The aberrant cell walls of boron-deficient bean root nodules have no covalently bound hydroxyproline-/proline-rich proteins

Boron seed coating combined with seed inoculation with boron tolerant bacteria (Bacillus sp. MN-54) and maize stalk biochar improved growth and productivity of maize (Zea mays L.) on saline soil

Salinity exerts significant negative impacts on growth and productivity of crop plants and numerous management practices are used to improve crop performance under saline environments. Micronutrients, plant growth promoting bacteria and biochar are known to improve crop productivity under stressful environments. Maize (Zea mays L.) is an important cereal crop and its productivity is adversely impacted by salinity. Although boron (B) application, seed inoculation with boron-tolerant bacteria (BTB) and biochar are known to improve maize growth under stressful environments, there is less information on their combined impact in enhancing maize productivity on saline soils. This study investigated the impact of B seed coating combined with seed inoculation with BTB + biochar on maize productivity under saline soil. Four B seed coating levels [0.0 (no seed coating), 1.0, 1.5, 2.0 g B kg-1 seed], and individual or combined application of 5 % (w/w) maize stalk biochar, and seed inoculation with Bacillus sp. MN-54 BTB were included in the study. Different growth and yield attributes and grain quality were significantly improved by seed coating with 1.5 B kg-1 seed coupled with biochar + BTB. Seed coating with 1.5 B kg-1 seed combined with biochar + BTB improved stomatal conductance by 32 %, photosynthetic rate by 15 %, and transpiration ratio by 52 % compared to seed coating (0 B kg-1 seed) combined with biochar only. Similarly, the highest plant height (189 cm), number of grain rows cob-1 (15.5), grain yield (54.9 g plant-1), biological yield (95.5 g plant-1), and harvest index (57.6 %) were noted for B seed coating (1.5 g B kg-1 seed) combined with biochar + BTB inoculation. The same treatment resulted in the highest grain protein and B contents. It is concluded that B seed coating at 1.5 g B kg-1 seed combined with biochar + BTB inoculation could significantly improve yield and quality of maize crop on saline soils. However, further field experiments investigating the underlying mechanisms are needed to reach concrete conclusions and large-scale recommendations.

A critical review of plant adaptation to environmental boron stress: Uptake, utilization, and interplay with other abiotic and biotic factors

Boron (B) is an indispensable mineral nutrient for plants and is primarily taken up by roots mainly in the form of boric acid (H3BO3). Recently, research shows that B has a significant impact on plant growth and productivity due to its narrow range between deficiency and toxicity. Fertilization and other procedures to address B stress (deficiency and toxicity) in soils are generally expensive and time-consuming. Over the past 20 years, substantial studies have been conducted to investigate the mechanisms underlying B acquisition and the molecular regulation of B stress in plants. In this review, we discuss the effects of B stress on plant growth, physiology, and biochemistry, and finding on enhancing plant tolerance from the perspective of plant B uptake, transport, and utilization. We also refer to recent results demonstrating the interactions among B and other biological and abiotic factors, including nitrogen, phosphorus, aluminum, and microorganisms. Finally, emerging trends in this field are discussed.

What Can Boron Deficiency Symptoms Tell Us about Its Function and Regulation?

On the eve of the 100th anniversary of Dr. Warington's discovery of boron (B) as a nutrient essential for higher plants, "boronists" have struggled to demonstrate a role beyond its structural function in cell walls dimerizing pectin molecules of rhamnogalacturonan II (RGII). In this regard, B deficiency has been associated with a plethora of symptoms in plants that include macroscopic symptoms like growth arrest and cell death and biochemical or molecular symptoms that include changes in cell wall pore size, apoplast acidification, or a steep ROS production that leads to an oxidative burst. Aiming to shed light on B functions in plant biology, we proposed here a unifying model integrating the current knowledge about B function(s) in plants to explain why B deficiency can cause such remarkable effects on plant growth and development, impacting crop productivity. In addition, based on recent experimental evidence that suggests the existence of different B ligands other than RGII in plant cells, namely glycolipids, and glycoproteins, we proposed an experimental pipeline to identify putative missing ligands and to determine how they would integrate into the above-mentioned model.

Interaction between Boron and Other Elements in Plants

Boron (B) is an essential mineral nutrient for growth of plants, and B deficiency is now a worldwide problem that limits production of B deficiency-sensitive crops, such as rape and cotton. Agronomic practice has told that balanced B and other mineral nutrient fertilizer applications is helpful to promote crop yield. In recent years, much research has reported that applying B can also reduce the accumulation of toxic elements such as cadmium and aluminum in plants and alleviate their toxicity symptoms. Therefore, the relation between B and other elements has become an interesting issue for plant nutritionists. Here we summarize the research progress of the interaction between B and macronutrients such as nitrogen, phosphorus, calcium, potassium, magnesium, and sulfur, essential micronutrients such as iron, manganese, zinc, copper, and molybdenum, and beneficial elements such as sodium, selenium, and silicon. Moreover, the interaction between B and toxic elements such as cadmium and aluminum, which pose a serious threat to agriculture, is also discussed in this paper. Finally, the possible physiological mechanisms of the interaction between B and other elements in plants is reviewed. We propose that the cell wall is an important intermediary between interaction of B and other elements, and competitive inhibition of elements and related signal transduction pathways also play a role. Currently, research on the physiological role of B in plants mainly focuses on its involvement in the structure and function of cell walls, and our understanding of the details for interactions between B and other elements also tend to relate to the cell wall. However, we know little about the metabolic process of B inside cells, including its interactions with other elements. More research is needed to address the aforementioned research questions in future.

