The Arabidopsis minE mutation causes new plastid and FtsZ1 localization phenotypes in the leaf epidermis

Abstract

Plastids in the leaf epidermal cells of plants are regarded as immature chloroplasts that, like mesophyll chloroplasts, undergo binary fission. While mesophyll chloroplasts have generally been used to study plastid division, recent studies have suggested the presence of tissue- or plastid type-dependent regulation of plastid division. Here, we report the detailed morphology of plastids and their stromules, and the intraplastidic localization of the chloroplast division-related protein AtFtsZ1-1, in the leaf epidermis of an Arabidopsis mutant that harbors a mutation in the chloroplast division site determinant gene AtMinE1. In atminE1, the size and shape of epidermal plastids varied widely, which contrasts with the plastid phenotype observed in atminE1 mesophyll cells. In particular, atminE1 epidermal plastids occasionally displayed grape-like morphology, a novel phenotype induced by a plastid division mutation. Observation of an atminE1 transgenic line harboring an AtMinE1 promoter::AtMinE1-yellow fluorescent protein fusion gene confirmed the expression and plastidic localization of AtMinE1 in the leaf epidermis. Further examination revealed that constriction of plastids and stromules mediated by the FtsZ1 ring contributed to the plastid pleomorphism in the atminE1 epidermis. These results illustrate that a single plastid division mutation can have dramatic consequences for epidermal plastid morphology, thereby implying that plastid division and morphogenesis are differentially regulated in epidermal and mesophyll plastids.

Isolation and analysis of a stromule‐overproducing Arabidopsis mutant suggest the role of PARC6 in plastid morphology maintenance in the leaf epidermis

“…As previously reported (Fujiwara et al ), epidermal plastids in minE1 assumed various morphologies, such as swollen giant plastids and grape‐shaped plastid clusters. Although one of the outstanding features of epidermal plastids in parc6‐5 was the multiple occurrence of stromule‐like anastomoses (or bridges) that connected the sub‐plastid bodies within a single plastid, such structures were less evident in minE1 parc6‐5 , as well as in minE1 .…”

“…These lines of evidence suggest that non‐photosynthetic plastids can divide without the machinery required for conventional chloroplast division (Pyke , Miyagishima et al ). In support of this hypothesis, differential impacts of mutations in the A. thaliana genes for chloroplast division‐associated proteins on chloroplasts and other plastid types were also reported, describing almost normal plastid size and number in some non‐green cells of the minD1 ( arc11 ), arc3 and mcd1 mutants (Fujiwara et al , Wang et al , Fujiwara et al ) and enhanced formation of stromules (see below) from non‐mesophyll plastids in the arc3 , arc5 , arc6 and minE1 mutants (Holzinger et al , Kojo et al , Fujiwara et al ).…”

“…Considering the need for efficient microscopic screening from a large plant population, the leaf epidermis of transgenic A. thaliana with cyan fluorescent protein (CFP)‐labeled plastids (Itoh et al ) was chosen as the material, because of its easy access for plastid observation (Fujiwara et al ). Although plastids in leaf epidermal cells are categorized as chloroplasts (Barton et al ), their morphology, especially with respect to division and stromule formation, is more similar to that of non‐green plastids than to that of mesophyll chloroplasts (Fujiwara et al ). Thus, we propose that leaf epidermal plastids may serve as a useful model for studying the dynamic morphology of non‐mesophyll plastids, including active stromule formation.…”

Stromules: Probing Formation and Function

“…Mutations in the ARC3 and ARC6 genes that reduce chloroplast division in Arabidopsis also result in increased stromule length (Holzinger et al, 2008). ARC3 mutants act through minE (Angel et al, 2013), and stromule frequency is increased in Arabidopsis minE1 mutants, which are affected in chloroplast division site placement (Fujiwara et al, 2015). FtsZ rings could theoretically provide constrictions that narrow incipient plastid projections into stromules, but experimental evidence for such a process is currently lacking.…”

“…A role of the dynamin-related GTPase ARC5 in production of stromule-derived vesicles merits further investigation. The possible involvement of FtsZ in "stromule fission" has been proposed (Fujiwara et al, 2015). The ultimate fate of the broken stromule "tips" is unknown-are they taken up by plastids, or do they enter other organelles within the cells?…”