Chaperonins are a class of molecular chaperones that assist in the folding and assembly of a wide range of substrates. In plants, chloroplast chaperonins are composed of two different types of subunits, Cpn60α and Cpn60β, and duplication of Cpn60α and Cpn60β genes occurs in a high proportion of plants. However, the importance of multiple Cpn60α and Cpn60β genes in plants is poorly understood. In this study, we found that loss-of-function of CPNA2 (AtCpn60α2), a gene encoding the minor Cpn60α subunit in Arabidopsis thaliana, resulted in arrested embryo development at the globular stage, whereas the other AtCpn60α gene encoding the dominant Cpn60α subunit, CPNA1 (AtCpn60α1), mainly affected embryonic cotyledon development at the torpedo stage and thereafter. Further studies demonstrated that CPNA2 can form a functional chaperonin with CPNB2 (AtCpn60β2) and CPNB3 (AtCpn60β3), while the functional partners of CPNA1 are CPNB1 (AtCpn60β1) and CPNB2. We also revealed that the functional chaperonin containing CPNA2 could assist the folding of a specific substrate, KASI (β-ketoacyl-[acyl carrier protein] synthase I), and that the KASI protein level was remarkably reduced due to loss-of-function of CPNA2. Furthermore, the reduction in the KASI protein level was shown to be the possible cause for the arrest of cpna2 embryos. Our findings indicate that the two Cpn60α subunits in Arabidopsis play different roles during embryo development through forming distinct chaperonins with specific AtCpn60β to assist the folding of particular substrates, thus providing novel insights into functional divergence of Cpn60α subunits in plants.

The endosperm and embryo originate from the fertilized central cell and egg cell through a programmed series of cell division and differentiation events. Characterization of more vital genes involved in endosperm and embryo development can help us to understand the regulatory mechanism in more depth. In this study, we found that loss of NAA10 and NAA15, the catalytic and auxiliary subunits of Arabidopsis thaliana N-terminal acetyltransferase A (AtNatA), respectively, led to severely delayed and incomplete endosperm cellularization, accompanied by disordered cell division in the early embryo. Studies on the marker genes/lines of the endosperm (AGL62-GFP, pDD19::GFP, pDD22::NLS-GFP and N9185) and embryo (STM, FIL, SCR and WOX5) in naa10/naa15 mutants showed that expression patterns of these markers were significantly affected, which were tightly associated with the defective feature of endosperm cellularization and embryo cell differentiation. Subsequently, embryonic complementation rescued the abortive embryos, but failed to initiate endosperm cellularization properly, further confirming the essential role of AtNatA in both endosperm and embryo development. Moreover, repression of AGL62 in naa10 and naa15 restored the endosperm cellularization, suggesting that NAA10/NAA15 functions in initiation of endosperm cellularization by inhibiting the expression of AGL62 in Arabidopsis. Therefore, NAA10 and NAA15 could be considered as crucial factors involved in promoting endosperm cellularization in Arabidopsis.

Legume-rhizobium symbiosis in root nodules fixes nitrogen to satisfy the plant’s nitrogen demands. The nodules’ demand for energy is thought to determine nitrogen fixation rates. How this energy state is sensed to modulate nitrogen fixation is unknown. Here, we identified two soybean ( Glycine max ) cystathionine β-synthase domain–containing proteins, nodule AMP sensor 1 (GmNAS1) and NAS1-associated protein 1 (GmNAP1). In the high–nodule energy state, GmNAS1 and GmNAP1 form homodimers that interact with the nuclear factor-Y C (NF-YC) subunit (GmNFYC10a) on mitochondria and reduce its nuclear accumulation. Less nuclear GmNFYC10a leads to lower expression of glycolytic genes involved in pyruvate production, which modulates phosphoenolpyruvate allocation to favor nitrogen fixation. Insight into these pathways may help in the design of leguminous crops that have improved carbon use, nitrogen fixation, and growth.

Pentatricopeptide repeat (PPR) proteins form a large protein family and have diverse functions in plant development. Here, we identified an ALBINO EMBRYO AND SEEDLING (AES) gene that encodes a P-type PPR protein expressed in various tissues, especially the young leaves of Arabidopsis (Arabidopsis thaliana). Its null mutant aes exhibited collapsed chloroplast membrane system, reduced pigment content and photosynthetic activity, decreased transcript levels of PEP (plastid-encoded polymerase) -dependent chloroplast genes, and defective RNA splicing. Further work revealed that AES could directly bind to psbB-psbT, psbH-petB, rps8-rpl36, clpP, ycf3, and ndhA in vivo and in vitro and that the splicing efficiencies of these genes and the expression levels of ycf3, ndhA, and cis-tron psbB-psbT-psbH-petB-petD decreased dramatically, leading to defective PSI, PSII and Cyt b6f in aes. Moreover, AES could be transported into the chloroplast stroma via the TOC-TIC channel with the assistance of Tic110 and cpSRP54 and may recruit HCF244, SOT1, and CAF1 to participate in the target RNA process. These findings suggested that AES is an essential protein for the assembly of photosynthetic complexes, providing insights into the splicing of psbB operon (psbB-psbT-psbH-petB-petD), ycf3, and ndhA, as well as maintaining chloroplast homeostasis.