Elucidating the impact of boron fertilization on soil physico-chemical and biological entities under cauliflower-cowpea-okra cropping system in an Eastern Himalayan acidic Inceptisol

Information on the role of boron (B) on soil physico-chemical and biological entities is scarce, and the precise mechanism in soil is still obscure. Present field investigation aimed to assessing the implication of direct and residual effect of graded levels of applied-B on soil biological entities and its concomitant impact on crop productivity. The treatments comprised of five graded levels of B with four replications. To assess the direct effect of B-fertilization, cauliflower was grown as a test crop wherein, B-fertilization was done every year. For assessment of succeeding residual effects of B-fertilization, cowpea and okra were grown as test crops and, B-fertilization was phased out in both crops. The 100% recommended dose of NPK (RDF) along with FYM was uniformly applied to all crops under CCOCS. Results indicated that the direct effect of B had the edge over residual effect of B in affecting soil physico-chemical and biological entities under CCOCS. Amongst the graded levels of B, application of the highest B level (2 kg ha^–1) was most prominent in augmenting microbiological pools in soil at different crop growth stages. The order of B treatments in respect of MBC, MBN, and soil respiration at different crop growth stages was 2.0 kg B ha^–1 > 1.5 kg B ha^–1 > 1.0 kg B ha^–1 > 0.5 kg B ha^–1 > 0 kg B ha^–1, respectively. Moreover, maximum recoveries of potentially mineralizable-C (PMC) and potentially mineralizable-N (PMN) were noticed under 2 kg B ha^–1. Analogous trend was recorded in soil microbial populations at different crop growth stages. Similarly, escalating B levels up to 2 kg B ha^–1 exhibited significantly greater soil enzymatic activities viz., arylsulphatase (AS), dehydrogenase (DH), fluorescein diacetate (FDA) and phosphomonoesterase (PMA), except urease enzyme (UE) which showed an antagonistic effect of applied-B in soil. Greater geometric mean enzyme activity (GMEA) and soil functional diversity index were recorded under 2 kg B ha^–1 in CCOCS, at all crop growth stages over control. The inclusive results indicated that different soil physico-chemical and biological properties CCOCS can be invariably improved by the application of graded levels of B up to 2 kg B ha^–1 in an acid Inceptisol.

Structure and Development of the Legume-Rhizobial Symbiotic Interface in Infection Threads

The intracellular infection thread initiated in a root hair cell is a unique structure associated with Rhizobium-legume symbiosis. It is characterized by inverted tip growth of the plant cell wall, resulting in a tunnel that allows invasion of host cells by bacteria during the formation of the nitrogen-fixing root nodule. Regulation of the plant-microbial interface is essential for infection thread growth. This involves targeted deposition of the cell wall and extracellular matrix and tight control of cell wall remodeling. This review describes the potential role of different actors such as transcription factors, receptors, and enzymes in the rearrangement of the plant-microbial interface and control of polar infection thread growth. It also focuses on the composition of the main polymers of the infection thread wall and matrix and the participation of reactive oxygen species (ROS) in the development of the infection thread. Mutant analysis has helped to gain insight into the development of host defense reactions. The available data raise many new questions about the structure, function, and development of infection threads.

Identification of boric acid as a novel chemoattractant and elucidation of its chemoreceptor in Ralstonia pseudosolanacearum Ps29

Chemotaxis enables bacteria to move toward more favorable environmental conditions. We observed chemotaxis toward boric acid by Ralstonia pseudosolanacearum Ps29. At higher concentrations, the chemotactic response of R. pseudosolanacearum toward boric acid was comparable to or higher than that toward L-malate, indicating that boric acid is a strong attractant for R. pseudosolanacearum. Chemotaxis assays under different pH conditions suggested that R. pseudosolanacearum recognizes B(OH)₃ (or B(OH₃) + B(OH)₄⁻) but not B(OH)₄⁻ alone. Our previous study revealed that R. pseudosolanacearum Ps29 harbors homologs of all 22R. pseudosolanacearum GMI1000 mcp genes. Screening of 22 mcp single-deletion mutants identified the RS_RS17100 homolog as the boric acid chemoreceptor, which was designated McpB. The McpB ligand-binding domain (LBD) was purified in order to characterize its binding to boric acid. Using isothermal titration calorimetry, we demonstrated that boric acid binds directly to the McpB LBD with a K_D (dissociation constant) of 5.4 µM. Analytical ultracentrifugation studies revealed that the McpB LBD is present as a dimer that recognizes one boric acid molecule.

Boron-deficiency-responsive microRNAs and their targets in Citrus sinensis leaves

BACKGROUND

MicroRNAs play important roles in the adaptive responses of plants to nutrient deficiencies. Most research, however, has focused on nitrogen (N), phosphorus (P), sulfur (S), copper (Cu) and iron (Fe) deficiencies, limited data are available on the differential expression of miRNAs and their target genes in response to deficiencies of other nutrient elements. In this study, we identified the known and novel miRNAs as well as the boron (B)-deficiency-responsive miRNAs from citrus leaves in order to obtain the potential miRNAs related to the tolerance of citrus to B-deficiency.

METHODS

Seedlings of 'Xuegan' [Citrus sinensis (L.) Osbeck] were supplied every other day with B-deficient (0 μM H3BO3) or -sufficient (10 μM H3BO3) nutrient solution for 15 weeks. Thereafter, we sequenced two small RNA libraries from B-deficient and -sufficient (control) citrus leaves, respectively, using Illumina sequencing.

RESULTS

Ninety one (83 known and 8 novel) up- and 81 (75 known and 6 novel) down-regulated miRNAs were isolated from B-deficient leaves. The great alteration of miRNA expression might contribute to the tolerance of citrus to B-deficiency. The adaptive responses of miRNAs to B-deficiency might related to several aspects: (a) attenuation of plant growth and development by repressing auxin signaling due to decreased TIR1 level and ARF-mediated gene expression by altering the expression of miR393, miR160 and miR3946; (b) maintaining leaf phenotype and enhancing the stress tolerance by up-regulating NACs targeted by miR159, miR782, miR3946 and miR7539; (c) activation of the stress responses and antioxidant system through down-regulating the expression of miR164, miR6260, miR5929, miR6214, miR3946 and miR3446; (d) decreasing the expression of major facilitator superfamily protein genes targeted by miR5037, thus lowering B export from plants. Also, B-deficiency-induced down-regulation of miR408 might play a role in plant tolerance to B-deficiency by regulating Cu homeostasis and enhancing superoxide dismutase activity.

CONCLUSIONS

Our study reveals some novel responses of citrus to B-deficiency, which increase our understanding of the adaptive mechanisms of citrus to B-deficiency at the miRNA (post-transcriptional) level.

