Arabidopsis Ubiquitin-Conjugating Enzymes UBC4, UBC5, and UBC6 Have Major Functions in Sugar Metabolism and Leaf Senescence

Cytosolic fructose-1,6-bisphosphatase isoform mediates metabolic adjustments in bean fruit pericarp to support seed growth

Seed development requires substantial metabolic resources and is influenced by adverse environmental conditions. However, the ability of plants to produce viable seeds under restrictive conditions suggests the existence of mechanisms that make this process less sensitive to environmental stress. Uncovering their regulation could lead to the development of genotypes better adapted to stressful conditions. Plant response to stress is complex, and the contribution of organs such as the fruit pericarp to stress tolerance mechanism may have been underestimated. The bean fruit pericarp, a photosynthetic structure that contributes to seed development, can synthesize starch from surplus sucrose, which is later degraded during the rapid seed growth phase. This metabolic flexibility may be crucial for supporting seed growth when the photosynthate supply is reduced. To explore this possibility, we disrupted phloem continuity at the pedicel level in fruits about to enter the seed reserve accumulation stage. We used the capacity of the pericarp to incorporate 14CO2 to investigate changes in its metabolism. Our findings reveal that, in response to reduced photosynthate availability, the fruit pericarp did not increase 14CO2 fixation. However, the amount of 14C used for starch synthesis decreased, while the proportion used for soluble sugars synthesis increased. This shift resulted in an increase in 14C-products transported to seeds was accompanied by a significant increase in the activity of cytosolic fructose 1,6-bisphosphatase. Our results indicate that photosynthate restriction accelerates the degradation of pericarp storage proteins, and the increase in cFBPase activity could be crucial in converting the carbon produced in carbohydrates.

The CUL3A-LFH1-UBC15 ubiquitin ligase complex mediates SHORT VEGETATIVE PHASE degradation to accelerate flowering at high ambient temperature

Ambient temperature affects flowering time in plants, and the MADS-box transcription factor SHORT VEGETATIVE PHASE (SVP) plays a crucial role in the response to changes in ambient temperature. SVP protein stability is regulated by the 26S proteasome pathway and decreases at high ambient temperature, but the details of SVP degradation are unclear. Here, we show that SVP degradation at high ambient temperature is mediated by the CULLIN3-RING E3 ubiquitin ligase (CRL3) complex in Arabidopsis thaliana. We identified a previously uncharacterized protein that interacts with SVP at high ambient temperature and contains a BTB/POZ domain. We named this protein LATE FLOWERING AT HIGH TEMPERATURE 1 (LFH1). Single mutants of LFH1 or CULLIN3A (CUL3A) showed late flowering specifically at 27°C. LFH1 protein levels increased at high ambient temperature. We found that LFH1 interacts with CUL3A in the cytoplasm and is important for SVP-CUL3A complex formation. Mutations in CUL3A and/or LFH1 led to increased SVP protein stability at high ambient temperature, suggesting that the CUL3-LFH1 complex functions in SVP degradation. Screening E2 ubiquitin-conjugating enzymes (UBCs) using RING-BOX PROTEIN 1 (RBX1), a component of the CRL3 complex, as bait identified UBC15. ubc15 mutants also showed late flowering at high ambient temperature. In vitro and in vivo ubiquitination assays using recombinant CUL3A, LFH1, RBX1, and UBC15 showed that SVP is highly ubiquitinated in an ATP-dependent manner. Collectively, these results indicate that the degradation of SVP at high ambient temperature is mediated by a CRL3 complex comprising CUL3A, LFH1, and UBC15.