The gibberellin biosynthetic genes AtKAO1 and AtKAO2 have overlapping roles throughout Arabidopsis development

Ent‐kaurenoic acid oxidase (KAO), a class of cytochrome P450 monooxygenases of the subfamily CYP88A, catalyzes the conversion of ent‐kaurenoic acid (KA) to gibberellin (GA) GA₁₂, the precursor of all GAs, thereby playing an important role in determining GA concentration in plants. Past work has demonstrated the importance of KAO activity for growth in various plant species. In Arabidopsis, this enzyme is encoded by two genes designated KAO1 and KAO2. In this study, we used various approaches to determine the physiological roles of KAO1 and KAO2 throughout plant development. Analysis of gene expression pattern reveals that both genes are mainly expressed in germinating seeds and young developing organs, thus suggesting functional redundancy. Consistent with this, kao1 and kao2 single mutants are indistinguishable from wild‐type plants. By contrast, the kao1 kao2 double mutant exhibits typical non‐germinating GA‐dwarf phenotypes, similar to those observed in the severely GA‐deficient ga1‐3 mutant. Phenotypic characterization and quantitative analysis of endogenous GA contents of single and double kao mutants further confirm an overlapping role of KAO1 and KAO2 throughout Arabidopsis development.     

ABI4 downregulates expression of the sodium transporter HKT1;1 in Arabidopsis roots and affects salt tolerance

A plant's ability to cope with salt stress is highly correlated with their ability to reduce the accumulation of sodium ions in the shoot. Arabidopsis mutants affected in the ABSCISIC ACID INSENSITIVE (ABI) 4 gene display increased salt tolerance, whereas ABI4‐overexpressors are hypersensitive to salinity from seed germination to late vegetative developmental stages. In this study we demonstrate that abi4 mutant plants accumulate lower levels of sodium ions and higher levels of proline than wild‐type plants following salt stress. We show higher HKT1;1 expression in abi4 mutant plants and lower levels of expression in ABI4‐overexpressing plants, resulting in reduced accumulation of sodium ions in the shoot of abi4 mutants. HKT1;1 encodes a sodium transporter which is known to unload sodium ions from the root xylem stream into the xylem parenchyma stele cells. We have shown recently that ABI4 is expressed in the root stele at various developmental stages and that it plays a key role in determining root architecture. Thus ABI4 and HKT1;1 are expressed in the same cells, which suggests the possibility of direct binding of ABI4 to the HKT1;1 promoter. In planta chromatin immunoprecipitation and in vitro electrophoresis mobility shift assays demonstrated that ABI4 binds two highly related sites within the HKT1;1 promoter. These sites, GC(C/G)GCTT(T), termed ABI4‐binding element (ABE), have also been identified in other ABI4‐repressed genes. We therefore suggest that ABI4 is a major modulator of root development and function.     

Distinct and overlapping roles of two gibberellin 3-oxidases in Arabidopsis development

Gibberellin (GA) 3-oxidase, a class of 2-oxoglutarate-dependent dioxygenases, catalyzes the conversion of precursor GAs to their bioactive forms, thereby playing a direct role in determining the levels of bioactive GAs in plants. Gibberellin 3-oxidase in Arabidopsis is encoded by a multigene family consisting of at least four members, designated AtGA3ox1 to AtGA3ox4. It has yet to be investigated how each AtGA3ox gene contributes to optimizing bioactive GA levels during growth and development. Using quantitative real-time PCR analysis, we have shown that each AtGA3ox gene exhibits a unique organ-specific expression pattern, suggesting distinct developmental roles played by individual AtGA3ox members. To investigate the sites of synthesis of bioactive GA in plants, we generated transgenic Arabidopsis that carried AtGA3ox1-GUS and AtGA3ox2-GUS fusions. Comparisons of the GUS staining patterns of these plants with that of AtCPS-GUS from previous studies revealed the possible physical separation of the early and late stages of the GA pathway in roots. Phenotypic characterization and quantitative analysis of the endogenous GA content of ga3ox1 and ga3ox2 single and ga3ox1/ga3ox2 double mutants revealed distinct as well as overlapping roles of AtGA3ox1 and AtGA3ox2 in Arabidopsis development. Our results show that AtGA3ox1 and AtGA3ox2 are responsible for the synthesis of bioactive GAs during vegetative growth, but that they are dispensable for reproductive development. The stage-specific severe GA-deficient phenotypes of the ga3ox1/ga3ox2 mutant suggest that AtGA3ox3 and AtGA3ox4 are tightly regulated by developmental cues; AtGA3ox3 and AtGA3ox4 are not upregulated to compensate for GA deficiency during vegetative growth of the double mutant.     

