Summary: Plant interactions with arbuscular mycorrhizal fungi have long attracted interest for their potential to promote more efficient use of mineral resources in agriculture. Their use, however, remains limited by a lack of understanding of the processes that determine the outcome of the symbiosis. In this study, the impact of host genotype on growth response to mycorrhizal inoculation was investigated in a panel of diverse maize lines. A panel of 30 maize lines was evaluated with and without inoculation with arbuscular mycorrhizal fungi. The line Oh43 was identified to show superior response and, along with five other reference lines, was characterized in greater detail in a split‐compartment system, using ³³P to quantify mycorrhizal phosphorus uptake. Changes in relative growth indicated variation in host capacity to profit from the symbiosis. Shoot phosphate content, abundance of root‐internal and ‐external fungal structures, mycorrhizal phosphorus uptake, and accumulation of transcripts encoding plant PHT1 family phosphate transporters varied among lines. Superior response in Oh43 is correlated with extensive development of root‐external hyphae, accumulation of specific Pht1 transcripts and high phosphorus uptake by mycorrhizal plants. The data indicate that host genetic factors influence fungal growth strategy with an impact on plant performance.

Desiccation tolerance (DT) is a remarkable process that allows seeds in the dry state to remain viable for long periods of time that in some instances exceed 1,000 y. It has been postulated that seed DT evolved by rewiring the regulatory and signaling networks that controlled vegetative DT, which itself emerged as a crucial adaptive trait of early land plants. Understanding the networks that regulate seed desiccation tolerance in model plant systems would provide the tools to understand an evolutionary process that played a crucial role in the diversification of flowering plants. In this work, we used an integrated approach that included genomics, bioinformatics, metabolomics, and molecular genetics to identify and validate molecular networks that control the acquisition of DT in Arabidopsis seeds. Two DT-specific transcriptional subnetworks were identified related to storage of reserve compounds and cellular protection mechanisms that act downstream of the embryo development master regulators LEAFY COTYLEDON 1 and 2, FUSCA 3, and ABSCICIC ACID INSENSITIVE 3. Among the transcription factors identified as major nodes in the DT regulatory subnetworks, PLATZ1, PLATZ2, and AGL67 were confirmed by knockout mutants and overexpression in a desiccation-intolerant mutant background to play an important role in seed DT. Additionally, we found that constitutive expression of PLATZ1 in WT plants confers partial DT in vegetative tissues. Significance Seed desiccation tolerance (DT) is one of the most fascinating processes of higher plants, and has played a fundamental role in the evolution of land plants. DT allows plant seeds to remain viable in the dry state for years and even centuries. What the key transcription factors (TFs) are that activate the mechanisms that allow plant seeds to maintain cellular and DNA integrity for centuries remains largely unknown. In this paper, we report the identification of the TFs that act as major nodes of the transcriptional networks that regulate the acquisition of seed DT. We also report the functional validation of several of the major regulators of seed DT in plants.

Fruits and seeds are the major food source on earth. Both derive from the gynoecium and, therefore, it is crucial to understand the mechanisms that guide the development of this organ of angiosperm species. In Arabidopsis, the gynoecium is composed of two congenitally fused carpels, where two domains: medial and lateral, can be distinguished. The medial domain includes the carpel margin meristem (CMM) that is key for the production of the internal tissues involved in fertilization, such as septum, ovules, and transmitting tract. Interestingly, the medial domain shows a high cytokinin signaling output, in contrast to the lateral domain, where it is hardly detected. While it is known that cytokinin provides meristematic properties, understanding on the mechanisms that underlie the cytokinin signaling pattern in the young gynoecium is lacking. Moreover, in other tissues, the cytokinin pathway is often connected to the auxin pathway, but we also lack knowledge about these connections in the young gynoecium. Our results reveal that cytokinin signaling, that can provide meristematic properties required for CMM activity and growth, is enabled by the transcription factor SPATULA (SPT) in the medial domain. Meanwhile, cytokinin signaling is confined to the medial domain by the cytokinin response repressor ARABIDOPSIS HISTIDINE PHOSPHOTRANSFERASE 6 (AHP6), and perhaps by ARR16 (a type-A ARR) as well, both present in the lateral domains (presumptive valves) of the developing gynoecia. Moreover, SPT and cytokinin, probably together, promote the expression of the auxin biosynthetic gene TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1 (TAA1) and the gene encoding the auxin efflux transporter PIN-FORMED 3 (PIN3), likely creating auxin drainage important for gynoecium growth. This study provides novel insights in the spatiotemporal determination of the cytokinin signaling pattern and its connection to the auxin pathway in the young gynoecium. Most of our food comes from fruits and seeds, derived from a fertilized gynoecium. Therefore, understanding the mechanisms that control gynoecium development is of crucial importance. The Arabidopsis gynoecium has two fused carpels, with a medial domain between them, and a lateral domain consisting of the carpel walls. All the tissues that are involved in reproduction arise from the carpel margin meristem in the medial domain. The phytohormone cytokinin provides meristematic activity to cells, and interestingly, in a young gynoecium, the medial, but not the lateral, domain presents strong cytokinin signaling. One question that comes to mind is how this pattern is defined. This work demonstrates that the transcription factor SPATULA enables cytokinin signaling at the medial domain, while cytokinin signaling repressors are present in the lateral domain. A second question is whether and how cytokinin in the medial domain communicates with auxin, an important phytohormone for tissue differentiation. We found that cytokinin and SPT activate auxin biosynthesis and transport genes. The integration of these findings gives the first gene regulatory network acting during early gynoecium development. This network is most likely conserved in flowering plants, and can provide insights of molecular processes that are key for food production.

Synchronized communication between gametophytic and sporophytic tissue is crucial for successful reproduction, and hormones seem to have a prominent role in it. Here, we studied the role of the Arabidopsis (Arabidopsis thaliana) cytochrome P450 CYP78A9 enzyme during reproductive development. First, controlled pollination experiments indicate that CYP78A9 responds to fertilization. Second, while CYP78A9 overexpression can uncouple fruit development from fertilization, the cyp78a8 cyp78a9 loss-of-function mutant has reduced seed set due to outer ovule integument development arrest, leading to female sterility. Moreover, CYP78A9 has a specific expression pattern in inner integuments in early steps of ovule development as well as in the funiculus, embryo, and integuments of developing seeds. CYP78A9 overexpression did not change the response to the known hormones involved in flower development and fruit set, and it did not seem to have much effect on the major known hormonal pathways. Furthermore, according to previous predictions, perturbations in the flavonol biosynthesis pathway were detected in cyp78a9, cyp78a8 cyp78a9, and empty siliques (es1-D) mutants. However, it appeared that they do not cause the observed phenotypes. In summary, these results add new insights into the role of CYP78A9 in plant reproduction and present, to our knowledge, the first characterization of metabolite differences between mutants in this gene family.