Juan J. Camacho-Cristóbal

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.

Auxin and ethylene are involved in the responses of root system architecture to low boron supply in Arabidopsis seedlings

Changes in root architecture are one of the adaptive strategies used by plants to compensate for nutrient deficiencies in soils. In this work, the temporal responses of Arabidopsis (Arabidopsis thaliana) root system architecture to low boron (B) supply were investigated. Arabidopsis Col-0 seedlings were grown in 10 µM B for 5 days and then transferred to a low B medium (0.4 µM) or control medium (10 µM) for a 4-day period. Low B supply caused an inhibition of primary root (PR) growth without altering either the growth or number of lateral roots (LRs). In addition, low B supply induced root hair formation and elongation in positions close to the PR meristem not observed under control conditions. The possible role of auxin and ethylene in the alteration of root system architecture elicited by low B supply was also studied by using two Arabidopsis reporter lines (DR5:GUS and EBS:GUS) and two Arabidopsis mutants with impaired auxin and ethylene signaling (aux1-22 and ein2-1). Low B supply increased auxin reporter DR5:GUS activity in PR tip, suggesting that low B alters the pattern of auxin distribution in PR tip. Moreover, PR elongation in aux1-22 mutant was less sensitive to low B treatment than in wild-type plants, which suggests that auxin resistant 1 (AUX1) participates in the inhibition of PR elongation under low B supply. From all these results, a hypothetical model to explain the effect of low B treatment on PR growth is proposed. We also show that ethylene, via ethylene-insensitive 2 (EIN2) protein, is involved in the induction of root hair formation and elongation under low B treatment.

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.

PRD, an Arabidopsis AINTEGUMENTA-like gene, is involved in root architectural changes in response to phosphate starvation

Changes in root architecture are one of the adaptive strategies used by plants to compensate for local phosphate (Pi) deficiency in soils. Root architecture variables triggered by Pi availability are well documented in Arabidopsis (Arabidopsis thaliana), but the molecular mechanisms behind these adaptive responses remain to be elucidated. By the use of transcriptomic and quantitative RT-PCR analysis, we observed that an AINTEGUMENTA-like gene, named PRD for Phosphate Root Development, was rapidly repressed in roots under low Pi conditions. The physiological function of the PRD gene was analyzed through the null allele mutant prd, which displayed less development of primary and lateral roots under Pi-starvation conditions than wild-type plants. Complementation of the prd mutant with the wild-type gene led to a similar response to Pi starvation as wild-type plants, indicating the complete rescue of the mutant phenotype. These results suggest that PRD gene is involved in the regulation of root architectural responses to Pi starvation by controlling primary and lateral root elongation. This model is in agreement with the tissue-specific pattern of PRD gene expression, which was observed to occur specifically in the apex in both the primary and lateral roots. However, Pi influx, anionic profiles and root expression of genes typically induced by Pi starvation, such as high affinity Pi transporters (PHT1;1 and PHT1;4) and an acid phosphatase (AtACP5), were similar in wild type and prd plants in response to Pi starvation. These results support the hypothesis that the PRD gene is not a checkpoint for Pi-starvation responses, but acts specifically as a regulator of root architectural responses to Pi starvation.

Boron deficiency inhibits root cell elongation via an ethylene/auxin/ROS-dependent pathway in Arabidopsis seedlings

Highlight The rapid inhibition of root cell elongation in response to boron deficiency is mediated by an ethylene/auxin/ROS dependent pathway

Is Ca2+ involved in the signal transduction pathway of boron deficiency? New hypotheses for sensing boron deprivation.

Plants sense and transmit nutrient-deprivation signals to the nucleus. This increasingly interesting research field advances knowledge of signal transduction pathways for mineral deficiencies. The understanding of this topic for most micronutrients, especially boron (B), is more limited. Several hypotheses have been proposed to explain how a B deprivation signal would be conveyed to the nucleus, which are briefly summarized in this review. These hypotheses do not explain how so many metabolic and physiological processes quickly respond to B deficiency. Short-term B deficiency affects the cytosolic Ca(2+) levels as well as root expression of genes involved in Ca(2+) signaling. We propose and discuss that Ca(2+) and Ca(2+)-related proteins - channels/transporters, sensor relays, and sensor responders - might have major roles as intermediates in a transduction pathway triggered by B deprivation. This hypothesis may explain how plants sense and convey the B-deprivation signal to the nucleus and modulate physiological responses. The possible role of arabinogalactan-proteins in the B deficiency signaling pathway is also taken into account.

