Evolution of Plant Sucrose Uptake Transporters

In angiosperms, sucrose uptake transporters (SUTs) have important functions especially in vascular tissue. Here we explore the evolutionary origins of SUTs by analysis of angiosperm SUTs and homologous transporters in a vascular early land plant, Selaginella moellendorffii, and a non-vascular plant, the bryophyte Physcomitrella patens, the charophyte algae Chlorokybus atmosphyticus, several red algae and fission yeast, Schizosaccharomyces pombe. Plant SUTs cluster into three types by phylogenetic analysis. Previous studies using angiosperms had shown that types I and II are localized to the plasma membrane while type III SUTs are associated with vacuolar membrane. SUT homologs were not found in the chlorophyte algae Chlamydomonas reinhardtii and Volvox carterii. However, the characean algae Chlorokybus atmosphyticus contains a SUT homolog (CaSUT1) and phylogenetic analysis indicated that it is basal to all other streptophyte SUTs analyzed. SUTs are present in both red algae and S. pombe but they are less related to plant SUTs than CaSUT1. Both Selaginella and Physcomitrella encode type II and III SUTs suggesting that both plasma membrane and vacuolar sucrose transporter activities were present in early land plants. It is likely that SUT transporters are important for scavenging sucrose from the environment and intracellular compartments in charophyte and non-vascular plants. Type I SUTs were only found in eudicots and we conclude that they evolved from type III SUTs, possibly through loss of a vacuolar targeting sequence. Eudicots utilize type I SUTs for phloem (vascular tissue) loading while monocots use type II SUTs for phloem loading. We show that HvSUT1 from barley, a type II SUT, reverted the growth defect of the Arabidopsis atsuc2 (type I) mutant. This indicates that type I and II SUTs evolved similar (and interchangeable) phloem loading transporter capabilities independently

Arabidopsis bHLH100 and bHLH101 Control Iron Homeostasis via a FIT-Independent Pathway

Iron deficiency induces a complex set of responses in plants, including developmental and physiological changes, to increase iron uptake from soil. In Arabidopsis, many transporters involved in the absorption and distribution of iron have been identified over the past decade. However, little is known about the signaling pathways and networks driving the various responses to low iron. Only the basic helix–loop–helix (bHLH) transcription factor FIT has been shown to control the expression of the root iron uptake machinery genes FRO2 and IRT1. Here, we characterize the biological role of two other iron-regulated transcription factors, bHLH100 and bHLH101, in iron homeostasis. First direct transcriptional targets of FIT were determined in vivo. We show that bHLH100 and bHLH101 do not regulate FIT target genes, suggesting that they play a non-redundant role with the two closely related bHLH factors bHLH038 and bHLH039 that have been suggested to act in concert with FIT. bHLH100 and bHLH101 play a crucial role in iron-deficiency responses, as attested by their severe growth defects and iron homeostasis related phenotypes on low-iron media. To gain further insight into the biological role of bHLH100 and bHLH101, we performed microarray analysis using the corresponding double mutant and showed that bHLH100 and bHLH101 likely regulate genes involved in the distribution of iron within the plant. Altogether, this work establishes bHLH100 and bHLH101 as key regulators of iron-deficiency responses independent of the master regulator FIT and sheds light on new regulatory networks important for proper growth and development under low iron conditions

Arabidopsis bHLH100 and bHLH101 Control Iron Homeostasis via a FIT-Independent Pathway

<div><p>Iron deficiency induces a complex set of responses in plants, including developmental and physiological changes, to increase iron uptake from soil. In Arabidopsis, many transporters involved in the absorption and distribution of iron have been identified over the past decade. However, little is known about the signaling pathways and networks driving the various responses to low iron. Only the basic helix–loop–helix (bHLH) transcription factor FIT has been shown to control the expression of the root iron uptake machinery genes <em>FRO2</em> and <em>IRT1</em>. Here, we characterize the biological role of two other iron-regulated transcription factors, bHLH100 and bHLH101, in iron homeostasis. First direct transcriptional targets of FIT were determined <em>in vivo</em>. We show that bHLH100 and bHLH101 do not regulate FIT target genes, suggesting that they play a non-redundant role with the two closely related bHLH factors bHLH038 and bHLH039 that have been suggested to act in concert with FIT. bHLH100 and bHLH101 play a crucial role in iron-deficiency responses, as attested by their severe growth defects and iron homeostasis related phenotypes on low-iron media. To gain further insight into the biological role of bHLH100 and bHLH101, we performed microarray analysis using the corresponding double mutant and showed that bHLH100 and bHLH101 likely regulate genes involved in the distribution of iron within the plant. Altogether, this work establishes bHLH100 and bHLH101 as key regulators of iron-deficiency responses independent of the master regulator FIT and sheds light on new regulatory networks important for proper growth and development under low iron conditions.</p> </div

Phylogeny and identification of <i>bhlh100/bhlh101</i> double mutant.

<p>Phylogenetic tree of closely related bHLH genes involved in the iron-deficiency response. The tree was created using MUSCLE alignment of protein sequences and neighbor joining (bioNJ) on phylogeny.fr <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0044843#pone.0044843-Gascuel1" target="_blank">[20]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0044843#pone.0044843-Edgar1" target="_blank">[22]</a>.</p

35S::FIT:GR <i>fit-2</i> plants express functional FIT protein.

<p>Two-week-old plants grown on 1/2 MS in the absence (A) or presence (B) of 30 µM dexamethasone (DEX).</p

<i>bhlh100/bhlh101</i> plants accumulate less chlorophyll and less iron than wild-type.

<p>(A) Chlorophyll content expressed per gram of fresh weight (FW) was measured in extracts of plants grown on 1/2 MS without added iron (–Fe) or supplemented with 100 µM Fe (+Fe). (B) Iron was measured in plants grown on 1/2 MS without added iron (–Fe) or in the presence of 100 µM Fe (+Fe). * indicates a statistically significant difference between wild-type (WT) and <i>bhlh100/bhlh101</i>. (C) Seed iron content of wild-type (WT) and <i>bhlh100/bhlh101</i>.</p

Expression of selected FIT-independent genes in <i>bhlh100/bhlh101</i> and wild-type backgrounds.

<p>Relative gene expression level of <i>bHLH038</i> (A), <i>bHLH039</i> (B), <i>ZIF</i> (C), and <i>MTP3</i> (D) in wildtype (WT) and <i>bhlh100/bhlh101</i> plants grown on 1/2 MS without iron (black) or in the presence of 100 µM Fe (white). Error bars indicate standard error (n = 3–4).</p

Heatmap of differentially expressed genes in roots of wild-type and <i>bhlh100/bhlh101</i> plants.

<p>Genes found to be differentially expressed between the +Fe conditions (100 µM Fe) and the –Fe conditions for both wild-type (WT) and <i>bhlh100/bhlh101</i> plants were clustered using a Pearson correlation into 12 clusters. Clusters with significantly enriched GO terms (left) or known iron-regulated genes (right) were numbered.</p

<i>bhlh100/bhlh101</i> plants flower later than wild-type.

<p>(A) Number of leaves of greenhouse-grown wild-type (WT) and <i>bhlh100/bhlh101</i> double mutant plants. Leaves were counted at the time of bolting (n = 25 for each genotype; p-value = 0.0170). (B) Greenhouse-grown plants displaying late-flowering phenotype.</p