Borate Cross-Linked/Total Rhamnogalacturonan II Ratio in Cell Walls for the Biochemical Diagnosis of Boron Deficiency in Hydroponically Grown Pumpkin

Abstract

Boron (B) is an essential trace element for vascular plants, 1,2 and B deficiency inhibits the growth of root and shoot tips. 3 The B deficiency in field crops has been reported for various species and in many countries. 4 B deficiency is diagnosed by plant symptoms and by soil and plant analysis for good crop management. 5In plant analysis for mineral nutrition diagnosis, 6,7 the total mineral content of leaves is frequently determined to compare with the critical content. Based on the role and behavior of minerals in plants, the fractionated analysis of active forms (e.g., water-soluble) and biochemical analysis of metal species related to the metal function (e.g., metalloenzyme) has also been proposed. Water-soluble B is reported to be possible to diagnose the B nutritional status in plants. 8 However, a biochemical diagnosis method for B has not yet been developed because the function of B in plants at the molecular level has not been well resolved until recent years.In the last decade, much progress has been made in studying the chemical form and function of B in plants (Fig. 1).1,2 It has been demonstrated that the water-soluble B in plants is present as boric acid, and as a borate ester with low-molecular-weight diol compounds (1:1 and 1:2), 9,10 and that the amount of watersoluble B decreases with a decrease in the B supply. 8,11 Waterinsoluble B has been revealed to exist in cell walls as a dimeric rhamnogalacturonan II-borate (dRG-II-B) complex, in which a borate-apiose (1:2) ester cross-links two monomeric RG-IIs. [12][13][14][15][16] The pectic polysaccharide RG-II is covalently bound to homogalacturonan, which is a major component of pectin. 17The borate cross-linked RG-II or dRG-II-B/total RG-II ratio in cell walls of B-sufficient and deficient plants is reported to be 0.8 -0.9 and 0 -0.2, respectively. 18,19 These studies have established that the major function of B in plants is to covalently cross-link RG-II by a borate ester, and thereby to form a pectic network. Such cross-linked pectin macromolecules contribute to the strength and integrity of cell walls. [18][19][20] Considering this B function, it is expected that the dRG-II-B to total RG-II ratio in cell walls would be a useful biochemical index to diagnose B deficiency in plants. Thus, in the present study, we analyzed the dRG-II-B to total RG-II ratio as well as the water-soluble B content of tissues of pumpkin hydroponically grown under various low-B conditions to evaluate their applicability to the diagnosis of plant B deficiency. Materials and Methods Plant material and growth conditionsPumpkin seeds (Cucurbita maxima Duchesne cv. TokyoKabocha, Sakata Seed Co., Yokohama, Japan) were sown in washed rock-wool 66R (Nitto Boseki Co., Tokyo, Japan) and grown in a growth chamber Research Center, National Agriculture and Food Research Organization, Tsukuba, Japan **Forestry and Forest Products Research Institute, Tsukuba, Japan Boron (B) is an essential micronutrient for vascular plants. The function of B has been demonstrated to cro...

Roles of BOR2, a Boron Exporter, in Cross Linking of Rhamnogalacturonan II and Root Elongation under Boron Limitation in Arabidopsis

“…In a study of pumpkin roots exposed to low-B conditions, RG-II-B dimer formation decreased along with the root dry weight (Matsunaga and Ishii, 2006). In pumpkin roots subjected to B-depleted conditions (no addition of B), the fraction of RG-II forming the borate complex was about 55% (Matsunaga and Ishii, 2006). Under these conditions, the pumpkin root dry weight was about 50% lower than under the B-sufficient conditions.…”

“…B is distributed primarily to cell walls prior to its accumulation in the soluble fraction of Arabidopsis roots under low-B conditions (Noguchi et al, 2000). The RG-II-B complex is known to occur in radish roots (Kobayashi et al, 1996) and pumpkin (Cucurbita maxima) roots (Matsunaga and Ishii, 2006). In a study of pumpkin roots exposed to low-B conditions, RG-II-B dimer formation decreased along with the root dry weight (Matsunaga and Ishii, 2006).…”

“…The RG-II-B complex is known to occur in radish roots (Kobayashi et al, 1996) and pumpkin (Cucurbita maxima) roots (Matsunaga and Ishii, 2006). In a study of pumpkin roots exposed to low-B conditions, RG-II-B dimer formation decreased along with the root dry weight (Matsunaga and Ishii, 2006). In pumpkin roots subjected to B-depleted conditions (no addition of B), the fraction of RG-II forming the borate complex was about 55% (Matsunaga and Ishii, 2006).…”

“…After centrifugation, the supernatant was filtered through a 0.2-or 0.45-mm membrane. They were subjected to size-exclusion HPLC/refractive index detector (Hitachi High Technologies Corporation) and analyzed as described in Matsunaga and Ishii (2006). The relative proportions of RG-II-B dimer to total RG-II were estimated from the peak area corresponding to RG-II-B dimer to the integration of the respective peak area of RG-II monomer and RG-II-B dimer.…”

Analysis of Boron and B-pectin Complex Content in the Abscission Zone during Floral and Fruit Abscission in Tobacco (Nicotiana tabacum)

“…Matsunaga and Ishii (2006) reported a method for assessing B deficiency by analyzing the formation of cross-links of the pectin RG-II domain that function as B carriers. They also showed that 90-100% of dRG-II-B corresponds to sufficient B, 50-80% corresponds to underlying deficiency, and less than 40% corresponds to B depletion and disturbances in growth.…”