Auxin transport affects a variety of important growth and developmental processes in plants, including the regulation of shoot and root branching. The asymmetrical localization of auxin influx and efflux carriers within the plasma membrane establishes the auxin gradient and facilitates its transport. REH1, a rice EIR1 (Arabidopsis ethylene insensitive root 1)-like gene, is a putative auxin efflux carrier. Phylogenetic analysis of 32 members of the PIN family, taken from across different species, showed that in terms of evolutionary relationship, OsPIN1 is closer to the PIN1 family than to the PIN2 family. It is, therefore, renamed as OsPIN1 in this study. OsPIN1 was expressed in the vascular tissues and root primordial in a manner similar to AtPIN1. Adventitious root emergence and development were significantly inhibited in the OsPIN1 RNA interference (RNAi) transgenic plants, which was similar to the phenotype of NPA (N-1-naphthylphalamic acid, an auxin-transport inhibitor)-treated wild-type plants. α-naphthylacetic acid (α-NAA) treatment was able to rescue the mutated phenotypes occurring in the RNAi plants. Overexpression or suppression of the OsPIN1 expression through a transgenic approach resulted in changes of tiller numbers and shoot/root ratio. Taken together, these data suggest that OsPIN1 plays an important role in auxin-dependent adventitious root emergence and tillering.

Soybean accounts for more than half of the global production of oilseed and more than a quarter of the protein used globally for human food and animal feed. Soybean domestication involved parallel increases in seed size and oil content, and a concomitant decrease in protein content. However, science has not yet discovered whether these effects were due to selective pressure on a single gene or multiple genes. Here, re-sequencing data from >800 genotypes revealed a strong selection during soybean domestication on GmSWEET10a. The selection of GmSWEET10a conferred simultaneous increases in soybean-seed size and oil content as well as a reduction in the protein content. The result was validated using both near-isogenic lines carrying substitution of haplotype chromosomal segments and transgenic soybeans. Moreover, GmSWEET10b was found to be functionally redundant with its homologue GmSWEET10a and to be undergoing selection in current breeding, leading the the elite allele GmSWEET10b, a potential target for present-day soybean breeding. Both GmSWEET10a and GmSWEET10b were shown to transport sucrose and hexose, contributing to sugar allocation from seed coat to embryo, which consequently determines oil and protein contents and seed size in soybean. We conclude that past selection of optimal GmSWEET10a alleles drove the initial domestication of multiple soybean-seed traits and that targeted selection of the elite allele GmSWEET10b may further improve the yield and seed quality of modern soybean cultivars.

The antagonistic interaction between iron (Fe) and phosphorus (P) has been noted in the area of plant nutrition. To understand the physiology and molecular mechanisms of this interaction, we studied the growth performance, nutrient concentration, and gene expression profiles of root and shoot segments derived from 10-d-old rice (Oryza sativa) seedlings under four different nutrient conditions: (1) full strength of Fe and P (+Fe+P); (2) full strength of P and no Fe (−Fe+P); (3) full strength of Fe and no P (+Fe−P); and (4) without both Fe and P (−Fe−P). While removal of Fe in the growth medium resulted in very low shoot and root Fe concentrations, the chlorotic symptoms and retarded seedling growth were only observed on seedlings grown in the presence of P. Microarray data showed that in roots, 7,628 transcripts were significantly changed in abundance in the absence of Fe alone. Interestingly, many of these changes were reversed if P was also absent (−Fe−P), with only approximately 15% overlapping with –Fe alone (–Fe+P). Analysis of the soluble Fe concentration in rice seedling shoots showed that P deficiency resulted in significantly increased Fe availability within the plants. The soluble Fe concentration under –Fe–P conditions was similar to that under +Fe+P conditions. These results provide evidence that the presence of P can affect Fe availability and in turn can influence the regulation of Fe-responsive genes.