Physiological and Molecular Aspects of Tolerance to Environmental Constraints in Grain and Forage Legumes

Despite the agronomical and environmental advantages of the cultivation of legumes, their production is limited by various environmental constraints such as water or nutrient limitation, frost or heat stress and soil salinity, which may be the result of pedoclimatic conditions, intensive use of agricultural lands, decline in soil fertility and environmental degradation. The development of more sustainable agroecosystems that are resilient to environmental constraints will therefore require better understanding of the key mechanisms underlying plant tolerance to abiotic constraints. This review provides highlights of legume tolerance to abiotic constraints with a focus on soil nutrient deficiencies, drought, and salinity. More specifically, recent advances in the physiological and molecular levels of the adaptation of grain and forage legumes to abiotic constraints are discussed. Such adaptation involves complex multigene controlled-traits which also involve multiple sub-traits that are likely regulated under the control of a number of candidate genes. This multi-genetic control of tolerance traits might also be multifunctional, with extended action in response to a number of abiotic constraints. Thus, concrete efforts are required to breed for multifunctional candidate genes in order to boost plant stability under various abiotic constraints.

Tree foliar chemistry in an African savanna and its relation to life history strategies and environmental filters

Understanding the relative importance of environment and life history strategies in determining leaf chemical traits remains a key objective of plant ecology. We assessed 20 foliar chemical properties among 12 African savanna woody plant species and their relation to environmental variables (hillslope position, precipitation, geology) and two functional traits (thorn type and seed dispersal mechanism). We found that combinations of six leaf chemical traits (lignin, hemi-cellulose, zinc, boron, magnesium, and manganese) predicted the species with 91% accuracy. Hillslope position, precipitation, and geology accounted for only 12% of the total variance in these six chemical traits. However, thorn type and seed dispersal mechanism accounted for 46% of variance in these chemical traits. The physically defended species had the highest concentrations of hemi-cellulose and boron. Species without physical defense had the highest lignin content if dispersed by vertebrates, but threefold lower lignin content if dispersed by wind. One of the most abundant woody species in southern Africa, Colophospermum mopane, was found to have the highest foliar concentrations of zinc, phosphorus, and δ(13)C, suggesting that zinc chelation may be used by this species to bind metallic toxins and increase uptake of soil phosphorus. Across all studied species, taxonomy and physical traits accounted for the majority of variability in leaf chemistry.

Metabolic profiling reveals altered pattern of central metabolism in navel orange plants as a result of boron deficiency

We focused on the changes of metabolite profiles in navel orange plants under long-term boron (B) deficiency using a gas chromatography-mass spectrometry (GC-MS) approach. Curling of the leaves and leaf chlorosis were observed only in the upper leaves (present before start of the treatment) of B-deficient plants, while the lower leaves (grown during treatment) did not show any visible symptoms. The metabolites with up-accumulation in B-deficient leaves were mainly proline, l-ornithine, lysine, glucoheptonic acid, fucose, fumarate, oxalate, quinate, myo-inositol and allo-inositol, while the metabolites with down-accumulation in B-deficient leaves were mainly serine, asparagine, saccharic acid, citrate, succinate, shikimate and phytol. The levels of glucose and fructose were increased only in the upper leaves by B deficiency, while starch content was increased in all the leaves and in roots. The increased levels of malate, ribitol, gluconic acid and glyceric acid occurred only in the lower leaves of B-deficient plants. The increased levels of phenols only in the upper leaves indicated that the effects of B on phenol metabolism in citrus plants may be a consequence of disruptions in leaf structure. Metabolites with opposite reactions in upper and lower leaves were mainly glutamine, glycine and pyrrole-2-carboxylic acid. To our knowledge, the phenomena of allo-inositol even higher than myo-inositol occurred characterized for the first time in this species. These results suggested that the altered pattern of central metabolism may be either specific or adaptive responses of navel orange plants to B deficiency.

Effects of boron nutrition and water stress on nitrogen fixation, seed δ15N and δ13C dynamics, and seed composition in soybean cultivars differing in maturities

Therefore, the objective of the current research was to investigate the effects of foliar B nutrition on seed protein, oil, fatty acids, and sugars under water stress conditions. A repeated greenhouse experiment was conducted using different maturity group (MG) cultivars. Plants were well-watered with no foliar B (W - B), well-watered with foliar B (W + B), water-stressed with no foliar B (WS - B), and water-stressed with foliar B (WS + B). Foliar B was applied at rate of 0.45 kg · ha(-1) and was applied twice at flowering and at seed-fill stages. The results showed that seed protein, sucrose, fructose, and glucose were higher in W + B treatment than in W - B, WS + B, and WS - B. The increase in protein in W + B resulted in lower seed oil, and the increase of oleic in WS - B or WS + B resulted in lower linolenic acid. Foliar B resulted in higher nitrogen fixation and water stress resulted in seed δ (15)N and δ (13)C alteration. Increased stachyose indicated possible physiological and metabolic changes in carbon and nitrogen pathways and their sources under water stress. This research is beneficial to growers for fertilizer management and seed quality and to breeders to use (15)N/(14)N and (13)C/(12)C ratios and stachyose to select for drought tolerance soybean.

Recovery and fine structure variability of RGII sub-domains in wine (Vitis vinifera Merlot)

BACKGROUND AND AIMS

Rhamnogalacturonan II (RGII) is a structurally complex pectic sub-domain composed of more than 12 different sugars and 20 different linkages distributed in five side chains along a homogalacturonan backbone. Although RGII has long been described as highly conserved over plant evolution, recent studies have revealed variations in the structure of the polysaccharide. This study examines the fine structure variability of RGII in wine, focusing on the side chains A and B obtained after sequential mild acid hydrolysis. Specifically, this study aims to differentiate intrinsic structural variations in these RGII side chains from structural variations due to acid hydrolysis.

METHODS

RGII from wine (Vitis vinifera Merlot) was sequentially hydrolysed with trifluoroacetic acid (TFA) and the hydrolysis products were separated by anion-exchange chromatography (AEC). AEC fractions or total hydrolysates were analysed by MALDI-TOF mass spectrometry.

KEY RESULTS

The optimal conditions to recover non-degraded side chain B, side chain A and RGII backbone were 0·1 m TFA at 40 °C for 16 h, 0·48 m TFA at 40 °C for 16 h (or 0·1 m TFA at 60 °C for 8 h) and 0·1 m TFA at 60 °C for 16 h, respectively. Side chain B was particularly prone to acid degradation. Side chain A and the RGII GalA backbone were partly degraded by 0·1 m TFA at 80 °C for 1-4 h. AEC allowed separation of side chain B, methyl-esterified side chain A and non-methyl-esterified side chain A. The structure of side chain A and the GalA backbone were highly variable.

