensor to map GA signaling activity in the SAM. We show that high GA signaling is found primarily in cells located between organ primordia that are the precursors of internodes. By gain- and loss-of-function approaches, we further demonstrate that GAs regulate cell division plane orientation to establish the typical cellular organization of internodes, thus contributing to internode specification in the SAM.

Gibberellin and abscisic acid transporters facilitate endodermal suberin formation in Arabidopsis.

The plant hormone gibberellin (GA) regulates multiple developmental processes. It accumulates in the root elongating endodermis, but how it moves into this cell file and the significance of this accumulation are unclear. Here we identify three NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER (NPF) transporters required for GA and abscisic acid (ABA) translocation. We demonstrate that NPF2.14 is a subcellular GA/ABA transporter, presumably the first to be identified in plants, facilitating GA and ABA accumulation in the root endodermis to regulate suberization. Further, NPF2.12 and NPF2.13, closely related proteins, are plasma membrane-localized GA and ABA importers that facilitate shoot-to-root GA12 translocation, regulating endodermal hormone accumulation. This work reveals that GA is required for root suberization and that GA and ABA can act non-antagonistically. We demonstrate how the clade of transporters mediates hormone flow with cell-file-specific vacuolar storage at the phloem unloading zone, and slow release of hormone to induce suberin formation in the maturation zone.

ABA homeostasis and long-distance translocation are redundantly regulated by ABCG ABA importers.

The effects of abscisic acid (ABA) on plant growth, development, and response to the environment depend on local ABA concentrations. Here, we show that in Arabidopsis, ABA homeostasis is regulated by two previously unknown ABA transporters. Adenosine triphosphate­binding cassette subfamily G member 17 (ABCG17) and ABCG18 are localized to the plasma membranes of leaf mesophyll and cortex cells to redundantly promote ABA import, leading to conjugated inactive ABA sinks, thus restricting stomatal closure. ABCG17 and ABCG18 double knockdown revealed that the transporters encoded by these genes not only limit stomatal aperture size, conductance, and transpiration while increasing water use efficiency but also control ABA translocation from the shoot to the root to regulate lateral root emergence. Under abiotic stress conditions, ABCG17 and ABCG18 are transcriptionally repressed, promoting active ABA movement and response. The transport mechanism mediated by ABCG17 and ABCG18 allows plants to maintain ABA homeostasis under normal growth conditions.