Structural Role of Boron in the Cell Wall and its Involvement in Other Physiological Functions

Boron (B), an essential nutrient for vascular plants, is located mostly (about 80–90% of dry weight) in the plant cell wall forming borate ester cross-linked rhamnogalacturonan II (RG-II) dimmer, this complex being essential to the structure and function of the cell wall.1,2 To date, the primordial function of this micronutrient is undoubtedly its structural role in the cell wall.3,4 Nonetheless, it has been reported that B seems to exert different effects on very diverse processes in vascular plants, such as root elongation, indole-3-acetic acid oxidase, sugar translocation, carbohydrate metabolism, nucleic acid synthesis, and pollen tube growth.3,5,6 Boron also appears to affect membrane potential, plasmalemma-bound enzymes and ion fluxes across membranes,7–9 cytoskeletal proteins,10,11 accumulation of phenolics and polyamines,8,12,13 and nitrogen metabolism.14,15 Although the symptoms due to B deficiency are rapid and clear,5,6,9 however, results from long-term B deficiency hinder the progress in the knowledge on the primary physiological roles of B in vascular plants. Often, the observed effects are consequence of secondary effects of B deficiency and this fact, along with the very small amount required of this element, become B probably the least understood of all essential nutrients in vascular plants.16,17 The mechanisms underlying the various metabolic disorders caused by cell wall defects under B deficiency are indeed unknown.18

Very recently our group has shown that a short-term boron-deficient treatment led to a decline in root and, especially, leaf nitrate concentrations in tobacco plants15 as a result of a lower net nitrate-uptake rate. B deficiency seems to decrease net nitrate uptake by repressing expression of root plasma membrane H⁺-ATPase gene15 and, consequently, by decreasing the proton electrochemical gradient across the plasma membrane necessary to cotransport protons and nitrate inwards. Root asparagine concentration increased under B deprivation,15 which was also observed for other mineral deficiencies.19 Moreover, asparagine synthetase (AS) genes are also overexpressed under osmotic and salt stresses,20 and the levels of AS transcripts and free asparagine increased in plants stressed by excess of cadmium.21 All these data point out that induction of AS could be a common response of plants to cope with various stresses in order to reassimilate, in cooperation with glutamine synthetase-glutamate synthase cycle,15,22 the ammonium released by the enhanced protein and amino acid catabolism in stressed plants, thus preventing ammonium accumulation and toxicity in the plant tissues.

Therefore, it is really surprising that the deficiency of B—an element whose primary role is in the cell wall structure—is able to trigger diverse effects on so ample number of physiological processes in plants, particularly taking into account that B is very largely located at the cell wall that is, outside the protoplast.23,24

Does Boron Act as a Cellular Signal Too?

It has been suggested that changes in B concentrations may lead to a mechanical cascade of signals starting by an altered conformation of membrane-bound proteins, and extending into the cytoplasm via the cell wall-plasma membrane-cytoskeleton continuum4 (Fig. 1). This proposal is supported by the fact that B deprivation enhanced rapidly levels of cytoskeletal proteins (actin and tubulin) in Arabidopsis10 and maize11 roots, which led to an altered polymerization pattern of their cytoskeletal assemblies. Accordingly, the primary action of boron deprivation would be on an apoplastic target, which signals deeper into the cell via endocytosis-mediated pectin signaling along putative cell wall-plasma membrane-cytoskeleton continuum.25

Interestingly, it has been also reported that B deficiency induces the expression of stress-responsive genes18 and NIP5;1 gene26,27 that codes a protein essential for B uptake when this micronutrient is limiting.26 It has been proposed that a quick signal transfer from the cell wall to the cytoplasm could be involved for gene induction after B removal.18

Another tentative working hypothesis to explain the possible function of B as a cellular signal is illustrated in Figure 1 together with the above-mentioned hypothesis. In addition to the structural role of B stabilizing molecules with cis-diol groups in the cell wall (e.g., cross-linked RG-II dimmer)1,2 or other cis-diol containing molecules located at any cellular place, independently of their function,17 B could also affect the interaction between transcription factors (TF) and target genes. The TF interaction with the target gene sequence might be modulated by intracellular B levels. This interaction could be by either the direct linkage of B to the TF or through molecules capable of binding B and thereby linking with the TF. So, depending on the target gene and the type of TF (activator or repressor) the linkage or not with B could regulate gene expression (Fig. 1). This hypothesis might be more complicated taking into account the possibility of the networks of TF that can act in plant stress responses.28 The proteins encoded by these B-modulated genes, or eventually their absence, would act in diverse physiological processes at any cellular level. For instance, B might be essential in the cell wall not only for borate-ester bridges between RG-II monomers, but also as a signal to allow the expression of enzymes needed in the synthesis and assembly of cell wall, such as arabinogalactan proteins and expansins.4,29 In addition, this tentative hypothesis would be consistent with the results reported by other authors on gene expression under different B supply.18,27

How could intracellular B carry out this signaling function from a mechanistic point of view? TF, in the same way that other proteins such as glycoproteins, can contain not only hydroxylated ligands, such as sugar moieties, but also other as serine or threonine residues that might form ester-like complexes with borate.4 Obviously, the cytosolic concentration of B needed to exert its signaling function would be very low, which is compatible with the negligible levels found into the protoplast for this micronutrient.23,24

Curiously, calcium (Ca) is another element very important in the cell wall structure since it is readily bound by the free carboxyl groups and links pectin chains together into expanded, highly hydrated gel networks.29 Moreover the role of Ca as a cellular signal in many pathways and even as a second messenger in plant responses to pathogen attacks has been highlighted.30 Therefore, B could share with Ca three physiological features: (a) a structural role in the cell wall, (b) its very scarce mobility and solubility (very low cytosolic concentration), and (c) a signaling function. Hence, despite Ca and B are a macronutrient and a micronutrient, respectively, both of them could play similar roles as structural and signaling elements in plant cells.

In summary, we propose the hypothesis that B may exert its primary function not only through stabilization of molecules containing cis-diol groups1,2,16,17 but also as a cellular signal capable of interacting with TF, which could explain why many physiological processes are affected when vascular plants are subjected to B deficiency.