Root Responses to Boron Deficiency Mediated by Ethylene

Low boron (B) supply alters the architecture of the root system in Arabidopsis thaliana seedlings, leading to a reduction in the primary root growth and an increase in the length and number of root hairs. At short-term (hours), B deficiency causes a decrease in the cell elongation of the primary root, resulting in a lower growth. Experimental approaches using ethylene insensitive Arabidopsis mutants, inhibitors of ethylene response, and GUS reporter lines suggest that ethylene is involved in these responses of the primary root to B deficiency. Furthermore, it has been shown that auxin participates in the inhibition of cell elongation under short-term B deprivation. These results support that an interaction between ethylene and auxin plays an important role in controlling the primary root elongation, in which a number of genes related to the synthesis, transport, and signaling of both phytohormones could modulate this effect. Evidence for a root cross-talk among both hormones and other possible intermediates (abscisic acid, calcium sensors, and reactive oxygen species) in response to B deficiency is provided and discussed.

Transcription factors as potential participants in the signal transduction pathway of boron deficiency

Boron (B) plays a well-known structural role in the cell wall, however the way of perceiving B deficiency by roots and transmitting this environmental signal to the nucleus to elicit a response is not well established. It is known that the direct interaction between Ca2+ sensors and transcription factors (TFs) is a necessary step to regulate the expression of downstream target genes in some signaling pathways. Interestingly, B deprivation affected gene expressions of several TFs belonging to MYB, WRKY, and bZIP families, as well as expressions of Ca2+-related genes such as several CML (calmodulin-like protein) and CPK (Ca2+-dependent protein kinase) genes. Taken together, these results suggest that B deficiency could affect the expression of downstream target genes by alteration of a calcium signaling pathway in which the interaction between CMLs and/or CPKs with TFs (activator or repressor) would be a crucial step, which would explain why some genes are upregulated whereas others are repressed upon B deprivation.

Boron deficiency increases expressions of asparagine synthetase, glutamate dehydrogenase and glutamine synthetase genes in tobacco roots irrespective of the nitrogen source

Abstract Nitrate-ammonium (N-A) and ammonium-nitrate (A-N) transition experiments were performed with tobacco (Nicotiana tabacum, cv Gatersleben) plants to study whether the increased expression of ammonium assimilation-related genes found under short-term boron (B) deficiency is maintained when the nitrogen source changes. Asparagine synthetase (AS), glutamate dehydrogenase (GDH) and glutamine synthetase (GS) gene overexpressions in roots under B deprivation were detected in both nitrogen transitions, although GS and, especially, AS up-regulations were observed earlier in the ammonium-nitrate transition. Transcript levels were inversely correlated with hexose contents after 24 h of B deprivation. Our results suggest that tobacco roots respond to short-term B deficiency by increasing the expression of AS, GDH and GS genes irrespectively of the nitrogen source. A hypothetical model to explain these results is proposed and discussed. Thus, the combined action of these enzymes would avoid the accumulation of ammonia and help maintain the activity of the tricarboxylic acid cycle when tobacco roots are affected by B deficiency, the plant organ that first senses this stress. Therefore, under B deficiency, they would act as an ammonium detoxifying mechanism, being a complement to the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway. We propose that the protective role of these enzymes could be extended to changing nitrogen-supply conditions.

Effects of phosphate on in vitro nitrate reductase activity from tobacco leaves

Abstract Extracts from tobacco (Nicotiana tabacum L. cv. Gatersleben 1) leaves harvested 30 min before the beginning of the light period (‘dark extract’) and after 4 h illumination (‘light extract’) were used to study the effects of phosphate (Pi) on in vitro nitrate reductase (NR, E.C. 1.6.6.1) activity. Addition of 25 mM Pi increased (−Mg2+)NR activity from ‘light and dark extracts’, this effect being specific of this anion and not due to an increased ionic strength. However, (+Mg2+)NR activity decreased when the extracts were preincubated with 25 mM Pi. This Pi-inactivation was insensitive to staurosporine, which indicates that inactivation was not the result of NR protein phosphorylation. NR activity (±Mg2+) from ‘dark extracts’ increased when preincubated for 1 h and reached similar activities as those from ‘light extracts’ at the end of the preincubation. This increase of NR activity from ‘dark extracts’ was abolished in the presence of okadaic acid, reflecting a dephosphorylation of NR protein by endogenous phosphatases in a time-dependent process. These data are discussed concluding that Pi affects the in vitro NR activity in tobacco plants.