Enhanced resistance to soybean cyst nematode Heterodera glycines in transgenic soybean by silencing putative CLE receptors

CLE peptides are small extracellular proteins important in regulating plant meristematic activity through the CLE‐receptor kinase‐WOX signalling module. Stem cell pools in the SAM (shoot apical meristem), RAM (root apical meristem) and vascular cambium are controlled by CLE signalling pathways. Interestingly, plant‐parasitic cyst nematodes secrete CLE‐like effector proteins, which act as ligand mimics of plant CLE peptides and are required for successful parasitism. Recently, we demonstrated that Arabidopsis CLE receptors CLAVATA1 (CLV1), the CLAVATA2 (CLV2)/CORYNE (CRN) heterodimer receptor complex and RECEPTOR‐LIKE PROTEIN KINASE 2 (RPK2), which transmit the CLV3 signal in the SAM, are required for perception of beet cyst nematode Heterodera schachtii CLEs. Reduction in nematode infection was observed in clv1, clv2, crn, rpk2 and combined double and triple mutants. In an effort to develop nematode resistance in an agriculturally important crop, orthologues of Arabidopsis receptors including CLV1, CLV2, CRN and RPK2 were identified from soybean, a host for the soybean cyst nematode Heterodera glycines. For each of the receptors, there are at least two paralogues in the soybean genome. Localization studies showed that most receptors are expressed in the root, but vary in their level of expression and spatial expression patterns. Expression in nematode‐induced feeding cells was also confirmed. In vitro direct binding of the soybean receptors with the HgCLE peptide was analysed. Knock‐down of the receptors in soybean hairy roots showed enhanced resistance to SCN. Our findings suggest that targeted disruption of nematode CLE signalling may be a potential means to engineer nematode resistance in crop plants.     

MOL1 is required for cambium homeostasis in Arabidopsis

Plants maintain pools of pluripotent stem cells which allow them to constantly produce new tissues and organs. Stem cell homeostasis in shoot and root tips depends on negative regulation by ligand–receptor pairs of the CLE peptide and leucine‐rich repeat receptor‐like kinase (LRR‐RLK) families. However, regulation of the cambium, the stem cell niche required for lateral growth of shoots and roots, is poorly characterized. Here we show that the LRR‐RLK MOL1 is necessary for cambium homeostasis in Arabidopsis thaliana. By employing promoter reporter lines, we reveal that MOL1 is active in a domain that is distinct from the domain of the positively acting CLE41/PXY signaling module. In particular, we show that MOL1 acts in an opposing manner to the CLE41/PXY module and that changing the domain or level of MOL1 expression both result in disturbed cambium organization. Underlining discrete roles of MOL1 and PXY, both LRR‐RLKs are not able to replace each other when their expression domains are interchanged. Furthermore, MOL1 but not PXY is able to rescue CLV1 deficiency in the shoot apical meristem. By identifying genes mis‐expressed in mol1 mutants, we demonstrate that MOL1 represses genes associated with stress‐related ethylene and jasmonic acid hormone signaling pathways which have known roles in coordinating lateral growth of the Arabidopsis stem. Our findings provide evidence that common regulatory mechanisms in different plant stem cell niches are adapted to specific niche anatomies and emphasize the importance of a complex spatial organization of intercellular signaling cascades for a strictly bidirectional tissue production.     

