Mutations in a Golden2-Like Gene Cause Reduced Seed Weight in Barley albino lemma 1 Mutants

Abstract

The albino lemma 1 (alm1) mutants of barley (Hordeum vulgare L.) exhibit obvious chlorophyll-deficient hulls. Hulls are seed-enclosing tissues on the spike, consisting of the lemma and palea. The alm1 phenotype is also expressed in the pericarp, culm nodes and basal leaf sheaths, but leaf blades and awns are normal green. A single recessive nuclear gene controls tissue-specific alm1 phenotypic expression. Positional cloning revealed that the ALM1 gene encodes a Golden 2-like (GLK) transcription factor, HvGLK2, belonging to the GARP subfamily of Myb transcription factors. This finding was validated by genetic evidence indicating that all 10 alm1 mutants studied had a lesion in functionally important regions of HvGLK2, including the three alpha-helix domains, an AREAEAA motif and the GCT box. Transmission electron microscopy revealed that, in lemmas of the alm1.g mutant, the chloroplasts lacked thylakoid membranes, instead of stacked thylakoid grana in wild-type chloroplasts. Compared with wild type, alm1.g plants showed similar levels of leaf photosynthesis but reduced spike photosynthesis by 34%. The alm1.g mutant and the alm1.a mutant showed a reduction in 100-grain weight by 15.8% and 23.1%, respectively. As in other plants, barley has HvGLK2 and a paralog, HvGLK1. In flag leaves and awns, HvGLK2 and HvGLK1 are expressed at moderate levels, but in hulls, HvGLK1 expression was barely detectable compared with HvGLK2. Barley alm1/Hvglk2 mutants exhibit more severe phenotypes than glk2 mutants of other plant species reported to date. The severe alm1 phenotypic expression in multiple tissues indicates that HvGLK2 plays some roles that are nonredundant with HvGLK1.

Chloroplast development in green plant tissues: the interplay between light, hormone, and transcriptional regulation

“…The g2 mutants are pale because they have smaller chloroplasts with incorrectly formed thylakoids in bundle sheath cells (Langdale & Kidner, 1994; Rossini et al ., 2001). The GLK genes belong to the Golden2, ARR‐B and PSR1 (GARP) transcription factor family (Fitter et al ., 2002) and have now been studied in many species including Arabidopsis, rice, tomato, sorghum, Gynandropsis gynandra , barley and the moss Physcomitrium patens (Fitter et al ., 2002; Yasumura et al ., 2005; Powell et al ., 2012; Wang et al ., 2013; Taketa et al ., 2021). In each of these species GLK genes exist as a homologous pair named GLK1 and GLK2 and glk1 glk2 double mutants have a pale‐green phenotype associated with smaller chloroplasts, reduced accumulation of thylakoid membranes and defects in grana stacking (Rossini et al ., 2001; Fitter et al ., 2002; Yasumura et al ., 2005; Waters et al ., 2008, 2009b).…”

GOLDEN2‐LIKE transcription factors: A golden ticket to improve crops?

“…The role of GLKs in regulating photosynthesis in fruit-associated tissues has been extended to cereals, with the identification of barley albino lemma 1 (alm1) as a mutant of HvGLK2 (Taketa et al, 2021).…”

“…Alm1 mutants have a characteristic phenotype with whitish spikes, mostly due to the lack of chlorophyll in the hull tissues, which is accompanied by reduced seed weight (Taketa et al, 2021). In rice, recent research by Zheng et al (2022) has shown that OsGLK1 must be downregulated in anther tapetum cells indicating a role before fertilisation that can affect anther development, pollen viability and seed setting.…”

Genome-Wide Identification and Characterization of GARP Transcription Factor Gene Family Members Reveal Their Diverse Functions in Tea Plant (Camellia sinensis)

“…GARP family TFs are involved in very diverse functions in across physiological processes and stress responses in plants, such as growth and development, chloroplast development, nutrient sensing, floral transition, hormone signaling, and stress responses ( Safi et al, 2017 ). Specifically, GLK genes in different species are well known to control chloroplast structure formation, and mutations in these genes cause albino leaf phenotypes and affect photosynthesis by reducing chlorophyll accumulation ( Fitter et al, 2002 ; Powell et al, 2012 ; Nadakuduti et al, 2014 ; Chen et al, 2016 ; Nagatoshi et al, 2016 ; Lupi et al, 2019 ; Taketa et al, 2021 ). NIGT1/HRS1/HHO subgroup TFs constitute a type of NO 3 – -inducible TF-encoding genes in plants and function as negative regulators in the Arabidopsis response to nitrogen starvation by repressing the expression of other key NO 3 – -inducible genes, including NRT2.1 ( Sawaki et al, 2013 ; Medici et al, 2015 ; Kiba et al, 2018 ; Maeda et al, 2018 ; Ueda et al, 2020b ; Wang et al, 2020d ).…”