Summary: Arabidopsis thaliana SPX (SYG/PHO81/XPR1) domain genes have recently been shown to be involved in the phosphate (Pi) signaling pathway. We show here that a rice (Oryza sativa) SPX gene, OsSPX1, is specifically induced by Pi starvation in roots. Suppression of OsSPX1 by RNA interference resulted in severe signs of toxicity caused by the over‐accumulation of Pi, similar to that found in OsPHR2 (phosphate starvation response transcription factor 2) overexpressors and pho2 (phosphate‐responsive mutant 2). Quantitative RT‐PCR showed that expression of OsSPX1 was strongly induced in OsPHR2 overexpression and pho2 mutant plants, indicating that OsSPX1 occurs downstream of OsPHR2 and PHO2. The expression of 10 genes associated with the phosphate‐starvation signal pathways was analyzed. Expression of OsPT2 (phosphate transporter 2) and OsPT8 was significantly induced in OsSPX1‐RNAi (OsSPX1‐Ri) plants, suggesting that over‐accumulation of Pi in OsSPX1‐Ri plants results from an increase in Pi transport. In contrast, overexpression of OsSPX1 suppressed the induction of expression by Pi starvation of all 10 phosphate starvation‐induced genes tested: IPS1 (induced by phosphate starvation 1), IPS2, OsPAP10 (purple acid phosphatase 10), OsSQD2 (sulfoquinovosyldiacylglycerol 2), miR399d and miR399j (microRNA 399), OsPT2, OsPT3, OsPT6 and OsPT8. This suggests that OsSPX1 acts via a negative feedback loop to optimize growth under phosphate‐limited conditions.

• Proteins possessing the SPX domain are found in several proteins involved in inorganic phosphate (Pi) transport and signalling in yeast and plants. Although the functions of several SPX‐domain protein subfamilies have recently been uncovered, the role of the SPX‐MFS subfamily is still unclear. • Using quantitative RT‐PCR analysis, we studied the regulation of SPX‐MFS gene expression by the central regulator, OsPHR2 and Pi starvation. The function of OsSPX‐MFS1 in Pi homeostasis was analysed using an OsSPX‐MFS1 mutant (mfs1) and osa‐miR827 overexpression line (miR827‐Oe). Finally, heterologous complementation of a yeast mutant impaired in Pi transporter was used to assess the capacity of OsSPX‐MFS1 to transport Pi. • Transcript analyses revealed that members of the SPX‐MFS family were mainly expressed in the shoots, with OsSPX‐MFS1 and OsSPX‐MFS3 being suppressed by Pi deficiency, while OsSPX‐MFS2 was induced. Mutation in OsSPX‐MFS1 (mfs1) and overexpression of the upstream miR827 (miR827‐Oe) plants impaired Pi homeostasis in the leaves. In addition, studies in yeast revealed that OsSPX‐MFS1 may be involved in Pi transport. • The results suggest that OsSPX‐MFS1 is a key player in maintaining Pi homeostasis in the leaves, potentially acting as a Pi transporter.

Soybean (Glycine max) seed is primarily composed of a mature embryo that provides a major source of protein and oil for humans and other animals. Early in development, the tiny embryos grow rapidly and acquire large quantities of sugars from the liquid endosperm of developing seeds. An insufficient supply of nutrients from the endosperm to the embryo results in severe seed abortion and yield reduction. Hence, an understanding of the molecular basis and regulation of assimilate partitioning involved in early embryo development is important for improving soybean seed yield and quality. Here, we used expression profiling analysis to show that two paralogous sugar transporter genes from the SWEET (Sugars Will Eventually be Exported Transporter) family, GmSWEET15a and GmSWEET15b, were highly expressed in developing soybean seeds. In situ hybridization and quantitative real-time PCR showed that both genes were mainly expressed in the endosperm at the cotyledon stage. GmSWEET15b showed both efflux and influx activities for sucrose in Xenopus oocytes. In Arabidopsis (Arabidopsis thaliana), knockout of three AtSWEET alleles is required to see a defective, but not lethal, embryo phenotype, whereas knockout of both GmSWEET15 genes in soybean caused retarded embryo development and endosperm persistence, resulting in severe seed abortion. In addition, the embryo sugar content of the soybean knockout mutants was greatly reduced. These results demonstrate that the plasma membrane sugar transporter, GmSWEET15, is essential for embryo development in soybean by mediating Suc export from the endosperm to the embryo early in seed development.