Transcription Factor IAA27 Positively Regulates P Uptake through Promoted Adventitious Root Development in Apple Plants.

Phosphate (P) deficiency severely limits the growth and production of plants. Adventitious root development plays an essential role in responding to low phosphorus stress for apple plants. However, the molecular mechanisms regulating adventitious root growth and development in response to low phosphorus stress have remained elusive. In this study, a mutation (C-T) in the coding region of the apple AUXIN/INDOLE-3-ACETIC ACID 27 (IAA27) gene was identified. MdIAA27T-overexpressing transgenic apple improved the tolerance to phosphorus deficiency, which grew longer and denser adventitious roots and presented higher phosphorous content than the control plants under low phosphorus conditions, while the overexpression of MdIAA27C displayed the opposite trend. Moreover, the heterologous overexpression of MdIAA27 in tobacco yielded the same results, supporting the aforementioned findings. In vitro and in vivo assays showed that MdIAA27 directly interacted with AUXIN RESPONSE FACTOR (ARF8), ARF26 and ARF27, which regulated Small Auxin-Up RNA 76 (MdSAUR76) and lateral organ boundaries domain 16 (MdLBD16) transcription. The mutation in IAA27 resulted in altered interaction modes, which in turn promoted the release of positive ARFs to upregulate SAUR76 and LBD16 expression in low phosphorus conditions. Altogether, our studies provide insights into how the allelic variation of IAA27 affects adventitious root development in response to low phosphorus stress.

Siderophore production in pseudomonas SP. strain SP3 enhances iron acquisition in apple rootstock.

AIMS: The purpose of this study was to analyse the effects of siderophore-producing bacteria and bacterial siderophore on the iron nutrition of apple rootstocks under iron-deficient conditions. METHODS AND RESULTS: We isolated three Pseudomonas strains, SP1, SP2 and SP3 from the rhizosphere of the Fe-efficient apple rootstocks using the chrome azurol S agar plate assay. We found that all three strains had the ability to secrete indole acetic acid-like compounds and siderophores, especially SP3. When Fe-inefficient rootstocks treated with SP3 were grown in alkaline soil, an increase in the biomass, root development, and Fe concentration was observed in the plants. In addition, SP3 secreted pyoverdine, a siderophore that can chelate Fe3+ to enhance the bioavailability of Fe for plants. We purified the pyoverdine from the SP3 culture supernatant. Hydroponic experiments were conducted with a Fe-deficient solution supplemented with pyoverdine, resulting in a reduction in the chlorosis caused by Fe deficiency and marked improvement in Fe uptake. CONCLUSIONS: Under iron-deficient conditions, Pseudomonas sp. strain SP3 can effectively promote apple rootstock growth and improve plant iron nutrition by secreting siderophores that enhance Fe availability. SIGNIFICANCE AND IMPACT OF THE STUDY: This study showed that plant growth-promoting rhizobacteria from Fe-efficient plants have the potential to improve iron nutrition in Fe-inefficient plants, and Fe-siderophore chelates can be used as an effective source of iron for apple plants. Based on these findings, it may be possible to develop biological agents such as siderophore-producing bacteria for sustainable agricultural and horticultural production.

Downregulation of the auxin transporter gene SlPIN8 results in pollen abortion in tomato.