Oryza sativa Lysine‐Histidine‐type Transporter 1 functions in root uptake and root‐to‐shoot allocation of amino acids in rice

In agricultural soils, amino acids can represent vital nitrogen (N) sources for crop growth and yield. However, the molecular mechanisms underlying amino acid uptake and allocation are poorly understood in crop plants. This study shows that rice (Oryza sativa L.) roots can acquire aspartate at soil concentration, and that japonica subspecies take up this acidic amino acid 1.5‐fold more efficiently than indica subspecies. Genetic association analyses with 68 representative japonica or indica germplasms identified rice Lysine‐Histidine‐type Transporter 1 (OsLHT1) as a candidate gene associated with the aspartate uptake trait. When expressed in yeast, OsLHT1 supported cell growth on a broad spectrum of amino acids, and effectively transported aspartate, asparagine and glutamate. OsLHT1 is localized throughout the rice root, including root hairs, epidermis, cortex and stele, and to the leaf vasculature. Knockout of OsLHT1 in japonica resulted in reduced root uptake of amino acids. Furthermore, in ¹⁵N‐amino acid‐fed mutants versus wild‐type, a higher percentage of ¹⁵N remained in roots instead of being allocated to the shoot. ¹⁵N‐ammonium uptake and subsequently the delivery of root‐synthesized amino acids to Oslht1 shoots were also significantly decreased, which was accompanied by reduced shoot growth. These results together provide evidence that OsLHT1 functions in both root uptake and root to shoot allocation of a broad spectrum of amino acids in rice.     

Activation of gibberellin 20-oxidase 2 undermines auxin-dependent root and root hair growth in NaCl-stressed <Emphasis Type="Italic">Arabidopsis</Emphasis> seedlings

Although salt stress mainly disturbs plant root growth by affecting the biosynthesis and signaling of phytohormones, such as gibberellin (GA) and auxin, the exact mechanisms of the crosstalk between these two hormones remain to be clarified. Indole-3-acetic acid (IAA) is a biologically active auxin molecule. In this study, we investigated the role of Arabidopsis GA20-oxidase 2 (GA20ox2), a final rate-limiting enzyme of active GA biosynthesis, in IAA-directed root growth under NaCl stress. Under the NaCl treatment, seedlings of a loss-of-function ga20ox2-1 mutant exhibited primary root and root hair elongation, altered GA₄ accumulation, and decreased root Na⁺ contents compared with the wild-type, transgenic GA20ox2-complementing, and GA20ox2-overexpression plant lines. Concurrently, ga20ox2-1 alleviated the tissue-specific inhibition of NaCl on IAA generation by YUCCAs, IAA transport by PIN1 and PIN2, and IAA accumulation in roots, thereby explaining how NaCl increased GA20ox2 expression in shoots but disrupted primary root and root hair growth in wild-type seedlings. In addition, a loss-of-function pin2 mutant impeded GA20ox2 expression, indicating that GA20ox2 function requires PIN2 activity. Thus, the activation of GA20ox2 retards IAA-directed primary root and root hair growth in response to NaCl stress.     

Shoot phytochrome B modulates reactive oxygen species homeostasis in roots via abscisic acid signaling in Arabidopsis

