Abstract
Molecular, genetic, and electrophysiological evidence indicates that at least one of the plant Glu receptor-like molecules, GLR3.4, functions as an amino acid-gated Ca²⁺channel at the plasma membrane. The aspect of plant physiology, growth, or development to which GLR3.4 contributes is an open question. Protein localization studies performed here provide important information. In roots, GLR3.4 and the related GLR3.2 protein were present primarily in the phloem, especially in the vicinity of the sieve plates. GLR3.3 was expressed in most cells of the growing primary root but was not enriched in the phloem, including the sieve plate area. GLR3.2 and GLR3.4 physically interacted with each other better than with themselves as evidenced by a biophotonic assay performed in human embryonic kidney cells and Nicotiana benthamiana leaf cells. GLR3.3 interacted poorly with itself or the other two GLRs. Mutations in GLR3.2, GLR3.4, or GLR3.2 and GLR3.4 caused the same and equally severe phenotype, namely, a large overproduction and aberrant placement of lateral root primordia. Loss of GLR3.3 did not affect lateral root primordia. These results support the hypothesis that apoplastic amino acids acting through heteromeric GLR3.2/GLR3.4 channels affect lateral root development via Ca²⁺ signaling in the phloem.
Highlights
GLR3.2, GLR3.3, and GLR3.4 fused to green fluorescent protein (GFP), each fusion being expressed in the respective glr knockout mutant and under the control of its native promoter
The results presented in this article establish a role for phloemlocalized GLR3.2/GLR3.4 heteromeric, amino acid–gated Ca2+
The consequence of mutating this control mechanism is ectopic hyperproduction of lateral root primordia. These results could be interpreted to mean that GLR3.2/GLR3.4 is a negative regulator, restricting primordia numbers and position along the root axis by
Summary
In the years since GLUTAMATE RECEPTOR-LIKE (GLR) genes were discovered in plants (Lam et al, 1998; Lacombe et al, 2001), research has focused on their phylogeny and evolution (Chiu et al., 1999, 2002; Turano et al, 2001), expression patterns and transcriptional responses (Meyerhoff et al, 2005; Roy et al, 2008), roles in carbon:nitrogen balance (Kang and Turano, 2003), abscisic acid sensing (Kang et al, 2004), and contributions to ionic relations, including Ca2+ signaling (Kim et al, 2001; Qi et al, 2006; Stephens et al, 2008; Cho et al, 2009). The last category relates closely to the presumed molecular function of GLRs because the homologous ionotropic Glu receptors (iGluRs) in animals combine as heterotetramers to form amino acid–gated ion channels with varying permeability to Na+, K+, and Ca2+ (Traynelis et al, 2010).
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