Root hairs and rhizoids are cells with rooting functions in land plants. We describe two basic helix-loop-helix transcription factors that control root hair development in the sporophyte (2n) of the angiosperm Arabidopsis thaliana and rhizoid development in the gametophytes (n) of the bryophyte Physcomitrella patens. The phylogeny of land plants supports the hypothesis that early land plants were bryophyte-like and possessed a dominant gametophyte and later the sporophyte rose to dominance. If this hypothesis is correct, our data suggest that the increase in morphological complexity of the sporophyte body in the Paleozoic resulted at least in part from the recruitment of regulatory genes from gametophyte to sporophyte.

Postmitotic cell growth defines cell shape and size during development. However, the mechanisms regulating postmitotic cell growth in plants remain unknown. Here we report the discovery of a basic helix-loop-helix (bHLH) transcription factor called RSL4 (ROOT HAIR DEFECTIVE 6-LIKE 4) that is sufficient to promote postmitotic cell growth in Arabidopsis thaliana root-hair cells. Loss of RSL4 function resulted in the development of very short root hairs. In contrast, constitutive RSL4 expression programmed constitutive growth, resulting in the formation of very long root hairs. Hair-cell growth signals, such as auxin and low phosphate availability, modulate hair cell extension by regulating RSL4 transcript and protein levels. RSL4 is thus a regulator of growth that integrates endogenous developmental and exogenous environmental signals that together control postmitotic growth in root hairs. The control of postmitotic growth by transcription factors may represent a general mechanism for regulating cell size across diverse organisms.

PHR2, a central regulator of phosphate signaling in rice, enhanced the expression of phosphate starvation-induced (PSI) genes and resulted in the enhancement of Pi acquisition under Pi deficiency stress. This occurred via PHR2 binding to a cis-element named the PHR1 binding sequences. However, the transcription level of PHR2 was not responsive to Pi starvation. So how is activity of transcription factor PHR2 adjusted to adapt diverse Pi status? Here, we identify an SPX family protein, Os-SPX4 (SPX4 hereafter), involving in Pi starvation signaling and acting as a negative regulator of PHR2. SPX4 is shown to be a fast turnover protein. When Pi is sufficient, through its interaction with PHR2, SPX4 inhibits the binding of PHR2 to its cis-element and reduces the targeting of PHR2 to the nucleus. However, when plants grow under Pi deficiency, the degradation of SPX4 is accelerated through the 26S proteasome pathway, thereby releasing PHR2 into the nucleus and activating the expression of PSI genes. Because the level of SPX4 is responsive to Pi concentration and SPX4 interacts with PHR2 and regulates its activity, this suggests that SPX4 senses the internal Pi concentration under diverse Pi conditions and regulates appropriate responses to maintain Pi homeostasis in plants.

SummaryFour genes of Arabidopsis (At5g20150, At2g26660, At2g45130 and At5g15330) encoding no conservative region other than an SPX domain (SYG1, Pho81 and XPR1) were named AtSPX1-AtSPX4. The various subcellular localizations of their GFP fusion proteins implied function variations for the four genes. Phosphate starvation strongly induced expression of AtSPX1 and AtSPX3 with distinct dynamic patterns, while AtSPX2 was weakly induced and AtSPX4 was suppressed. Expression of the four AtSPX genes was reduced to different extents in the Arabidopsis phr1 and siz1 mutants under phosphate starvation, indicating that they are part of the phosphate-signaling network that involves SIZ1/PHR1. Over-expression of AtSPX1 increased the transcript levels of ACP5, RNS1 and PAP2 under both phosphate-sufficient and phosphate-deficient conditions, suggesting a potential transcriptional regulation role of AtSPX1 in response to phosphate starvation. Partial repression of AtSPX3 by RNA interference led to aggravated phosphate-deficiency symptoms, altered P allocation and enhanced expression of a subset od phosphate-responsive genes including AtSPX1. Our results indicate that both AtSPX1 and AtSPX3 play positive roles in plant adaptation to phosphate starvation, and AtSPX3 may have a negative feedback regulatory role in AtSPX1 response to phosphate starvation.

