Functional characterization of Citrus macrophylla BOR1 as a boron transporter

Abstract

Plants have evolved to develop an efficient system of boron uptake and transport using a range of efflux carriers named BOR proteins. In this work we isolated and characterized a boron transporter of citrus (Citrus macrophylla), which was named CmBOR1 for its high homology to AtBOR1. CmBOR1 has 4403 bp and 12 exons. Its coding region has 2145 bp and encodes for a protein of 714 amino acids. CmBOR1 possesses the molecular features of BORs such as an anion exchanger domain and the presence of 10 transmembrane domains. Functional analysis in yeast indicated that CmBOR1 has an efflux boron transporter activity, and transformants have increased tolerance to excess boron. CmBOR1 is expressed in leaves, stem and flowers and shows the greatest accumulation in roots. The transcript accumulation was significantly increased under boron deficiency conditions in shoots. In contrast, the accumulation of the transcript did not change in boron toxicity conditions. Finally, we observed that constitutive expression of CmBOR1 was able to increase tolerance to boron deficiency conditions in Arabidopsis thaliana, suggesting that CmBOR1 is a xylem loading boron transporter. Based on these results, it was determined that CmBOR1 encodes a boric acid/borate transporter involved in tolerance to boron deficiency in plants.

Physiological and genetic responses to boron deficiency inBrassica napus: A review

“…Recently, BOR1 has been reported to have a similar function in other species, including OsBOR1 in rice (Nakagawa et al 2007), TaBOR1 in wheat (Leaungthitikanchana et al 2013), VvBOR1 in grape (Vitis vinifera L.) (Perez-Castro et al 2012) and CmBOR1 in Citrus macrophylla Wester. (Cañon et al 2013). In B. napus, six genes, BnBOR1;1a, BnBOR1;1c, BnBOR1;2a, BnBOR1;2c, BnBOR1;3a and BnBOR1;3c, have now been isolated and characterized (Sun et al 2012).…”

Transcription factor BnaA9.WRKY47 contributes to the adaptation of Brassica napus to low boron stress by up‐regulating the boric acid channel gene BnaA3.NIP5;1

“…Under low B conditions, the transport pathways of B from the Arabidopsis root surface to the shoot includes at least three events: (1) NIP5;1, a major boric acid channel, efficiently facilitates B uptake from soil into the root (Takano et al, 2006;Wang et al, 2017); (2) BOR1, a B efflux transporter, is responsible for xylem loading of B (Noguchi et al, 1997;Takano et al, 2002); and (3) NIP6;1 and NIP7;1, homologues of NIP5;1, are required for proper distribution of boric acid in developing shoot tissues and anthers (Routray et al, 2018;Tanaka et al, 2008). Recently, more B transport-related genes, such as tassel-less1 (co-orthologue of AtNIP5;1) (Matthes et al, 2018) and rotten ear (co-orthologue of AtBOR1) (Chatterjee et al, 2014) in maize, OsNIP3;1 (a homologue of AtNIP5;1) (Hanaoka et al, 2014;Shao et al, 2018) and OsBOR1 (a homologue of AtBOR1) (Nakagawa et al, 2007) in rice, VvBOR1 in grapevine (P erez-Castro et al, 2012), CmBOR1 in citrus (Cañon et al, 2013) and TaBOR1s in wheat (Leaungthitikanchana et al, 2013), have been reported. These studies demonstrated that both boric acid channel NIPs (nodulin 26-like intrinsic proteins) and B transporter BORs are critical for plants to cope with B stress.…”

Identificación de caracteres fisiológicos y moleculares para la tolerancia a estreses abióticos en portainjertos de cítricos tetraploides.

“…De igual manera, se analizó la expresión del gen BOR1, que modula el transporte de B en plantas a nivel de la entrada al xilema, regulando la translocación de las raíces a la parte aérea (Takano et al, 2002). En cítricos, se ha realizado recientemente la caracterización funcional del gen CmBOR1 en C. macrophylla (Cañon et al, 2013). La cantidad de transcrito de éste gen en las raíces no varía al aplicarse una concentración alta de B en el medio y la actividad del transportador permite acumular eficientemente el B en las hojas, aun cuando la cantidad en el medio externo sea excesiva (Cañon et al, 2013;Martínez-Cuenca et al, 2015).…”

“…Los cebadores utilizados son los descritos en publicaciones previas Cañon et al, 2013) y se muestran en la Tabla 4.1. Se realizaron tres réplicas técnicas simultáneas de cada reacción y tres réplicas biológicas sucesivas.…”

“…Este canal se localiza en la membrana plasmática de las células del periciclo de la raíz y su sobreexpresión, en condiciones deficitarias de B, activa la translocación de B desde las raíces hacia la parte aérea de la planta, probablemente en forma de borato (Miwa et al, 2006). Sin embargo algunos estudios señalan que cuando existe exceso de B, la proteína BOR1 es eliminada de la membrana mediante endocitosis y degradada en las vacuolas (Takano et al, 2005b;Takano et al, 2010), lo que sugiere que su regulación puede ser uno de los mecanismos para evitar la toxicidad por B. Éste transportador ha sido recientemente caracterizado en cítricos al localizarse el gen ortólogo CmBOR1 en Citrus macrophylla Wester y verificarse su función (Cañon et al, 2013).…”