Underground roots normally reside in darkness. However, they are often exposed to ambient light that penetrates through cracks in the soil layers which can occur due to wind, heavy rain or temperature extremes. In response to light exposure, roots produce reactive oxygen species (ROS) which promote root growth. It is known that ROS‐induced growth promotion facilitates rapid escape of the roots from non‐natural light. Meanwhile, long‐term exposure of the roots to light elicits a ROS burst, which causes oxidative damage to cellular components, necessitating that cellular levels of ROS should be tightly regulated in the roots. Here we demonstrate that the red/far‐red light photoreceptor phytochrome B (phyB) stimulates the biosynthesis of abscisic acid (ABA) in the shoots, and notably the shoot‐derived ABA signals induce a peroxidase‐mediated ROS detoxification reaction in the roots. Accordingly, while ROS accumulate in the roots of the phyb mutant that exhibits reduced primary root growth in the light, such an accumulation of ROS did not occur in the dark‐grown phyb roots that exhibited normal growth. These observations indicate that mobile shoot‐to‐root ABA signaling links shoot phyB‐mediated light perception with root ROS homeostasis to help roots adapt to unfavorable light exposure. We propose that ABA‐mediated shoot‐to‐root phyB signaling contributes to the synchronization of shoot and root growth for optimal propagation and performance in plants.     

Developing xylem‐preferential expression of PdGA20ox1, a gibberellin 20‐oxidase 1 from Pinus densiflora,improves woody biomass production in a hybrid poplar

Woody biomass has gained popularity as an environmentally friendly, renewable and sustainable resource for liquid fuel production. Here, we demonstrate biotechnological improvement of the quantity and quality of woody biomass by employing developing xylem (DX)‐preferential production of gibberellin (GA), a phytohormone that positively regulates stem growth. First, for the proof of concept experiment, we produced transgenic Arabidopsis plants expressing GA20‐oxidase, a key enzyme in the production of bioactive GAs, from Pinus densiflora (PdGA20ox1) under the control of either a constitutive 35S promoter, designated 35S::PdGA20ox1, or a DX‐specific promoter (originated from poplar), designated DX15::PdGA20ox1. As we hypothesized, both transgenic Arabidopsis plants (35S::PdGA20ox1 and DX15::PdGA20ox1) exhibited an accelerated stem growth that resulted in a large increase of biomass, up to 300% compared to wild‐type control plants, together with increased secondary wall thickening and elongation of fibre cells. Next, we applied our concept to the production of transgenic poplar trees. Both transgenic poplar trees (35S::PdGA20ox1 and DX15::PdGA20ox1) showed dramatic increases in biomass, up to 300%, with accelerated stem growth and xylem differentiation. Cell wall monosaccharide composition analysis revealed that in both Arabidopsis and poplar, glucose and xylose contents were significantly increased. However, undesirable phenotypes of 35S::PdGA20ox1 poplar, including poor root growth and leaf development, were found. Interestingly, DX15::PdGA20ox1 poplar resulted in a reduction of undesirable phenotypes. Our results indicate that the controlled production of GAs through a tissue‐specific promoter can be utilized as an efficient biotechnological tool for producing enhanced plant biomass, minimizing unwanted effects.     

AtPER1 enhances primary seed dormancy and reduces seed germination by suppressing the ABA catabolism and GA biosynthesis in Arabidopsis seeds

Seed is vital to the conservation of germplasm and plant biodiversity. Seed dormancy is an adaptive trait in numerous seed‐plant species, enabling plants to survive under stressful conditions. Seed dormancy is mainly controlled by abscisic acid (ABA) and gibberellin (GA) and can be classified as primary and secondary seed dormancy. The primary seed dormancy is induced by maternal ABA. Here we found that AtPER1, a seed‐specific peroxiredoxin, is involved in enhancing primary seed dormancy. Two loss‐of‐function atper1 mutants, atper1‐1 and atper1‐2, displayed suppressed primary seed dormancy accompanied with reduced ABA and increased GA contents in seeds. Furthermore, atper1 mutant seeds were insensitive to abiotic stresses during seed germination. The expression of several ABA catabolism genes (CYP707A1, CYP707A2, and CYP707A3) and GA biosynthesis genes (GA20ox1, GA20ox3, and KAO3) in atper1 mutant seeds was increased compared to wild‐type seeds. The suppressed primary seed dormancy of atper1‐1 was completely reduced by deletion of CYP707A genes. Furthermore, loss‐of‐function of AtPER1 cannot enhance the seed germination ratio of aba2‐1 or ga1‐t, suggesting that AtPER1‐enhanced primary seed dormancy is dependent on ABA and GA. Additionally, the level of reactive oxygen species (ROS) in atper1 mutant seeds was significantly higher than that in wild‐type seeds. Taken together, our results demonstrate that AtPER1 eliminates ROS to suppress ABA catabolism and GA biosynthesis, and thus improves the primary seed dormancy and make the seeds less sensitive to adverse environmental conditions.     