CONCLUSIONS

Several modifications to the RGII structure of wine were identified. The observed dearabinosylation and deacetylation were primarily the consequence of acidic treatment, while variation in methyl-esterification, methyl-ether linkages and oxidation reflect natural diversity. The physiological significance of this variability, however, remains to be determined.

Covalent bond formation between amino acids and lignin: cross-coupling between proteins and lignin

The present study characterized the products formed from the reaction of amino acids and in turn, proteins, with lignin resulting in cross-coupling. When added to reaction mixtures containing coniferyl alcohol, horseradish peroxidase and H2O2, three amino acids (Cys, Tyr, and Thr) are able to form adducts. The low molecular weight products were analyzed by HPLC and from each reaction mixture, one product was isolated and analyzed by LC/MS. LC/MS results are consistent with bond formation between the polar side-chain of these amino acids with Cα. These results are consistent with the cross-coupling of Cys, Tyr and Thr through a quinone methide intermediate. In addition to the free amino acids, it was found that the cross-coupling of proteins with protolignin through Cys or Tyr residues. The findings provide a mechanism by which proteins and lignin can cross-couple in the plant cell wall.

Effects of foliar boron application on seed composition, cell wall boron, and seed δ(15)N and δ(13)C isotopes in water-stressed soybean plants

Limited information is available on the effects of foliar boron (B) application on soybean seed composition. The objective of this research was to investigate the effects of foliar B on seed composition (protein, oil, fatty acids, and sugars). Our hypothesis was that since B is involved in nitrogen and carbon metabolism, it may impact seed composition. A repeated greenhouse experiment was conducted where half of the soybean plants was exposed to water stress (WS) and the other half was well-watered. Foliar boron (FB) in the form of boric acid was applied twice at a rate of 1.1 kg ha(-1). The first application was during flowering stage, and the second application was during seed-fill stage. Treatments were water stressed plants with no FB (WS-B); water stressed plants with FB (WS+B); watered plants without FB (W-B), and watered plants with FB (W+B). The treatment W-B was used as a control. Comparing with WS-B plants, B concentration was the highest in leaves and seed of W+B plants (84% increase in leaves and 73% in seed). Seeds of W+B plants had higher protein (11% increase), oleic acid (27% increase), sucrose (up to 40% increase), glucose, and fructose comparing with W-B. However, seed stachyose concentrations increased by 43% in WS-B plants seed compared with W-B plants. Cell wall (structural) B concentration in leaves was higher in all plants under water stress, especially in WS-B plants where the percentage of cell wall B reached up to 90%. Water stress changed seed δ(15)N and δ(13)C values in both B applied and non-B applied plants, indicating possible effects on nitrogen and carbon metabolism. This research demonstrated that FB increased B accumulation in leaves and seed, and altered seed composition of well-watered and water stressed plants, indicating a possible involvement of B in seed protein, and oleic and linolenic fatty acids. Further research is needed to explain mechanisms of B involvement in seed protein and fatty acids.

Is boron a prebiotic element? A mini-review of the essentiality of boron for the appearance of life on earth

Boron is probably a prebiotic element with special importance in the so-called "sugars world". Boron is not present on Earth in its elemental form. It is found only in compounds, e.g., borax, boric acid, kernite, ulexite, colemanite and other borates. Volcanic spring waters sometimes contain boron-based acids (e.g., boric, metaboric, tetraboric and pyroboric acid). Borates influence the formation of ribofuranose from formaldehyde that feeds the "prebiotic metabolic cycle". The importance of boron in the living world is strongly related to its implications in the prebiotic origins of genetic material; consequently, we believe that throughout the evolution of life, the primary role of boron has been to provide thermal and chemical stability in hostile environments. The complexation of boric acid and borates with organic cis-diols remains the most probable chemical mechanism for the role of this element in the evolution of the living world. Because borates can stabilize ribose and form borate ester nucleotides, boron may have provided an essential contribution to the "pre-RNA world".

Boron deficiency and transcript level changes

Boron (B) is an essential element for plant growth whose deficiency causes an alteration in the expression of a wide range of genes involved in several physiological processes. However, our understanding of the signal transduction pathways that trigger the B-deficiency responses in plants is still poor. The aims of this review are (i) to summarize the genes whose transcript levels are affected by B deficiency and (ii) to provide an update on recent findings that could help to understand how the signal(s) triggered by B deficiency is transferred to the nucleus to modulate gene expression. In this contribution we review the effects of B deficiency on the transcript level of genes related to B uptake and translocation, maintenance of cell wall and membrane function, nitrogen assimilation and stress response. In addition, we discuss the possible mediation of calcium, arabinogalactan-proteins and other cis-diol containing compounds in the signaling mechanisms that transfer the signal of B deficiency to nuclei. Finally, we conclude that the advance in the knowledge of the molecular basis of B deficiency response in plants will allow improving the tolerance of crops to B deficiency stress.

Borate promotes the formation of a complex between legume AGP-extensin and Rhamnogalacturonan II and enhances production of Rhizobium capsular polysaccharide during infection thread development in Pisum sativum symbiotic root nodules

The capacity to bind to biomolecules is considered to be the basis for any physiological role of boron (B). Legume arabinogalactan protein-extensin (AGPE), a major component of the infection thread matrix of legume nodules is a potential B-ligand. Therefore, its role in infection threads development was investigated in Pisum sativum grown under B deficiency. Using the AGPE-specific antibody MAC265, immunochemical analysis revealed that a 175 kDa MAC265 antigen was abundant in +B but much weaker in -B nodule extracts. A B-dependent complex involving AGPE and rhamnogalacturonan II (RGII) could be co-purified using anti-RGII antiserum. Following fractionation of -B nodules, MAC265 antigens were mostly associated with the bacterial pellet. Immunogold staining confirmed that AGPE was closely associated with the surface of rhizobia in the lumen of threads in -B nodules whereas in +B nodules, AGPE was separated from the bacterial surface by a sheath of capsular polysaccharide. Interestingly, colonies of rhizobia grown in free-living culture without B developed low capsule production. Therefore, we propose that B could be important for apical growth of infection threads by strengthening thread wall through a B-dependent AGPE-RGII interaction and by promoting bacterial advance through a B-dependent production of a stable rhizobial capsule that prevents AGPE attachment.

