Arbuscular mycorrhizal fungi form the most wide-spread endosymbiosis with plants. There is very little host specificity in this interaction, however host preferences as well as varying symbiotic efficiencies have been observed. We hypothesize that secreted proteins (SPs) may act as fungal effectors to control symbiotic efficiency in a host-dependent manner. Therefore, we studied whether arbuscular mycorrhizal (AM) fungi adjust their secretome in a host- and stage-dependent manner to contribute to their extremely wide host range. We investigated the expression of SP-encoding genes of Rhizophagus irregularis in three evolutionary distantly related plant species, Medicago truncatula, Nicotiana benthamiana and Allium schoenoprasum. In addition we used laser microdissection in combination with RNA-seq to study SP expression at different stages of the interaction in Medicago. Our data indicate that most expressed SPs show roughly equal expression levels in the interaction with all three host plants. In addition, a subset shows significant differential expression depending on the host plant. Furthermore, SP expression is controlled locally in the hyphal network in response to host-dependent cues. Overall, this study presents a comprehensive analysis of the R. irregularis secretome, which now offers a solid basis to direct functional studies on the role of fungal SPs in AM symbiosis.

Oriented cell divisions are significant in plant morphogenesis because plant cells are embedded in cell walls and cannot relocate. Cell divisions follow various regular orientations, but the underlying mechanisms have not been clarified. We propose that cell-shape-dependent self-organization of cortical microtubule arrays is able to provide a mechanism for determining planes of early tissue-generating divisions and may form the basis for robust control of cell division orientation in the embryo. To show this, we simulate microtubules on actual cell surface shapes, from which we derive a minimal set of three rules for proper array orientation. The first rule captures the effects of cell shape alone on microtubule organization, the second rule describes the regulation of microtubule stability at cell edges, and the third rule includes the differential effect of auxin on local microtubule stability. These rules generate early embryonic division plane orientations and potentially offer a framework for understanding patterned cell divisions in plant morphogenesis.

Land plants can grow to tremendous body sizes, yet even the most complex architectures are the result of iterations of the same developmental processes: organ initiation, growth, and pattern formation. A central question in plant biology is how these processes are regulated and coordinated to allow for the formation of ordered, 3D structures. All these elementary processes first occur in early embryogenesis, during which, from a fertilized egg cell, precursors for all major tissues and stem cells are initiated, followed by tissue growth and patterning. Here we discuss recent progress in our understanding of this phase of plant life. We consider the cellular basis for multicellular development in 3D and focus on the genetic regulatory mechanisms that direct specific steps during early embryogenesis.

Auxin-dependent control of cytoskeleton and cell shape regulates division orientation in the Arabidopsis embryo Graphical abstract Highlights d Auxin responses regulate directional cell expansion in Arabidopsis embryos d Cell shape and division orientation are tightly coupled d Transcriptome analysis identifies MT-associated IQD proteins in division control d Cytoskeletal dynamics control division orientation

Summary 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. 14C‐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.