Significance: Flowering in many plants begins with the perception of daylength in leaves. For example, some plants flower in short days, others in long days. In Arabidopsis thaliana , the leaves then transmit a small protein, FLOWERING TIME T (FT), to the shoot apex in the food-conducting cells, the phloem. Arrival of FT causes the apex to transition from leaf to flower formation. In this paper, we show that only two files of phloem cells in A. thaliana veins synthesize FT. This is also true for the Maryland Mammoth cultivar of tobacco ( Nicotiana tabacum ). Killing the cells specifically affects downstream, but not upstream, genes, indicating that an extensive intercellular signaling system regulates FT synthesis in the phloem.

The remobilization of nonstructural carbohydrates (NSCs) reserved in rice (Oryza sativa) sheaths is essential for grain filling. This assimilate distribution between plant tissues and organs is determined by sucrose non-fermenting-1-related protein kinase 1 (SnRK1). However, the SnRK1-mediated mechanism regulating the sheath-to-panicle transport of NSCs in rice remains unknown. In this study, leaf cutting treatment was used to accelerate NSC transport in the rice sheaths. Accelerated NSC transport was accompanied by increased levels of OsSnRK1a mRNA expression, SnRK1a protein expression, catalytic subunit phosphorylation of SnRK1, and SnRK1 activity, indicating that SnRK1 activity plays an important role in sheath NSC transport. We also discovered that trehalose-6-phosphate, a signal of sucrose availability, slightly reduced SnRK1 activity in vitro. Since SnRK1 activity is mostly regulated by OsSnRK1a transcription in response to low sucrose content, we constructed an snrk1a mutant to verify the function of SnRK1 in NSC transport. NSCs accumulated in the sheaths of snrk1a mutant plants and resulted in a low seed setting rate and grain weight, verifying that SnRK1 activity is essential for NSC remobilization. Using phosphoproteomics and parallel reaction monitoring, we identified 20 SnRK1-dependent phosphosites that are involved in NSC transport. In addition, the SnRK1-mediated phosphorylation of the phosphosites directly affected starch degradation, sucrose metabolism, phloem transport, sugar transport across the tonoplast, and glycolysis in rice sheaths to promote NSC transport. Therefore, our findings reveal the importance, function, and possible regulatory mechanism of SnRK1 in the sheath-to-panicle transport of NSCs in rice.

Phloem loading is the first step in sucrose transport from source leaves to sink organs. The phloem loading strategy in rice remains unclear. To determine the potential phloem loading mechanism in rice, yeast invertase (INV) was overexpressed by a 35S promoter specifically in the cell wall to block sugar transmembrane loading in rice. The transgenic lines exhibited obvious phloem loading suppression characteristics accompanied by the accumulation of sucrose and starch, restricted vegetative growth and decreased grain yields. The decreased sucrose exudation rate with p-chloromercuribenzenesulfonic acid (PCMBS) treatment also indicated that rice actively transported sucrose into the phloem. OsSUT1 (SUCROSE TRANSPORTER 1) showed the highest mRNA levels of the plasma membrane-localized OsSUTs in source leaves. Cross sections of the OsSUT::GUS transgenic plants showed that the expression of OsSUT1 and OsSUT5 occurred in the phloem companion cells. Rice ossut1 mutants showed reduced growth and grain yield, supporting the hypothesis of OsSUT1 acting in phloem loading. Based on these results, we conclude that apoplastic phloem loading plays a major role in the export of sugar from rice leaves.

Background: Large panicle rice has a large sink capacity, but inferior spikelet filling is poor in this variety of rice due to asynchronous grain filling. The understanding of the factors that cause asynchronous grain filling will help to propose a model for how to regulate the rice inferior spikelets grain filling. Results: In this study, two large panicle rice varieties, W1844 and CJ03, with the same sink capacity but with differences in asynchronous grain filling were used. The difference in the grain filling rate between superior and inferior spikelets in W1844 was much smaller than that in CJ03. We found that superior spikelet filling was initiated earlier in W1844 than in CJ03. The source-to-sink translocation rate of sucrose during the grain filling stage was more efficient in W1844 than in CJ03, and the gene expression levels of sucrose transporters (OsSUTs) were higher in W1844 functional leaves than in those of CJ03. In addition, carbon output, the transport ratio, and the contribution rate from the stem and sheath to the panicle were much higher at the early filling stage than at later filling stages in W1844. Conclusion: Efficient sugar translocation can satisfy high sink strength, and also the strong sink activity can facilitate the sugar unloading in spikelets. All the above results indicate that an efficient sugar translocation rate at the early grain filling stage can improve sink strength and inferior grain filling initiation. Strategies to limit asynchronous grain filling in rice were also discussed based on our findings.

Background: Short internodes contribute to plant dwarfism, which is exceedingly beneficial for crop production. However, the underlying mechanisms of internode elongation are complicated and have been not fully understood.Results: Here, we report a maize dwarf mutant,dwarf2014(d2014), which displays shortened lower internodes. Map-based cloning revealed that thed2014gene is a novelbr2allele with a splicing variation, resulting in a higher expression ofBR2-T02instead of normalBR2-T01. Then, we found that the internode elongation ind2014/br2exhibited a pattern of inhibition-normality-inhibition (transient for the ear-internode), correspondingly, at the 6-leaf, 12-leaf and 14-leaf stages. Indeed,BR2encodes a P-glycoprotein1 (PGP1) protein that functions in auxin efflux, and our in situ hybridization assay showed thatBR2was mainly expressed in vascular bundles of the node and internode. Furthermore, significantly higher auxin concentration was detected in the stem apex ofd2014at the 6-leaf stage and strictly in the node region for the ear-internode at the 14-leaf stage. In such context, we propose that BR2/PGP1 transports auxin from node to internode through the vascular bundles, and excessive auxin accumulation in the node (immediately next to the intercalary meristem) region suppresses internode elongation ofd2014.Conclusions: These findings suggest that low auxin levels mediated by BR2/PGP1 in the intercalary meristem region are crucial for internode elongation.

Iron (Fe) is an essential mineral element required for plant growth, and when soil availability of Fe is low, plants show symptoms of severe deficiency. Under conditions of Fe deficiency, plants alter several processes to acquire Fe from soil. In this study, we used rice cultivars H 9405 with high Fe accumulation in seeds and Yang 6 with low Fe accumulation in seeds to study their physiological responses to different conditions of Fe availability. In both shoots and roots, the responses of ROS enzymes, leaf and root ultrastructure and photosynthetic system to iron deficiency in Yang 6 were much sensitive than those in H 9405. For the distribution of iron, the iron content was much higher in roots of Yang 6, in contrast to higher shoot content in H 9405. Differential responses were shown with the Fe content in roots and shoots, which were the opposite in the two varieties; thus, we proposed the existence of long-distance signals. Then split root and shoot removal experiments were used to demonstrate that a long-distance signal was involved in the iron-deficient rice plant, and the signal strength was highly correlated with the functional leaves.