Ligands of boron in Pisum sativum nodules are involved in regulation of oxygen concentration and rhizobial infection

Boron (B) is an essential nutrient for N(2)-fixing legume-rhizobia symbioses, and the capacity of borate ions to bind and stabilize biomolecules is the basis of any B function. We used a borate-binding-specific resin and immunostaining techniques to identify B ligands important for the development of Pisum sativum-Rhizobium leguminosarum 3841 symbiotic nodules. arabinogalactan-extensin (AGPE), recognized by MAC 265 antibody, appeared heavily bound to the resin in extracts derived from B-sufficient, but not from B-deficient nodules. MAC 265 stained the infection threads and the extracellular matrix of cortical cells involved in the oxygen diffusion barrier. In B-deprived nodules, immunolocalization of MAC 265 antigens was significantly reduced. Leghaemoglobin (Lb) concentration largely decreased in B-deficient nodules. The absence of MAC 203 antigens in B-deficient nodules suggests a high internal oxygen concentration, as this antibody detects an epitope on the lipopolysaccharide (LPS) of bacteroids typically expressed in micro-aerobically grown R. leguminosarum 3841. However, B-deprived nodules did not accumulate oxidized lipids and proteins, and revealed a decrease in the activity of the major antioxidant enzyme ascorbate peroxidase (APX). Therefore, B deficiency reduced the stability of nodule macromolecules important for rhizobial infection, and for regulation of oxygen concentration, resulting in non-functional nodules, but did not appear to induce oxidative damage in low-B nodules.

Boron deficiency results in induction of pathogenesis-related proteins from the PR-10 family during the legume-rhizobia interaction

Boron (B) deficiency has a strong effect on molecular and cellular plant-bacteria interactions during the development of the legume-rhizobia symbiosis, leading to reduced infection and early necrosis of nodules, resembling a pathogenic-like rather than a symbiotic interaction. Therefore, induction of pathogenesis-related (PRs) proteins was investigated here in legume root nodules. Following two-dimensional electrophoresis and MALDI-TOF spectrometry analysis of proteins extracted from Pisum sativum B-sufficient (+B) or B-deficient (-B) root nodules, two proteins from the family PR10, ABR17 and PR10.1, were identified as highly induced in -B nodules. Analysis of gene expression and the use of anti-ABR17 confirmed that induction occurred in B-deficient young nodules and increased during nodule development. ABR17 was also induced in -B nodules of Phaseolus vulgaris. Boron deficiency did not significantly increase the expression of these PR10 in uninfected plant tissues. Moreover, independent of B, induction was detected in senescent tissues, although at a level weaker than in -B nodules. The immunochemical study of ABR17 antigen distribution showed that it was localized in all tissues of poorly invaded B-deficient nodules and accumulated around bacteria, which showed advanced degradation. These results suggest that, under B deficiency, the rhizobia-legume dialogue fails and the bacterium is recognized as a pathogen by the plant, which reacts to prevent infection by inducing at least these two identified PR10 proteins.

Interactions between salinity and boron toxicity in tomato plants involve apoplastic calcium

The lack of consensus about the mutual relations between salinity and boron (B) toxicity with respect to the physiological response of plants necessitates investigation of the interactions of soluble B with salinity. In this investigation, the effect of B was compared with Ca in order to elucidate whether the two nutrients have similar effects and/or to elucidate a relationship under salinity. Following addition of B or Ca, salinity was applied to tomato plants and the cell wall and plasma membrane permeability, measured as water permeability and electrolyte leakage, in relation to amino acid and ion cell wall composition, were determined. As the relationship between B and salinity was complex, several hypotheses are established. The increase of aquaporin functionality due to the presence of B and Ca compared with NaCl-treated plants could be the most feasible, whereas there is currently no satisfactory explanation for the results for the cell wall amino acid composition. In addition, the elemental composition results revealed that, in addition the known interactions between B and Ca with respect to cell wall stability, Mg and Mn were also increased in NaCl+B and NaCl+Ca treatments, suggesting their possible involvement in the cell wall function necessary for plant growth.

Boron in plants: deficiency and toxicity

Boron (B) is an essential nutrient for normal growth of higher plants, and B availability in soil and irrigation water is an important determinant of agricultural production. To date, a primordial function of B is undoubtedly its structural role in the cell wall; however, there is increasing evidence for a possible role of B in other processes such as the maintenance of plasma membrane function and several metabolic pathways. In recent years, the knowledge of the molecular basis of B deficiency and toxicity responses in plants has advanced greatly. The aim of this review is to provide an update on recent findings related to these topics, which can contribute to a better understanding of the role of B in plants.

Boron dependent membrane glycoproteins in symbiosome development and nodule organogenesis: A model for a common role of boron in organogenesis

During the last two decades, we have analyzed the roles of boron (B) in the development of the legume-rhizobia symbiosis and nodule organogenesis. As in other plant tissues, B is needed for the maintenance of nodule cell wall structure. Moreover, several symbiotic events including rhizobial infection, nodule cell invasion and symbiosome development that involve membrane related functions (i.e., vesicle targeting, secretion, or cell surface interactions) are affected by B deficiency. Using anti-rhamnogalacturonan II (anti-RGII) antiserum and immunological techniques, we recently described membrane glycoproteins (RGII-glycoproteins) developmentally regulated in Pisum sativum nodules, which are not detected by the antibody in B-deficient nodules. RGII-glycoproteins appeared related with development processes involving extensive membrane synthesis, like symbiosome maturation or cell growth, both of them negatively affected by B deficiency. Here, we suggest that, besides maintaining cell wall structure, B is both stabilizing components of the membrane glycocalyx and promoting interactions between cell surfaces glycoconjugates that are important during the establishment of the symbiosis and during nodule development. Moreover, we hypothesize that B is playing a similar role during plant or animal embryogenesis and development.

Nitrogen metabolism and seed composition as influenced by glyphosate application in glyphosate-resistant soybean

Previous research has demonstrated that glyphosate can affect nitrogen fixation or nitrogen assimilation in soybean. This 2-year field study investigated the effects of glyphosate application of 1.12 and 3.36 kg of ae ha(-1) on nitrogen metabolism and seed composition in glyphosate-resistant (GR) soybean. There was no effect of glyphosate application on nitrogen fixation as measured by acetylene reduction assay, soybean yield, or seed nitrogen content. However, there were significant effects of glyphosate application on nitrogen assimilation, as measured by in vivo nitrate reductase activity (NRA) in leaves, roots, and nodules, especially at high rate. Transiently lower leaf nitrogen or (15)N natural abundance in high glyphosate application soybean supports the inhibition of NRA. With the higher glyphosate application level protein was significantly higher (10.3%) in treated soybean compared to untreated soybean. Inversely, total oil and linolenic acid were lowest at the high glyphosate application rate, but oleic acid was greatest (22%) in treated soybean. These results suggest that nitrate assimilation in GR soybean was more affected than nitrogen fixation by glyphosate application and that glyphosate application may alter nitrogen and carbon metabolism.

