Abstract
Background and AimsSulphur (S) is an essential macronutrient involved in numerous metabolic pathways required for plant growth. Crops of the plant family Brassicaceae require more S compared with other crops for optimum growth and yield, with most S ultimately sequestered in the mature seeds as the storage proteins cruciferin and napin, along with the unique S-rich secondary metabolite glucosinolate (GSL). It is well established that S assimilation primarily takes place in the shoots rather than roots, and that sulphate is the major form in which S is transported and stored in plants. We carried out a developmental S audit to establish the net fluxes of S in two lines of Brassica juncea mustard where seed GSL content differed but resulted in no yield penalty.MethodsWe quantified S pools (sulphate, GSL and total S) in different organs at multiple growth stages until maturity, which also allowed us to test the hypothesis that leaf S, accumulated as a primary S sink, becomes remobilized as a secondary source to meet the requirements of GSL as the dominant seed S sink.Key ResultsMaximum plant sulphate accumulation had occurred by floral initiation in both lines, at which time most of the sulphate was found in the leaves, confirming its role as the primary S sink. Up to 52 % of total sulphate accumulated by the low-GSL plants was lost through senesced leaves. In contrast, S from senescing leaves of the high-GSL line was remobilized to other tissues, with GSL accumulating in the seed from commencement of silique filling until maturity.ConclusionWe have established that leaf S compounds that accumulated as primary S sinks at early developmental stages in condiment type B. juncea become remobilized as a secondary S source to meet the demand for GSL as the dominant seed S sink at maturity.
Highlights
Sulphur (S) is an essential macronutrient involved in numerous metabolic pathways required for plant growth, including the synthesis of amino acids, proteins, co-enzymes, vitamins and secondary metabolites such as glucosinolates (GSLs) and sulphoflavonoids
Sulphide is used for the synthesis of cysteine, which is the precursor for the synthesis of methionine and other primary metabolites, including storage proteins, whereas phosphoadenosine 5′-phosphosulphate (PAPS) is used for sulphation of secondary metabolites, predominantly GSLs (Mugford et al, 2011)
Internal transport systems for sulphate and GSLs are well described in the context of sources and sinks for the model plant Arabidopsis, with only a few studies in canola using radiolabelled S supply to detect total S and sulphate remobilization (Balint and Rengel, 2011)
Summary
Sulphur (S) is an essential macronutrient involved in numerous metabolic pathways required for plant growth, including the synthesis of amino acids, proteins, co-enzymes, vitamins and secondary metabolites such as glucosinolates (GSLs) and sulphoflavonoids. Plants take up S from the soil in the form of sulphate, which is reduced to sulphide for further metabolism through S assimilation processes (Mugford et al, 2010). In the S assimilation process, sulphate is activated to adenosine 5′-phosphosulphate (APS), which is the branching point for subsequent steps in which APS is reduced to form sulphide and 3′-phosphoadenosine 5′-phosphosulphate (PAPS). Sulphide is used for the synthesis of cysteine, which is the precursor for the synthesis of methionine and other primary metabolites, including storage proteins, whereas PAPS is used for sulphation of secondary metabolites, predominantly GSLs (Mugford et al, 2011). Higher concentrations of GSLs are desired for providing the pungency to condiment mustard, while low concentrations have been selected for canola oil production
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