Significance: Using strawberry fruit as a model system, we uncover the mechanistic interactions between auxin, gibberellic acid (GA), and abscisic acid (ABA) that regulate the entire process of fruit development. Interlinked regulatory loops control ABA levels during fruit development. During the early stages, auxin/GA turns on a feedback loop to activate the removal of ABA via FveCYP707A4a -dependent catabolism needed for fruit growth. Down-regulation of auxin/GA results in the suppression of the feedback loop and the activation of the ABA biosynthesis-dependent feedforward loop, leading to a steep ABA accumulation for fruit ripening. The interlinked regulatory loops provide a conceptual framework that underlies the connection between the regulation of fruit growth and that of ripening as well as a molecular basis for manipulation of fruit sizes and ripening times.

Phytohormones and their interactions are critical for fruit development and are key topics in horticulture research. Auxin, together with gibberellic acid (GA), promotes cell division and expansion, thereby regulating fruit development and enlargement after fertilization. Auxin- and GA-related mutants show parthenocarpy (fruit formation without fertilization of the ovule) in many plant species, indicating that these hormones and possibly their interactions play a key role in the regulation of fruit initiation and development. Recent studies have shown clear molecular and genetic evidence that ARF/IAA and DELLA proteins interact with one another and regulate both auxin and GA signaling pathways in response to auxin and GA during fruit growth in horticultural plants such as tomato (the most studied fleshy fruit) and strawberry (the model for Rosaceae). These recent findings provide new insights into the mechanisms by which the plant hormones auxin and GA regulate fruit development.

The formation of cell polarity is essential for many developmental processes such as polar cell growth and spatial patterning of cell division. A plant-specific ROP ( R h o -like GTPases from P lants) subfamily of conserved Rho GTPase plays a crucial role in the regulation of cell polarity. However, the functional study of ROPs in angiosperm is challenging because of their functional redundancy. The Marchantia polymorpha genome encodes a single ROP gene, Mp ROP , providing an excellent genetic system to study ROP-dependent signaling pathways. Mp rop knockout mutants exhibited rhizoid growth defects, and MpROP was localized at the tip of elongating rhizoids, establishing a role for MpROP in the control of polar cell growth and its functional conservation in plants. Furthermore, the Mp rop knockout mutant showed defects in the formation of meristem notches associated with disorganized cell division patterns. These results reveal a critical function of Mp ROP in the regulation of plant development. Interestingly, these phenotypes were complemented not only by Mp ROP but also Arabidopsis At ROP2 , supporting the conservation of ROP’s function among land plants. Our results demonstrate a great potential for M. polymorpha as a powerful genetic system for functional and mechanistic elucidation of ROP signaling pathways during plant development.

The Arabidopsis thaliana RIC1, a key member of the ROP (Rho GTPase of Plants)-Interactive CRIB motif-containing protein family, has been characterized as a plant-specific microtubule-associated protein. RIC1 is thought to function as a key downstream effector of the small GTPase Rho-of-Plant 6 (ROP6), orchestrating the formation of well-ordered cortical microtubule arrays and promoting the interdigitated growth pattern of cotyledon or leaf epidermal pavement cells. However, the precise physicochemical properties governing RIC1's function in microtubule organization regulation remain known. Here, we demonstrate that RIC1 is a predicted intrinsically disordered protein that forms biomolecular condensates, which promote the polymerization of microtubule bundles in vitro. In Arabidopsis cotyledon pavement cells, RIC1 forms distinct condensate-like puncta, with a specific subset associating directly with cortical microtubules. RIC1 self-assembles into distinct spherical condensates under in vitro macromolecular crowding conditions. Remarkably, these RIC1 condensates co-condense with tubulin in vitro. We further demonstrate that RIC1 condensates elevate the local tubulin concentration, correlate with enhanced microtubule nucleation, and promote the polymerization of microtubule bundles in vitro. Moreover, the growing microtubule bundles actively reshape these condensates. Thus, RIC1 condensates may provide a specialized microenvironment that enables tubulin sequestration while simultaneously regulating microtubule nucleation and polymerization in vitro. This study provides crucial insights into the biomolecular condensation properties of the microtubule-associated protein RIC1, establishing a groundwork for future investigations into its in vivo functional mechanisms.