Developmentally regulated membrane glycoproteins sharing antigenicity with rhamnogalacturonan II are not detected in nodulated boron deficient Pisum sativum

The peribacteroid membrane (PBM) of symbiosomes from pea root nodules developed in the presence of boron (+B) was labelled by anti-rhamnogalacturonan II (RGII) (anti-rhamnogalacturonan II pectin polysaccharide) antiserum. However, in nodules from plants grown at low boron (-B), anti-RGII pectin polysaccharide did not stain PBMs. Given that RGII pectin binds to borate, and that symbiosomes differentiate aberrantly in -B nodules because of abnormal vesicle traffic, anti-RGII pectin polysaccharide antigens were further analysed. Following electrophoresis and electroblotting, anti-RGII pectin polysaccharide immunostained three bands in +B but not in -B nodule-derived PBMs. A similar banding pattern was observed after the immunostaining of membrane fractions from uninfected roots, indicating that anti-RGII pectin polysaccharide antigens are common to both peribacteroid and plasma membranes. Protease treatment of samples led to disappearance of anti-RGII pectin polysaccharide labelling, indicating that the three immunostained bands correspond to proteins or glycoproteins. The immunochemical study of RGII antigen distribution during nodule development showed that it is strongly present on the PBM of dividing (undifferentiated) symbiosomes but progressively disappeared during symbiosome maturation. In B-deficient nodules, PBMs were never decorated with RGII antigens, and there was an abnormal targeting of vesicles containing pectic polysaccharide (homogalacturanan) to cell membranes. Overall, these results indicate that RGII, boron and certain membrane (glyco)-proteins may interact closely and function cooperatively in membrane processes associated with symbiosome division and general cell growth.

Early responses to Nod factors and mycorrhizal colonization in a non-nodulating Phaseolus vulgaris mutant

Legumes can acquire nitrogen through a symbiotic interaction with rhizobial bacteria. The initiation of this process is determined by a molecular dialogue between the two partners. Legume roots exude flavonoids that induce the expression of the bacterial nodulation genes, which encode proteins involved in the synthesis and secretion of signals called Nod factors (NFs). NFs signal back to the plant root and trigger several responses, leading to bacterial invasion and nodule formation. Here, we describe the molecular and cellular characterization of a Phaseolus vulgaris non-nodulating mutant (NN-mutant). Root hair cells of the NN-mutant plant respond with swelling and branching when inoculated with Rhizobium etli, albeit without curling induction. Furthermore, neither initiation of cell division in the outer cortex, nor entrapment of bacteria nor infection thread formation was observed. Both the bean wild-type and the NN-mutant responded with elevated intracellular calcium changes in the root hairs. Although the NN-mutant is deficient in early nodulin gene expression when inoculated with R. etli, it can be effectively colonized by arbuscular mycorrhizal fungi (Glomus intraradices). Our data indicate that the P. vulgaris NN-mutant is not blocked at the NFs early perception stage, but at later downstream stages between Ca(2+) signaling and early nodulin induction. This supports the idea that both microsymbionts are perceived and trigger different downstream pathways in the host plant.

Why boron?

It is now more than 80 years since boron was convincingly demonstrated to be essential for normal growth of higher plants. However, its biochemical role is not well understood at the moment. Several recent reviews propose that B is implicated in three main processes: keeping cell wall structure, maintaining membrane function, and supporting metabolic activities. However, in the absence of conclusive evidence, the primary role of boron in plants remains elusive. Besides plants, growth of specific bacteria, such as heterocystous cyanobacteria and the recently reported actinomycetes of the genus Frankia, requires B, particularly for the stability of the envelopes that control the access of the nitrogenase-poisoning oxygen when they grow under N2-fixing conditions. Likewise, a role for B for animal embryogenesis and other developmental processes is being established. Finally, a new feature of the role of boron comes from signaling mechanisms for communication among bacteria and among legumes and rhizobia leading to N2-fixing symbiosis, and it is possible that new roles for B, based on its special chemistry and its interaction with Ca would appear in the world of signal transduction pathways. In conclusion, the diversity of roles played by B might indicate that either the micronutrient is involved in numerous processes or that its deficiency has a pleiotropic effect. The arising question is why such an element? Since all of the roles clearly established for B are related to its capacity to form diester bridges between cis-hydroxyl-containing molecules, we propose that the main reason for B essentiality is the stabilization of molecules with cis-diol groups turning them effective, irrespectively of their function.

Use of phenylboronic acids to investigate boron function in plants. Possible role of boron in transvacuolar cytoplasmic strands and cell-to-wall adhesion

The only defined physiological role of boron in plants is as a cross-linking molecule involving reversible covalent bonds with cis-diols on either side of borate. Boronic acids, which form the same reversible bonds with cis-diols but cannot cross-link two molecules, were used to selectively disrupt boron function in plants. In cultured tobacco (Nicotiana tabacum cv BY-2) cells, addition of boronic acids caused the disruption of cytoplasmic strands and cell-to-cell wall detachment. The effect of the boronic acids could be relieved by the addition of boron-complexing sugars and was proportional to the boronic acid-binding strength of the sugar. Experiments with germinating petunia (Petunia hybrida) pollen and boronate-affinity chromatography showed that boronic acids and boron compete for the same binding sites. The boronic acids appear to specifically disrupt or prevent borate-dependent cross-links important for the structural integrity of the cell, including the organization of transvacuolar cytoplasmic strands. Boron likely plays a structural role in the plant cytoskeleton. We conclude that boronic acids can be used to rapidly and reversibly induce boron deficiency-like responses and therefore are useful tools for investigating boron function in plants.

