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Nitrate represses the gibberellin pathway and subsequently activates the flowering repressors SMZ and SNZ, delaying flowering time in Arabidopsis., The reproductive success of plants largely depends on the correct programming of developmental phase transitions, particularly the shift from vegetative to reproductive growth. The timing of this transition is finely regulated by the integration of an array of environmental and endogenous factors. Nitrogen is the mineral macronutrient that plants require in the largest amount, and as such its availability greatly impacts on many aspects of plant growth and development, including flowering time. We found that nitrate signaling interacts with the age-related and gibberellic acid pathways to control flowering time in Arabidopsis thaliana. We revealed that repressors of flowering time belonging to the AP2-type transcription factor family including SCHLAFMUTZE (SMZ) and SCHNARCHZAPFEN (SNZ) are important regulators of flowering time in response to nitrate. Our results support a model whereby nitrate activates SMZ and SNZ via the gibberellin pathway to repress flowering time in Arabidopsis thaliana.

During vegetative and embryonic developmental transitions, plant cells are massively reorganized to support the activities that will take place during the subsequent developmental phase. Studying cellular and subcellular changes that occur during these short transitional periods can sometimes present challenges, especially when dealing with Arabidopsis thaliana embryo and seed tissues. As a complementary approach, cellular reprogramming can be used as a tool to study these cellular changes in another, more easily accessible, tissue type. To reprogram cells, genetic manipulation of particular regulatory factors that play critical roles in establishing or repressing the seed developmental program can be used to bring about a change of cell fate. During different developmental phases, vacuoles assume different functions and morphologies to respond to the changing needs of the cell. Lytic vacuoles (LVs) and protein storage vacuoles (PSVs) are the two main vacuole types found in flowering plants such as Arabidopsis. Although both are morphologically distinct and carry out unique functions, they also share some similar activities. As the co-existence of the two vacuole types is short-lived in plant cells, how they replace each other has been a long-standing curiosity. To study the LV to PSV transition, LEAFY COTYLEDON2 (LEC2), a key transcriptional regulator of seed development, was overexpressed in vegetative cells to activate the seed developmental program. At the cellular level, Arabidopsis leaf LVs were observed to convert to PSV-like organelles. This presents the opportunity for further research to elucidate the mechanism of LV to PSV transitions. Overall, this example demonstrates the potential usefulness of cellular reprogramming as a method to study cellular processes that occur during developmental transitions.

During seed development, the lytic vacuole (LV) is replaced by a protein storage vacuole (PSV) which specializes in accumulating seed storage proteins (SSPs). As seed protein reserves are mobilized upon germination, the PSV is once again replaced by the LV which takes on different roles in vegetative tissues. Cellular events occurring during these developmental transitions are not well understood, particularly, the transition between vacuole types. This research investigates whether PSVs can exist in leaves. To study vacuole transitions in leaves, an Arabidopsis thaliana line over-expressing the LEAFY COTYLEDON2 (LEC2) transcription factor was used. LEC2 is a master regulator of embryogenesis responsible for creating a cellular environment that promotes embryogenic development. Over-expression of LEC2 causes vegetative tissues to change their developmental fate to an embryonic state. LEC2 alters the leaf phenotype at the subcellular level; chloroplasts de-differentiated and contained more starch. The cytoplasm becomes filled with oil bodies, which are typically seed organelles. The large LV was replaced by small-sized vacuoles that accumulated protein deposits. Since LEC2 is responsible for activating the synthesis of SSPs during seed development, SSP accumulation was investigated in leaves. The major Arabidopsis SSP families were shown to accumulate within small sized vacuoles in leaf cells. By exploiting the developmental and tissue specific localization of two tonoplast intrinsic protein (TIP) isoforms, the small vacuoles were identified as PSVs. A time course following the cellular alterations and accumulation of seed proteins in leaves after induction of LEC2 activity with dexamethasone (DEX) revealed the appearance of embryonic characteristics as early as 4 days on DEX and became more prominent over time. Additionally, a dynamic view of the transition between vacuole types was observed using TIP isoforms fused to fluorescent markers. The morphology of leaf vacuoles was altered to resemble an amalgamation of a LV and PSV. Results suggest that as the LV transitions to a PSV, the tonoplast remodels before the large vacuole is replaced by smaller PSVs. The formation of PSVs in leaves in response to LEC2 over-expression is a novel approach to study vacuoles and will lead to a better understanding of their basic biology.