Guidong Liu

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.

Relationship between leaf chlorosis and different boron forms in Trifoliate orange seedlings under excessive boron supply

Abstract Boron (B) is an essential micronutrient element for plants, and it exists in many forms in plants. B toxicity is common in arid and semiarid regions, and inappropriate application of B fertilizer can lead to B toxicity. Trifoliate orange [Pincers trifoliata (L.) Raf] is a main citrus rootstock of the world, and is sensitive to excess B. Although it is known that leaf chlorosis is the typical visible symptom of B toxicity, it is still unclear how leaf chlorosis is related to different forms of B in Trifoliate orange seedlings. A hydroponic experiment was carried out with four B levels: B5 (5 μmol L–1), B10 (10 μmol L–1), B20 (20 μmol L–1) and B100 (100 μmol L–1). After a 117-d culture, free B, semi-bound B and bound B in root, stem and leaves were extracted and measured. The results showed that the seedling height, the number of leaves and the dry weight decreased with increasing B supply. Leaf B concentration was generally higher than those of roots and stems. New leaf B concentration showed no difference in different B treatments, but was lower than that of older leaves. B in root and stem was mostly in the form of bound B, and B in leaves was mainly in free B form. Moreover, the free B, semi-bound B and bound B had little changes in roots and stems with various B treatments, but free B in leaves significantly increased as leaf chlorosis was aggravated, indicating that leaf chlorosis is closely related with free B in leaves.

Different metabolite profile and metabolic pathway with leaves and roots in response to boron deficiency at the initial stage of citrus rootstock growth

Boron (B) is a microelement required for higher plants, and B deficiency has serious negative effect on metabolic processes. We concentrated on the changes in metabolite profiles of trifoliate orange leaves and roots as a consequence of B deficiency at the initial stage of growth by gas chromatography-mass spectrometry (GC-MS)-based metabolomics. Enlargement and browning of root tips were observed in B-deficient plants, while any obvious symptom was not recorded in the leaves after 30 days of B deprivation. The distinct patterns of alterations in metabolites observed in leaves and roots due to B deficiency suggest the presence of specific organ responses to B starvation. The accumulation of soluble sugars was occurred in leaves, which may be attributed to down-regulated pentose phosphate pathway (PPP) and amino acid biosynthesis under B deficiency, while the amount of most amino acids in roots was increased, indicating that the effects of B deficiency on amino acids metabolism in trifoliate orange may be a consequence of disruptions in root tissues and decreased protein biosynthesis. Several important products of shikimate pathway were also significantly affected by B deficiency, which may be related to abnormal growth of roots induced by B deficiency. Conclusively, our results revealed a global perspective of the discriminative metabolism responses appearing between B-deprived leaves and roots and provided new insight into the relationship between B deficiency symptom in roots and the altered amino acids profiling and shikimate pathway induced by B deficiency during seedling establishment.

Differential Changes in Cell-Wall Content and Boron and Calcium Concentration in Newhall Navel Orange Grafted on Two Rootstocks Differing in Boron-Deficiency Responses

The study aimed to determine if the variability of the effect of boron (B) deficiency on Newhall navel orange grafted on trifoliate orange (deficient-B-sensitive) and on citrange (deficient-B-tolerant) can be explained on the basis of changes in cell-wall content and in cell-wall B and calcium (Ca) concentrations. The plants were cultured in the nutrient solution (with or without B) for 65 days. Boron deficiency increased the cell-wall content in old leaves (leaves from last season) of trifoliate orange but had no impact on citrange. Boron deficiency did not reduce B concentration in cell walls of old leaves of citrange-grafted plants but increased their Ca concentration. For trifoliate-orange-grafted plants, however, B deficiency decreased the B concentration in cell walls of old leaves and did not increase their Ca concentration. The changes of B and Ca concentrations between with and without B supply in pectin were in good agreement with the changes of those in cell walls. The relatively greater ability of citrange-grafted plants to maintain B and Ca in the cell wall may contribute to their tolerance to low external B.

Changes in chemical composition and structure of root cell wall of citrus rootstock seedlings in response to boron deficiency by FTIR spectroscopy

ABSTRACT Boron (B) is an essential microelement for higher plants, and plays a role in cell wall formation. Citrange seedlings with different amounts of B were studied through Fourier-transform infrared spectroscopy (FTIR) analyses. The results showed that the growth and development of new roots were evidently inhibited by B deficiency. Boron-deficiency significantly increased cell wall biomass (CWB) as a percentage of root fresh weight and the ratio of B concentration in cell wall to the total B in roots. The findings from CWB FTIR spectra showed the band at 3429 cm−1 under control condition was shifted to 3442 cm−1 after being B deprived, suggesting that the mode of hydrogen bonding was changed by B deficiency. Boron deficiency clearly decreased the peak height of carboxylic ester band around 1741 cm−1, but increased that of COO− stretching band around 1400 cm−1, suggesting that the relative amount and degree of esterification of carboxylic groups was decreased and pectin content and structure was altered. These results demonstrate that changes in amount, structure, and assembly of root cell wall polymers may be either specific or adaptive responses of citrange seedlings to B deficiency and FTIR can be an appropriate method to study changes in cell wall under B deficiency.

Metabolic changes in roots of trifoliate orange [Poncirus trifoliate (L.) Raf.] as induced by different treatments of boron deficiency and resupply

Background and aimsBoron (B) deficiency is one of the major abiotic stresses to citrus. Trifoliate orange, as the most important citrus rootstock in China, is sensitive to B deficiency. B deficiency has serious negative effects on metabolic processes in roots, but whether the hazardous effects can be alleviated to some degree with B resupply remains unclear. The purpose of this study was to investigate the effects of B resupply on metabolites and metabolic pathway in B-starved trifoliate orange [Poncirus trifoliate (L.) Raf.] roots.MethodsGas chromatography-mass spectrometry (GC-MS) was used to analyze the metabolic profiles in roots under different treatments of B deprivation and B resupply.ResultsB deficiency induced an obvious enlargement of root tips and significant alterations in metabolites. Changes were also observed in basic or neutral amino acids, several organic acids and carbohydrates of B-starved roots. However, one week of B resupply quickly reversed the changes in the roots with the initiation of new roots across the inflated root tips, and the restored some of metabolites to a normal level, such as proline (Pro), linoleic acid, 6-deoxy-D-glucose and myo-inositol, indicating that these metabolites are very sensitive to B availability.ConclusionsThe data suggest that B resupply may contribute to the obvious, although not full, recovery of the root growth by reversing the central metabolite changes. This study provides a new insight into the relationship between root growth and the alteration on metabolites induced by B deficiency and B resupply, which can be expected to make an important contribution to the scientific application of B fertilizer.