Significance: Plants adapt to their fluctuating environment by monitoring surrounding light conditions through several photoreceptors, such as phytochrome. It is widely believed that upon absorbing red light, phytochrome induces plant light responses by regulating the transcription of numerous target genes. In this study, we provide clear evidence that phytochrome controls not only transcription, but also alternative splicing inArabidopsis. We reveal that 6.9% of the annotated genes in theArabidopsisgenome undergo rapid changes in their alternative splicing patterns in a red light- and phytochrome-dependent manner. Our results demonstrate that phytochrome simultaneously regulates two different aspects of gene expression, namely transcription and alternative splicing to mediate light responses in plants.

Polar transport of the phytohormone auxin through PIN-FORMED (PIN) auxin efflux carriers is essential for the spatiotemporal control of plant development. The Arabidopsis thaliana serine/threonine kinase D6 PROTEIN KINASE (D6PK) is polarly localized at the plasma membrane of many cells where it colocalizes with PINs and activates PIN-mediated auxin efflux. Here, we show that the association of D6PK with the basal plasma membrane and PINs is dependent on the phospholipid composition of the plasma membrane as well as on the phosphatidylinositol phosphate 5-kinases PIP5K1 and PIP5K2 in epidermis cells of the primary root. We further show that D6PK directly binds polyacidic phospholipids through a polybasic lysine-rich motif in the middle domain of the kinase. The lysine-rich motif is required for proper PIN3 phosphorylation and for auxin transport-dependent tropic growth. Polybasic motifs are also present at a conserved position in other D6PK-related kinases and required for membrane and phospholipid binding. Thus, phospholipid-dependent recruitment to membranes through polybasic motifs might not only be required for D6PK-mediated auxin transport but also other processes regulated by these, as yet, functionally uncharacterized kinases.

Organisms have evolved under gravitational force, and many sense the direction of gravity by means of statoliths in specialized cells. In flowering plants, starch-accumulating plastids, known as amyloplasts, act as statoliths to facilitate downstream gravitropism. The gravity-sensing mechanism has long been considered a mechanosensing process by which amyloplasts transmit forces to intracellular structures, but the molecular mechanism underlying this has not been elucidated. We show here that LAZY1-LIKE (LZY) family proteins involved in statocyte gravity signaling associate with amyloplasts and the proximal plasma membrane. This results in polar localization according to the direction of gravity. We propose a gravity-sensing mechanism by which LZY translocation to the plasma membrane signals the direction of gravity by transmitting information on the position of amyloplasts.

Summary: Plants monitor the light environment through informational photoreceptors that include phytochromes. In seedling de‐etiolation, phytochrome B (phyB), which is the most important member of the phytochrome family, interacts with transcription factors to regulate gene expression and transduce light signals. In this study, we identified rrc1 (reduced red‐light responses in cry1cry2 background 1), an Arabidopsis mutant that is impaired in phyB‐mediated light responses. A genetic analysis demonstrated that RRC1 affected light signaling in a phyB‐dependent manner. RRC1 encodes an ortholog of the human potential splicing factor SR140. The RRC1 polypeptide contains a C‐terminal arginine/serine‐rich (RS) domain that is important for the regulation of alternative splicing. Although the complete loss of RRC1 caused pleiotropic developmental abnormalities, the deletion of the RS domain specifically reduced phyB signaling and caused aberrant alternative splicing of several SR protein genes. Moreover, semi‐quantitative RT‐PCR analysis revealed that the alternative splicing patterns of some of the SR protein genes were altered in a red‐light‐dependent manner, and that these responses were reduced in both phyB and rrc1 mutants. These findings suggest that the regulation of alternative splicing by the RS domain of RRC1 plays an important role in phyB signal transduction.

Summary: The plant cell wall, while providing mechanical support to cells, also dynamically adjusts its composition and structure in response to cellular and environmental cues. Recent findings indicate that plants exposed to cold stress alter the composition of cell wall polysaccharides and that this altered status primes the plants to overcome future, more severe exposure. Here, we propose that the cell wall functions as a refined interface that retains information from past experiences, with such ‘memory’ being encoded as structural or compositional changes. This enables more efficient responses to future stimuli. Such a memory system may support various biological processes, in parallel with other systems such as chromatin‐based epigenetic memory. The cell wall is uniquely capable of storing higher‐order information related to the geometry of cells and tissues, mechanical cues, and environmental histories. In this context, we present emerging perspectives and representative cases of such memory stored in the plant cell wall, and we discuss plausible mechanisms through which the memory influences cellular responses.