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

Abstract

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.

Keywords: :

• arabidopsis
• boron deficiency
• calcium signaling
• Ca²⁺-dependent protein kinases
• calmodulin-like proteins
• transcription factors

The physiology of boron (B) is a topic of growing interest since both its deficiency as toxicity are important problems in agriculture affecting many land areas in the world. Despite the well known role of B as an essential component for the cell wall structure linking two rhamnogalacturonan II chains to stabilize them as dimers through diester bonds,^1–3 very few is known on the way of B deficiency is perceived by roots and conveyed to the nucleus to elicit a response to this mineral stress.

It has been proposed that lack of B would be recognized in cell wall as a structural alteration, which could lead to a lower mechanical strength of the wall, and this signal would be transmitted by cell wall-plasma membrane-cytoskeleton continuum with the participation of arabinogalactan proteins.^4,5 In addition, a damaged cell wall as a consequence of B deficiency also could cause a redox imbalance.⁶ Finally, transcription factors have also been suggested that would be involved in the mechanism of response to B deficiency since they would act in the regulation of gene expression.⁷

A very interesting result obtained by microarray is that B deprivation affected gene expressions of several transcription factors (TFs) belonging to MYB, WRKY, and bZIP families (Table 1). Thus, the expression of MYB14, MYB15, MYB78, WRKY38, WRKY40, and WRKY46 genes were upregulated, while bZIP34 and bZIP61 gene expressions were lower in plants subjected to B deprivation (Table 1). In addition, induction of WRKY6 gene has been reported to be essential for normal root growth under B deficiency, its expression being higher in B-deficient plants.⁸ The involvement of TFs in the signal cascade pathway of B deficiency would explain why this environmental stress can affect so many physiological processes in vascular plants.^9,10 Furthermore, this would also clarify the short-term effects caused by B deprivation, including the fact that some genes are upregulated whereas others are repressed.¹¹ The role of TFs (activator or repressor) would be essential to explain how target genes are regulated in one direction or another.⁷

Table 1. Expression levels of transcription factor genes in roots of Arabidopsis seedlings subjected (-) or not (+) to B deficiency for 24 h

GENE DESCRIPTION	FC (-B/+B)	TAIR ID
	24 h
Transcription factors (TFs)
MYB family
MYB domain protein 14 (MYB14)	1.84	At2g31180
MYB domain protein 15 (MYB15)	3.39	At3g23250
MYB domain protein 78 (MYB78)	1.94	At5g49620
WRKY family
WRKY DNA-binding protein 38 (WRKY38)	1.79	At5g22570
WRKY DNA-binding protein 40 (WRKY40)	2.34	At1g80840
WRKY DNA-binding protein 46 (WRKY46)	2.71	At2g46400
bZIP family
bZIP transcription factor 34 (bZIP34)	-2.53	At2g42380
bZIP transcription factor 61 (bZIP61)	-1.32	At3g58120

Differential expression levels were expressed on a fold change basis (FC, signal from B deficiency/signal from B sufficiency). Background correction and normalization of expression data were performed as already described.¹⁴ p-values for all FC shown were lower than 0.05 and considered statistically significant according to Student’s t-test.

In addition to the reported interactions between B and calcium,¹² recent studies go a step further and point out a possible relation between B and Ca²⁺ for gene expression.^11,13,14 Although B and Ca²⁺ belong to micronutrient and macronutrient groups, respectively, both play key roles in cell wall structure through their links with pectin chains. Moreover, it is well known that Ca²⁺ is a cellular signal in many pathways acting as a secondary messenger in plants,¹⁵ and it is becoming increasingly clear that B deprivation affects gene expressions involved in several processes.¹⁶ Hence, it would not be surprising that, according to our most recent hypothesis, genes encoding Ca²⁺-proteins would be playing a major role in the mechanism(s) through which plants are able to sense and transmit the B-deprivation signal.¹⁴

It is known that the direct interaction between Ca²⁺ sensors and TFs is a necessary step to regulate the expression of downstream target genes in some signaling pathways.^17-19 Thus, interactions between Ca²⁺-related proteins and TFs have been described.²⁰ Arabidopsis calmodulin-like protein 12 (CML12), whose gene expression is affected by B deficiency,¹⁴ is capable of binding to different TF families (MADS-box, MYB, WRKY, bZIP).²⁰ Other studies have also reported that several TFs, such as MYBs and WRKYs, are regulated by direct interaction with calmodulins (CaMs).^17,18 Besides, Ca²⁺-dependent protein kinases (CPKs) -other group of Ca²⁺ sensors- can interact with TFs in Arabidopsis, such as CPK4, CPK11, and CPK32.^21,22 In addition to TF genes (Table 1), we have reported that expressions of several CML and CPK genes were affected under B starvation.¹⁴ Therefore, all together, these reports and findings suggest that B deficiency could affect the expression of downstream target genes by alteration of a Ca²⁺ signaling cascade in which the interaction between CMLs and/or CPKs with TFs would be a main step.

Abbreviations:
CaM	=	calmodulin
CMLs	=	calmodulin-like proteins
CPKs	=	Ca2+-dependent protein kinases
TFs	=	transcription factors

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

The authors thank Ministerio de Economía y Competitividad (BFU2009–08397 and BFU2012–37445) and Junta de Andalucía (BIO-266 and P09-CVI-4721), Spain, for supporting this research.