As the classical sessile organisms, plants are exposed to a variety of environmental pressures such as abnormal temperature changes, drought, salt and alkali stress or pathogen infection. To cope with the fluctuating environmental stress conditions, plants gradually acquired systematic protection mechanisms to maintain normal life activities during the long-term evolutionary process. The heat shock protein (HSP) family, which includes HSP100; HSP70(DnaK); HSP90; HSP60; DNAJ proteins (also called J-protein or HSP40); and small HSP, is involved in the process of plants responding to abiotic stress ^[1][2][1,2]. Heat shock proteins are one of the most representative regulatory factors and also a kind of widely existing molecular chaperone [3], which acts at the frontline of defense against protein aggregation and plays an important role in helping plants cope with stressful environments [1].

Chloroplasts are not only organelles for plant photosynthesis, but also act as a general sensor for plants to perceive changes in the cellular or external environment [4]. When plants are subjected to adverse environmental effects such as high-temperature stress, the reactive oxygen species (ROS) accumulated inside of chloroplasts and proteins in chloroplasts are damaged or misfolded, which heavily affects the function of chloroplasts. In order to maintain the normal physiological functions of chloroplasts, plants need to rely on the chloroplast protein quality control system (cpPQC), such as the chloroplast heat shock proteins (cpHSP) [5], to degrade or reactivate the damaged or misfolded proteins. cpHSP70-1 is a major ATP-dependent chaperone that maintains proteostasis in chloroplasts, together with its co-chaperones: one is the co-chaperone chloroplast GrpE (CGE) [6]; the second is presumed to be the co-chaperone J-proteins similar to that in the cytoplasm [7]. Studies have shown that J-protein can improve the binding ability of HSP70 to substrate proteins [1].

All J-proteins contain the J-domain, which is a structure consisting of approximately 70 amino acids with an invariant histidine-proline-aspartic acid (HPD) tripeptide motif. According to their structural domain, J-proteins can be broadly divided into three categories ^[8][9][8,9]. All three classical types of J-proteins (class A, class B and class C types) contain J-domains for interaction with Hsp70 [10]. Some proteins only have a J-domain-like structure without the HPD tripeptide motif. These proteins are called J-like proteins [8]. It is reported that Arabidopsis contains two kinds of chloroplast heat shock protein 70 (cpHSP70), while this is true of at least 18 J-proteins and many J-like proteins in chloroplasts ^[8][11][12][8,11,12]. Except for the function as the co-chaperone of cpHSP70, some chloroplast J-proteins and J-like proteins have been demonstrated to play roles in many different biological processes ^[8][13][14][8,13,14].

2. DJA5 and DJA6

Both DJA5 and DJA6 have four domains: a J-domain; a glycine/phenylalanine-rich domain; a zinc-finger domain (also known as a cysteine-rich domain, CR domain); and a C-terminal domain ^[15][16][15,16]. They belong to the class A type of J protein. Both of them are localized in the chloroplast [8], and there are only four class A type of J-protein present in the plastid of Arabidopsis. The other two are DJA4 and DJA7. When DJA5 or DJA6 was knocked out, the single mutant was compensated by the other protein since the functions of DJA5 and DJA6 overlapped [14]. Therefore, the phenotype and the chloroplast ultrastructure of single mutants did not show obvious differences compared with that of WT. The dja5dja6 double knock-out mutant is seedling-lethal on soil; it presents an albino phenotype due to reduced chlorophyll under heterotrophic conditions. The leaves of dja5dja6 were gapped and the seedling developed tiny yellow pedicels, with only a few sterile flowers growing. On the ultrastructure, dja5dja6 possesses smaller and irregularly shaped chloroplasts and the thylakoid membrane is missing. The authors found that DJA5 and DJA6 are co-expressed with chloroplast sulfur utilization factor (SUF) system and the accumulation of chloroplast iron-sulfur (Fe-S) proteins is heavily affected in the dja5dja6 mutant. the protein content in the photosystem electron transport chain and the chloroplast SUF apparatus also decreased to a certain extent, which hindered the photosystem electron transport process. It can be seen that DJA5 and DJA6 are critical for maintaining a normal plastid shape, the normal phenotype of plant growth and Fe-S protein content [14].

3. DJC75

DJC75 (a J-protein, also named CRRJ and NdhT) and DNAJD15 (a J-like protein, also named CRRL and NdhU) are the subunits of the chloroplast NADH dehydrogenase-like (NDH) complex ^[17][18][19][22,23,24], which are required for the activity of NDH, functioning in cyclic electron transport ^[20][25]. In addition, DJC75 is essential for the accumulation of DNAJD15 and CRR31, an NDH activity-required protein.