Interplay between cytochrome c and gibberellins during Arabidopsis vegetative development

We studied the effect of reducing the levels of the mitochondrial electron carrier cytochrome c (CYTc) in Arabidopsis thaliana. Plants with CYTc deficiency have delayed growth and development, and reach flowering several days later than the wild‐type but with the same number of leaves. CYTc‐deficient plants accumulate starch and glucose during the day, and contain lower levels of active gibberellins (GA) and higher levels of DELLA proteins, involved in GA signaling. GA treatment abolishes the developmental delay and reduces glucose accumulation in CYTc‐deficient plants, which also show a lower raise in ATP levels in response to glucose. Treatment of wild‐type plants with inhibitors of mitochondrial energy production limits plant growth and increases the levels of DELLA proteins, thus mimicking the effects of CYTc deficiency. In addition, an increase in the amount of CYTc decreases DELLA protein levels and expedites growth, and this depends on active GA synthesis. We conclude that CYTc levels impinge on the activity of the GA pathway, most likely through changes in mitochondrial energy production. In this way, hormone‐dependent growth would be coupled to the activity of components of the mitochondrial respiratory chain.     

Auxin biosynthetic gene TAR2 is involved in low nitrogen‐mediated reprogramming of root architecture in Arabidopsis

In plants, the plasticity of root architecture in response to nitrogen availability largely determines nitrogen acquisition efficiency. One poorly understood root growth response to low nitrogen availability is an observed increase in the number and length of lateral roots (LRs). Here, we show that low nitrogen‐induced Arabidopsis LR growth depends on the function of the auxin biosynthesis gene TAR2 (tryptophan aminotransferase related 2). TAR2 was expressed in the pericycle and the vasculature of the mature root zone near the root tip, and was induced under low nitrogen conditions. In wild type plants, low nitrogen stimulated auxin accumulation in the non‐emerged LR primordia with more than three cell layers and LR emergence. Conversely, these low nitrogen‐mediated auxin accumulation and root growth responses were impaired in the tar2‐c null mutant. Overexpression of TAR2 increased LR numbers under both high and low nitrogen conditions. Our results suggested that TAR2 is required for reprogramming root architecture in response to low nitrogen conditions. This finding suggests a new strategy for improving nitrogen use efficiency through the engineering of TAR2 expression in roots.     

Perception of root‐active CLE peptides requires CORYNE function in the phloem vasculature

Arabidopsis root development is orchestrated by signaling pathways that consist of different CLAVATA3/EMBRYO SURROUNDING REGION (CLE) peptide ligands and their cognate CLAVATA (CLV) and BARELY ANY MERISTEM (BAM) receptors. How and where different CLE peptides trigger specific morphological or physiological changes in the root is poorly understood. Here, we report that the receptor‐like protein CLAVATA 2 (CLV2) and the pseudokinase CORYNE (CRN) are necessary to fully sense root‐active CLE peptides. We uncover BAM3 as the CLE45 receptor in the root and biochemically map its peptide binding surface. In contrast to other plant peptide receptors, we found no evidence that SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) proteins act as co‐receptor kinases in CLE45 perception. CRN stabilizes BAM3 expression and thus is required for BAM3‐mediated CLE45 signaling. Moreover, protophloem‐specific CRN expression complements resistance of the crn mutant to root‐active CLE peptides, suggesting that protophloem is their principal site of action. Our work defines a genetic framework for dissecting CLE peptide signaling and CLV/BAM receptor activation in the root.