Root branching under high salinity requires auxin-independent modulation of LATERAL ORGAN BOUNDARY DOMAIN 16 function.

Salinity stress constrains lateral root (LR) growth and severely affects plant growth. Auxin signaling regulates LR formation, but the molecular mechanism by which salinity affects root auxin signaling and whether salt induces other pathways that regulate LR development remains unknown. In Arabidopsis thaliana, the auxin-regulated transcription factor LATERAL ORGAN BOUNDARY DOMAIN 16 (LBD16) is an essential player in LR development under control conditions. Here, we show that under high-salt conditions, an alternative pathway regulates LBD16 expression. Salt represses auxin signaling but, in parallel, activates ZINC FINGER OF ARABIDOPSIS THALIANA 6 (ZAT6), a transcriptional activator of LBD16. ZAT6 activates LBD16 expression, thus contributing to downstream cell wall remodeling and promoting LR development under high-salt conditions. Our study thus shows that the integration of auxin-dependent repressive and salt-activated auxin-independent pathways converging on LBD16 modulates root branching under high-salt conditions.

Effective root responses to salinity stress include maintained cell expansion and carbon allocation.

Acclimation of root growth is vital for plants to survive salt stress. Halophytes are great examples of plants that thrive even under severe salinity, but their salt tolerance mechanisms, especially those mediated by root responses, are still largely unknown. We compared root growth responses of the halophyte Schrenkiella parvula with its glycophytic relative species Arabidopsis thaliana under salt stress and performed transcriptomic analysis of S. parvula roots to identify possible gene regulatory networks underlying their physiological responses. Schrenkiella parvula roots do not avoid salt and experience less growth inhibition under salt stress. Salt-induced abscisic acid levels were higher in S. parvula roots compared with Arabidopsis. Root transcriptomic analysis of S. parvula revealed the induction of sugar transporters and genes regulating cell expansion and suberization under salt stress. 14 C-labeled carbon partitioning analyses showed that S. parvula continued allocating carbon to roots from shoots under salt stress while carbon barely allocated to Arabidopsis roots. Further physiological investigation revealed that S. parvula roots maintained root cell expansion and enhanced suberization under severe salt stress. In summary, roots of S. parvula deploy multiple physiological and developmental adjustments under salt stress to maintain growth, providing new avenues to improve salt tolerance of plants using root-specific strategies.

Geometric cues forecast the switch from two- to three-dimensional growth in Physcomitrella patens.

During land colonization, plants acquired a range of body plan adaptations, of which the innovation of three-dimensional (3D) tissues increased organismal complexity and reproductivity. In the moss, Physcomitrella patens, a 3D leafy gametophore originates from filamentous cells that grow in a two-dimensional (2D) plane through a series of asymmetric cell divisions. Asymmetric cell divisions that coincide with different cell division planes and growth directions enable the developmental switch from 2D to 3D, but insights into the underlying mechanisms coordinating this switch are still incomplete. Using 2D and 3D imaging and image segmentation, we characterized two geometric cues, the width of the initial cell and the angle of the transition division plane, which sufficiently distinguished a gametophore initial cell from a branch initial cell. These identified cues were further confirmed in gametophore formation mutants. The identification of a fluorescent marker allowed us to successfully predict the gametophore initial cell with > 90% accuracy before morphological changes, supporting our hypothesis that, before the transition division, parental cells of the gametophore initials possess different properties from those of the branch initials. Our results suggest that the cell fate decision of the initial cell is determined in the parental cell, before the transition division.