Recovery of development and functionality of nodules and plant growth in salt-stressed Pisum sativum--Rhizobium leguminosarum symbiosis by boron and calcium

Nodules developed in Pisum sativum L. cv. Argona inoculated with Rhizobium leguminosarum bv. viciae 3841 and growing under saline conditions (75 mmol/L NaCl) are non functional and had abnormal structure. The infected cells contained a low amount of endophytic bacteria, compared to treatments without salt. Addition of B (up to 55.8 micromol/L) and Ca2+ (up to 2.72 mmol/L) increased bacterial population of host plant cells in salt-stressed nodules. Furthermore, symbiosomes developed inside the nodules from salt treated plants presented a degraded peribacteroid membrane. This effect was also prevented by combined addition of B and Ca2+. Given the importance of both nutrients in cell wall structure, the pectin fraction was studied by electron microscopy and immunological methods. Salt stress produced cells with walls dramatically altered or even degraded in several zones. Pectin polysaccharides, detected by JIM 5 monoclonal antibody, increased in cells under salinity. These effects resembled typical effects of B-deficiency reactions in cell walls, and the increase of both Ca2+ and especially B also prevented these alterations.

Boron requirement in the Discaria trinervis (Rhamnaceae) and Frankia symbiotic relationship. Its essentiality for Frankia BCU110501 growth and nitrogen fixation

The essentiality of boron (B) for nitrogen fixation in heterocystous cyanobacteria and rhizobial symbioses has been widely established. However, nothing is known about the possible involvement of the micronutrient in actinorhizal symbioses. Therefore, the effect of boron (B) deficiency on the establishment of the Discaria trinervis-Frankia BCU110501 symbiosis was investigated. Nodulation was diminished in B-deficient D. trinervis or in plants inoculated with Frankia grown in the absence of B. These poorly nodulated plants showed a reduction of shoot and root weight and small size. Because depletion of the micronutrient during growth of the actinomycete altered the infection capacity of Frankia, we also studied growth, structure and nitrogen fixation of free-living Frankia BCU110501. Growth was delayed in B-deficient BAP media (+N cultures), and completely inhibited in B-deprived N-free BAP media (-N cultures), suggesting that B is required to enhance growth of Frankia and essential for the development of nitrogen fixing activity. Ultrastructural study of B-deficient cells showed an alteration of filament walls both in +N and especially in -N cultures, indicating a possible role of the microelement in the maintenance of these structures. Moreover, the stability of vesicle envelopes was impaired in the absence of B and, hence, nitrogenase occurrence and nitrogen fixation were totally absent. The results show that B is required for both partners to establish an effective symbiosis.

Yellow lupine cyclophilin transcripts are highly accumulated in the nodule meristem zone

Cyclophilin (CyP) is one of the enzymes that act as peptidylprolyl cis-trans isomerases (EC 5.2.1.8). The cDNA and an intronless gene coding for cytosolic CyP have been isolated from yellow lupine. The deduced amino acid sequence of the characterized open reading frame shows approximately 80% homology with cytosolic CyP from other organisms. Southern blots of genomic DNA indicate that there is a small family of genes for CyP-related genes in the yellow lupine genome. RNA blot analyses demonstrate that CyP genes are expressed in all plant organs. The amount of CyP transcripts is dramatically increased in root nodules. In situ hybridization experiments indicate that CyP transcripts are localized mainly in meristematic tissues, with the highest level observed in the nodule meristem zone. The promoter of the sequenced gene contains 5' AAAGAT 3' and AT-rich motifs that are characteristic for some nodulin promoters.

Pectin: cell biology and prospects for functional analysis

Pectin is a major component of primary cell walls of all land plants and encompasses a range of galacturonic acid-rich polysaccharides. Three major pectic polysaccharides (homogalacturonan, rhamnogalacturonan-I and rhamnogalacturonan-II) are thought to occur in all primary cell walls. This review surveys what is known about the structure and function of these pectin domains. The high degree of structural complexity and heterogeneity of the pectic matrix is produced both during biosynthesis in the endomembrane system and as a result of the action of an array of wall-based pectin-modifying enzymes. Recent developments in analytical techniques and in the generation of anti-pectin probes have begun to place the structural complexity of pectin in cell biological and developmental contexts. The in muro de-methyl-esterification of homogalacturonan by pectin methyl esterases is emerging as a key process for the local modulation of matrix properties. Rhamnogalacturonan-I comprises a highly diverse population of spatially and developmentally regulated polymers, whereas rhamnogalacturonan-II appears to be a highly conserved and stable pectic domain. Current knowledge of biosynthetic enzymes, plant and microbial pectinases and the interactions of pectin with other cell wall components and the impact of molecular genetic approaches are reviewed in terms of the functional analysis of pectic polysaccharides in plant growth and development.

Pectin: cell biology and prospects for functional analysis

Pectin is a major component of primary cell walls of all land plants and encompasses a range of galacturonic acid-rich polysaccharides. Three major pectic polysaccharides (homogalacturonan, rhamnogalacturonan-I and rhamnogalacturonan-II) are thought to occur in all primary cell walls. This review surveys what is known about the structure and function of these pectin domains. The high degree of structural complexity and heterogeneity of the pectic matrix is produced both during biosynthesis in the endomembrane system and as a result of the action of an array of wall-based pectin-modifying enzymes. Recent developments in analytical techniques and in the generation of anti-pectin probes have begun to place the structural complexity of pectin in cell biological and developmental contexts. The in muro de-methyl-esterification of homogalacturonan by pectin methyl esterases is emerging as a key process for the local modulation of matrix properties. Rhamnogalacturonan-I comprises a highly diverse population of spatially and developmentally regulated polymers, whereas rhamnogalacturonan-II appears to be a highly conserved and stable pectic domain. Current knowledge of biosynthetic enzymes, plant and microbial pectinases and the interactions of pectin with other cell wall components and the impact of molecular genetic approaches are reviewed in terms of the functional analysis of pectic polysaccharides in plant growth and development.

Lectin-like glycoprotein PsNLEC-1 is not correctly glycosylated and targeted in boron-deficient pea nodules

Symbiosome development was studied in pea root nodules from plants growing in the absence of boron (B). Rhizobia released into the host cells of nodules from B-deficient plants developed to abnormal endophytic forms with an altered electrophoretic lipopolysaccharide pattern. Immunostaining after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electroblotting of nodule homogenates with antibodies that recognize glycoprotein components showed that two previously described lectin-like glycoproteins (PsNLEC-1A and PsNLEC-1B) did not harbor the carbohydrate epitope normally recognized by specific monoclonal antibodies. Material derived from B-deficient nodules, however, still contained three antigenic isoforms with similar electrophoretic mobilities to PsNLEC-1 isoforms A, B, and C. These could be detected following immunoblotting and immunostaining with a specific antiserum originating from the purified PsNLEC protein that had been heterologously expressed in Escherichia coli. Immunogold localization of PsNLEC-1 sugar epitopes in B-deficient nodules showed that they were associated mostly with cytoplasmic vesicles rather than normal localization in the symbiosome compartment of mature infected cells. These results suggest that a modification of the glycosyl-moieties of PsNLEC-1 and an alteration of vesicle targeting occur during the development of pea nodules in the absence of B, and that these changes are associated with the development of aberrant nonfunctional symbiosomes.

