The White Stripes Featuring ALBOSTRIANS, a Chloroplast-Localized CCT Domain Protein

Variegation in different organ types has been of great interest to biologists. The study of pigment variegation in maize (Zea mays) kernels led to the discovery of transposable elements (McClintock, 1950), whereas the study of eye pigment variegation in Drosophila has led to interesting insights into chromatin regulation (Elgin and Reuter, 2013). Leaf variegation, in which leaves contain both normal and defective chloroplasts, offers an excellent system to study chloroplast biogenesis and anterograde and retrograde signaling. The barley (Hordeum vulgare) mutant albostrians is a classic variegation mutant, and studies on it have stimulated different aspects of chloroplast research (Börner, 2017); however, the underlying gene has been unknown since its initial characterization more than 50 years ago (Hagemann and Scholz, 1962), until now.

In this issue of The Plant Cell, Li et al. (2019) have cloned the ALBOSTRIANS (HvAST) gene, which is identical to the CONSTANS, CO-LIKE, and TOC1 (CCT) MOTIF FAMILY 7 (HvCMF7) gene, and identified its product as the first CCT domain-containing protein to localize to the chloroplast. The authors initially identified HvCMF7 as the putative HvAST gene through map-based cloning of the classic mutant, now designated hvcmf7-1, which the authors predicted to encode a truncated protein. Subsequently, they generated a series of ethyl methanesulfonate-induced TILLING alleles of the candidate gene HvCMF7 and identified one mutant that segregated for albinism. This mutant, designated hvcmf7-2, harbored a lesion that introduced a premature stop codon, which was predicted to produce a truncated protein relative to both the HvCMF7 and hvcmf7-1 alleles. Notably, hvcmf7-1 and hvcmf7-2 were both deficient in chloroplast rRNA and lacked identifiable thylakoids and granum stacks in the cells of albino leaves. An allelism test confirmed that these mutants were alleles of the same gene. Furthermore, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9-mediated genome editing of “wild-type” barley plants introduced a lesion into the HvCMF7 gene, denoted hvcmf7-3, that generated a premature stop codon in a similar location to the hvcmf7-1 lesion, which also resulted in plants with variegated leaves (see figure). The closest homolog of HvCMF7 in Arabidopsis (Arabidopsis thaliana) is CHLOROPLAST IMPORT APPARATUS2 (AtCIA2), both of which encode CCT domain-containing proteins. AtCIA2 is a nucleus-localized transcription factor that promotes the expression of translocon and ribosomal genes (Sun et al., 2001); however, the localization of HvCMF7 to the chloroplasts (and possibly also to the nucleus) strongly suggests that it has a function outside of the nucleus. Interestingly the CCT domain of HvCMF7 is partially dispensable, as both hvcmf7-1 and hvcmf7-3 plants lack this protein-protein interaction domain and were still able to produce functional chloroplasts, albeit with reduced efficiency.

The variegated leaf phenotype from the CRISPR-Cas9-induced hvcmf7-3 allele. (Adapted from Li et al., 2019, Figure 5A)

Further experiments, such as protein localization and protein-protein interaction experiments, will help to reveal whether HvCMF7 has functions in common with AtCIA2 and what its functions in the chloroplast are. Now that the identity of this gene has been revealed, further examination of its functions is possible, which promises to lead to insights into chloroplast biogenesis and cross-communication between the nucleus and chloroplast and to new roles of CCT domain proteins.