We report here on a novel transcription factor with a basic helix-loop-helix domain for tolerance to inorganic phosphate (Pi) starvation in rice (Oryza sativa). The gene is designated OsPTF1. The expression of OsPTF1 is Pi starvation induced in roots while constitutively expressed in shoots, as shown by northern-blot analysis. Overexpression of OsPTF1 enhanced tolerance to Pi starvation in transgenic rice. Tillering ability, root and shoot biomass, and phosphorus content of transgenic rice plants were about 30% higher than those of the wild-type plants in Pi-deficient conditions in hydroponic experiments. In soil pot and field experiments, more than 20% increase in tiller number, panicle weight, and phosphorus content was observed in transgenic plants compared to wild-type plants at low-Pi levels. In Pi-deficient conditions, transgenic rice plants showed significantly higher total root length and root surface area, which results in a higher instantaneous Pi uptake rate over their wild-type counterparts. Microarray analysis for transgenic plants overexpressing OsPTF1 has been performed to investigate the downstream regulation of OsPTF1.

Phosphate transporters (PTs) mediate phosphorus uptake and are regulated at the transcriptional and posttranslational levels. In one key mechanism of posttranslational regulation, phosphorylation of PTs affects their trafficking from the endoplasmic reticulum (ER) to the plasma membrane. However, the kinase(s) mediating PT phosphorylation and the mechanism leading to ER retention of phosphorylated PTs remain unclear. In this study, we identified a rice (Oryza sativa) kinase subunit, CK2b3, which interacts with PT2 and PT8 in a yeast two-hybrid screen. Also, the CK2a3/b3 holoenzyme phosphorylates PT8 under phosphate-sufficient conditions. This phosphorylation inhibited the interaction of PT8 with PHOSPHATE TRANSPORTER TRAFFIC FACILITATOR1, a key cofactor regulating the exit of PTs from the ER to the plasma membrane. Additionally, phosphorus starvation promoted CK2b3 degradation, relieving the negative regulation of PT phosphorus-insufficient conditions. In accordance, transgenic expression of a nonphosphorylatable version of OsPT8 resulted in elevated levels of that protein at the plasma membrane and enhanced phosphorus accumulation and plant growth under various phosphorus regimes. Taken together, these results indicate that CK2a3/b3 negatively regulates PTs and phosphorus status regulates CK2a3/b3.

Glu receptors are known to function as Glu-activated ion channels that mediate mostly excitatory neurotransmission in animals. Glu receptor–like genes have also been reported in higher plants, although their function is largely unknown. We have identified a rice (Oryza sativa) Glu receptor–like gene, designated GLR3.1, in which mutation by T-DNA insertion caused a short-root mutant phenotype. Histology and DNA synthesis analyses revealed that the mutant root meristematic activity is distorted and is accompanied by enhanced programmed cell death. Our results supply genetic evidence that a plant Glu receptor–like gene, rice GLR3.1, is essential for the maintenance of cell division and individual cell survival in the root apical meristem at the early seedling stage.

Phosphorus (P), an essential macronutrient for all living cells, is indispensable for agricultural production. Although Arabidopsis (Arabidopsis thaliana) PHOSPHATE RESPONSE1 (PHR1) and its orthologs in other species have been shown to function in transcriptional regulation of phosphate (Pi) signaling and Pi homeostasis, an integrative comparison of PHR1-related proteins in rice (Oryza sativa) has not previously been reported. Here, we identified functional redundancy among three PHR1 orthologs in rice (OsPHR1, OsPHR2, and OsPHR3) using phylogenetic and mutation analysis. OsPHR3 in conjunction with OsPHR1 and OsPHR2 function in transcriptional activation of most Pi starvation-induced genes. Loss-of-function mutations in any one of these transcription factors (TFs) impaired root hair growth (primarily root hair elongation). However, these three TFs showed differences in DNA binding affinities and messenger RNA expression patterns in different tissues and growth stages, and transcriptomic analysis revealed differential effects on Pi starvation-induced gene expression of single mutants of the three TFs, indicating some degree of functional diversification. Overexpression of genes encoding any of these TFs resulted in partial constitutive activation of Pi starvation response and led to Pi accumulation in the shoot. Furthermore, unlike OsPHR2-overexpressing lines, which exhibited growth retardation under normal or Pi-deficient conditions, OsPHR3-overexpressing plants exhibited significant tolerance to low-Pi stress but normal growth rates under normal Pi conditions, suggesting that OsPHR3 would be useful for molecular breeding to improve Pi uptake/use efficiency under Pi-deficient conditions. We propose that OsPHR1, OsPHR2, and OsPHR3 form a network and play diverse roles in regulating Pi signaling and homeostasis in rice.