Boron and calcium distribution in nitrogen-fixing pea plants

In a glasshouse experiment, plants of Pisum sativum L. cv. Argona were grown hydroponically with different B and Ca levels, in order to elucidate a specific role for B and Ca on the N(2) fixation in this temperate legume. The treatments were as follows: control (9.3 µM B and 2 mM Ca), -B (without B and 2 mM Ca), -B+Ca (without B and 3.6 mM Ca), +Ca (9.3 µM B and 3.6 mM Ca), -Ca (9.3 µM B and 0.4 mM Ca) and -Ca+B (46.5 µM B and 0.4 mM Ca). The supply of -Ca and +Ca did not affect nitrogenase activity, but the weight of old shoots and total N content increased with the Ca treatment. No symptoms of B deficiency were detected in the plants of the -B and -B+Ca treatments, apart from weight reduction in young shoots and lower nitrogenase activity. The B concentration decreased in young shoots and roots of plants grown without B (-B), but there was a sharper decrease in the roots of -B+Ca plants and the levels of B in the young shoots were similar to the control levels. The B concentration in -Ca plants was reduced in the old shoot and in the root, while plant weight and N content increased in -Ca+B plants. The cell wall and total B concentrations in the nodules were 4-fold compared with those of the roots. By contrast, the Ca root wall was 2.5 times higher than the nodule levels although total pectin was higher in the nodule than in the root. Finally, the results obtained showed that a high supply of Ca could induce B mobilisation from root to shoot. On the other hand, the high B requirement found in pea plant nodules may contribute to explain the low nitrogenase activity detected under -B conditions.

The Pore Size of Non-Graminaceous Plant Cell Walls Is Rapidly Decreased by Borate Ester Cross-Linking of the Pectic Polysaccharide Rhamnogalacturonan II

The walls of suspension-cultured Chenopodium album L. cells grown continually for more than 1 year on B-deficient medium contained monomeric rhamnogalacturonan II (mRG-II) but not the borate ester cross-linked RG II dimer (dRG-II-B). The walls of these cells had an increased size limit for dextran permeation, which is a measure of wall pore size. Adding boric acid to growing B-deficient cells resulted in B binding to the wall, the formation of dRG-II-B from mRG-II, and a reduction in wall pore size within 10 min. The wall pore size of denatured B-grown cells was increased by treatment at pH </= 2.0 or by treatment with Ca(2+)-chelating agents. The acid-mediated increase in wall pore size was prevented by boric acid alone at pH 2.0 and by boric acid together with Ca(2+), but not by Na(+) or Mg(2+) ions at pH 1.5. The Ca(2+)-chelator-mediated increase in pore size was partially reduced by boric acid. Our results suggest that B-mediated cross-linking of RG-II in the walls of living plant cells generates a pectin network with a decreased size exclusion limit for polymers. The formation, stability, and possible functions of a borate ester cross-linked pectic network in the primary walls of nongraminaceous plant cells are discussed.

The boron requirement and cell wall properties of growing and stationary suspension-cultured chenopodium album L. cells

Suspension-cultured Chenopodium album L. cells are capable of continuous, long-term growth on a boron-deficient medium. Compared with cultures grown with boron, these cultures contained more enlarged and detached cells, had increased turbidity due to the rupture of a small number of cells, and contained cells with an increased cell wall pore size. These characteristics were reversed by the addition of boric acid (>/=7 &mgr;M) to the boron-deficient cells. C. album cells grown in the presence of 100 &mgr;M boric acid entered the stationary phase when they were not subcultured, and remained viable for at least 3 weeks. The transition from the growth phase to the stationary phase was accompanied by a decrease in the wall pore size. Cells grown without boric acid or with 7 &mgr;M boric acid were not able to reduce their wall pore size at the transition to the stationary phase. These cells could not be kept viable in the stationary phase, because they continued to expand and died as a result of wall rupture. The addition of 100 &mgr;M boric acid prevented wall rupture and the wall pore size was reduced to normal values. We conclude that boron is required to maintain the normal pore structure of the wall matrix and to mechanically stabilize the wall at growth termination.

BORON IN PLANT STRUCTURE AND FUNCTION

New and exciting developments in boron research in the past few years greatly contributed to better understanding of the role of boron in plants. Purification and identification of the first boron-polyol transport molecules resolved much of the controversy about boron phloem mobility. Isolation and characterization of the boron-polysaccharide complex from cell walls provided the first direct evidence for boron crosslinking of pectin polymers. Inhibition and recovery of proton release upon boron withdrawal and restitution in plant culture medium demonstrated boron involvement in membrane processes. Rapid boron-induced changes in membrane function could be attributed to boron-complexing membrane constituents. Boron may affect metabolic pathways by binding apoplastic proteins to cis-hydroxyl groups of cell walls and membranes, and by interfering with manganese-dependent enzymatic reactions. In addition, boron has been implicated in counteracting toxic effects of aluminum on root growth of dicotyledonous plants. Molecular investigations of boron nutrition have been initiated by the discovery of a novel mutant of Arabidopsis thaliana with an altered requirement for boron.

PLANT CELL WALL PROTEINS

The nature of cell wall proteins is as varied as the many functions of plant cell walls. With the exception of glycine-rich proteins, all are glycosylated and contain hydroxyproline (Hyp). Again excepting glycine-rich proteins, they also contain highly repetitive sequences that can be shared between them. The majority of cell wall proteins are cross-linked into the wall and probably have structural functions, although they may also participate in morphogenesis. On the other hand, arabinogalactan proteins are readily soluble and possibly play a major role in cell-cell interactions during development. The interactions of these proteins between themselves and with other wall components is still unknown, as is how wall components are assembled. The possible functions of cell wall proteins are suggested based on repetitive sequence, localization in the plant body, and the general morphogenetic pattern in plants.