KEY MESSAGE: SlPIN8 is expressed specifically within tomato pollen, and that it is involved in tomato pollen development and intracellular auxin homeostasis. The auxin (IAA) transport protein PIN-FORMED (PIN) plays key roles in various aspects of plant development. The biological role of the auxin transporter SlPIN8 in tomato development remains unclear. Here, we examined the expression pattern of the SlPIN8 gene in vegetative and reproductive organs of tomato. RNA interference (RNAi) transgenic lines specifically silenced for the SlPIN8 gene were generated to identify the role of SlPIN8 in pollen development. We found that SlPIN8 mRNA is expressed specifically within tomato pollen. In the anthers, the highest mRNA expression and ß-glucuronidase (GUS) activity of promoter-SlPIN8-GUS was detected during late stages of anther development, when pollen maturation occurred. The downregulation of SlPIN8 did not drastically affect the vegetative growth of tomato. However, in SlPIN8-RNAi transgenic plants, approximately 80% of the pollen grains were identified to be abnormal and lack viability; they were shriveled and flattened. Furthermore, the downregulation of SlPIN8 affected the gene expression of some anther development-specific proteins. SlPIN8-RNAi transgenic plants induced seedless fruits because of defective pollen function rather than defective female gametophyte function. In addition, SlPIN8 was found to localize to the endoplasmic reticulum, consistent with the changes in the auxin levels of SlPIN8-RNAi lines, whereas the level of free IAA was increased in SlPIN8-overexpressing protoplasts, indicating that SlPIN8 is involved in intracellular auxin homeostasis.

Characterization of MxFIT, an iron deficiency induced transcriptional factor in Malus xiaojinensis.

Iron deficiency often results in nutritional disorder in fruit trees. Transcription factors play an important role in the regulation of iron uptake. In this study, we isolated an iron deficiency response transcription factor gene, MxFIT, from an iron-efficient apple genotype of Malus xiaojinensis. MxFIT encoded a basic helix-loop-helix protein and contained a 966 bp open reading frame. MxFIT protein was targeted to the nucleus in onion epidermal cells and showed strong transcriptional activation in yeast cells. Spatiotemporal expression analysis revealed that MxFIT was up-regulated in roots under iron deficiency at both mRNA and protein levels, while almost no expression was detected in leaves irrespective of iron supply. Ectopic expression of MxFIT resulted in enhanced iron deficiency responses in Arabidopsis under iron deficiency and stronger resistance to iron deficiency. Thus, MxFIT might be involved in iron uptake and plays an important role in iron deficiency response.

The iron-regulated transporter, MbNRAMP1, isolated from Malus baccata is involved in Fe, Mn and Cd trafficking.

BACKGROUND AND AIMS: Iron deficiency is one of the most common nutritional disorders in plants, especially in fruit trees grown in calcareous soil. Malus baccata is widely used as an apple rootstock in north China and is highly resistant to low temperatures. There are few studies on iron absorption by this species at the molecular level. It is very important to understand the mechanism of iron uptake and transport in such woody plants. As a helpful tool, the aim of the present study was the cloning and functional analysis of NRAMP (natural resistance-associated macrophage protein) genes from the apple tree in relation to trafficking of micronutrients (Fe, Mn and Cd). METHODS: Reverse transcription-PCR (RT-PCR) combined with RACE (rapid amplification of cDNA ends) was adopted to isolate the full-length NRAMP1 cDNA. Southern blotting was used to test gene copy information, and northern blot was used to detect the gene's expression level. Complementation experiments using the yeast mutant strains DEY1453 and SLY8 were employed to confirm the iron- and manganese-transporting ability of NRAMP1 from apple, and inductively coupled plasma (ICP) spectrometry was used to measure Cd accumulation in yeast. NRAMP1-green fluorescent protein (GFP) fusion protein was used to determine the cellular localization in yeast. KEY RESULTS: A 2090 bp cDNA was isolated and named MbNRAMP1. It encodes a predicted polypeptide of 551 amino acids. MbNRAMP1 exists in the M. baccata genome as a single copy and was expressed mainly in roots. MbNRAMP1 rescued the phenotype of yeast mutant strains DEY1453 and SLY8, and also increased Cd2+ sensitivity and accumulation. MbNRAMP1 expression in yeast was largely influenced by iron status, and the expression pattern of MbNRAMP1-GFP varied with the environmental iron nutrition status. CONCLUSIONS: MbNRAMP1 encodes a functional metal transporter capable of mediating the distribution of ions as well as transport of the micronutrients, Fe and Mn, and the toxic metal, Cd.