4. AtJ20, AtJ8 and AtJ11

AtJ20 (DJC26, At4g13830) is a plastid J-protein containing J-domain only. J20 is able to interact with inactive and aggregated desoxyribose5-phosphate synthase (DXS), the first enzyme of the plastidic isoprenoid pathway [5]. It acts as an adaptor that provides its substrate, damaged DXS to cpHSP70s. Thereafter, cpHSP70 can deliver the irreversible inactive DXS to the Clp protease for degradation. Indeed, AtJ20 knock-out mutant accumulated high levels of DXS protein with reduced levels of DXS enzyme activity, while the transcription of DXS was not changed. j20-1, cphsp70-1 and cphsp70-2 mutants possess higher sensitivity to CLM, a specific DXS inhibitor, compared with that of the wild-type [5]. Under stress, especially heat-stress conditions, J20 promotes the degradation of DXS. On the other hand, cpHSP70 and reversible DXS can interact with the HSP100 chaperone ClpB3; the latter protein can synergistically contribute to refolding DXS back to its active form ^[21][30]. J20, J8 (AtDJC22, At1g80920) and J11 (DJC23, At4g36040) knock-out mutants were analyzed by Chen and coworkers ^[22][31]. These mutants all showed lower photosynthetic efficiency, the destabilization of photosystem II complexes, and unbalanced the redox reactions in chloroplasts. The AtJ8 knock-out mutant has a lighter effect on photosynthetic parameters than the AtJ11 or AtJ20 knock-out mutant ^[22][31]. It was assumed that AtJ8, AtJ11 and AtJ20 possess at least partially redundant functions, but also specific functions, respectively. There are three J-proteins with small molecular masses in Arabidopsis chloroplast, which are AtJ8, AtJ11 and AtJ20, respectively. These three J-proteins can assist HSP70 chaperone proteins to ensure the activity of Rubisco (Ribulose bisphosphate carboxylase oxygenase) by correctly folding and assembling the enzyme ^[22][23][31,32]. When one of the three J-proteins is knocked out, the activity of Rubisco, which fixes carbon dioxide in photosynthesis, is negatively affected. Therefore, the ability of atj11 or atj20 single mutant to fix carbon dioxide will be greatly reduced with the decrease in enzyme activity. The carbon dioxide assimilation ability of the atj8 mutant was slightly lower than that of the wild-type ^[22][31]. 

5. Choloroplast J-Proteins in Chlamydomonas reinhardtii

There are five chloroplast DnaJ homologs (CDJ) proteins in Chlamydomonas, namely CDJ1 to 5. CDJ1 is a plastidic protein containing a zinc-finger domain, which localizes to the soluble matrix part, thylakoid and low-density membrane of chloroplast. High temperature only can weakly induce the expression of the CDJ1 gene; therefore, the CDJ1 protein is only slightly increased under heat treatment. Solid experiment results showed that HSP90C and HSP70B form a complex in advance and then bind to CDJ1 ^[24][36]. The protein that interacts with both HSP70B and its cochaperone CDJ2 was identified by mass spectrometry as vesicle-induced protein (VIPP1) in plastids, which is essential for thylakoid biogenesis. Therefore, CDJ2 can specifically recognize and bind the substrate protein VIPP1 and recruit it to HSP70B, thus, participating in thylakoid membrane biogenesis ^[25][37]. CDJ3 and CDJ4 are weakly expressed and appear to be localized to the stroma and thylakoid membranes, respectively ^[26][38]. CDJ3 is strongly induced by light, and CDJ5 was also found to be light-inducible. The homologues of CDJ3-5 also can be found in green algae, moss and higher plants. CDJ3-5 all have special domains called bacterial-type ferredoxin domains. Since they all have redox-active Fe-S clusters, CDJ3-5 can activate ATPase activity on HSP70B through its J-domain and recruit HSP70B to participate in chloroplast Fe-S cluster biogenesis ^[27][39]. CDJ3-5 are most similar to the Fd domain-containing DJC76 clade, including DJC76, DJC77 and DJC82 of Arabidopsis [8]. Therefore, it is interesting to explore the function of DJC76 clade proteins, since DJA5 and DJA6 are involved in plastid Fe-S cluster biogenesis [14]. Determining the relationship between DJC76 clade proteins and DJA5/6 requires further analysis.

6. DJC31 and DJC62

The localization of DJC31 and DJC62 is relatively special, and whether they can be localized in chloroplasts is related to the integrity of their proteins. DJC31 and DJC62 are two structurally similar J-proteins that both carry clamp-type tetratricopeptide repeat domains (TPRs) and belong to the class C type of J-protein. When DJC31 or DJC62 in Arabidopsis was knocked out, the phenotype of the single mutant has little change compared with the wild-type. When both of them were knocked out, the morphology of roots, leaves, flowers and siliques were all abnormal, indicating that DJC31 and DJC62 are important for maintaining the morphology of plants [11]. In addition, djc31djc62 double mutant is more drought tolerant than the wild-type, and hypersensitive to ABA. Previously, DJC31 and DJC62 were predicted to localize either to the nucleus or the chloroplast ^[28][40]. Further, chloroplast import experiments found that truncated forms of DJC31 and DJC62 could be imported, indicating that both of them are located in the chloroplast [8]. Surprisely, Dittmer and co-authors recently discovered that both DJC31 and DJC62 are located to the endoplasmic reticulum membrane, which was validated by detecting the two proteins in isolated chloroplasts and microsomal membranes [11]. The TPR domains of DJC31 and DJC62 share the conserved K5N9-N6-K2R6 motif with the human HSP70 and HSP90 co-chaperone TPR2 (also known as DNAJC7).

7. Plastid-Localized J-Like Proteins

Orange (OR) was cloned from orange cauliflower mutant, melon fruit and carrot roots, which is required for carotenoid accumulation ^[29][30][31][42,43,44]. OR is a J-like protein that lacks the J-domain and the C-terminal domain of classic J-protein, and contains a DNAJ-type zinc-finger domain ^[29][30][42,43]. Plastid-localized OR is able to regulate a major rate-limiting enzyme of carotenoid biosynthesis, therefore, promoting carotenoid biosynthesis ^[32][45]. In addition, it also regulates plastid preprotein import by interacting with Tic40 and Tic110, two key translocons for preprotein importing [13]. A gain-of-function mutation endow OR the function of promoting chromoplast biogenesis ^[33][46]. OR^his variant directly interacts with ACCUMULATION AND REPLICATION OF CHLOROPLASTS3 (ARC3), and the interaction interferes with the interaction of ARC3 and PARALOG of ARC6 (PARC6) ^[34][47]. Both ARC3 and PARC6 are crucial regulators of plastid division, and their interaction is important for plastid division. Therefore, OR^his can also regulate chromoplast number. The nucleus-localized OR interacts with the transcription factor TCP14 and represses its transactivation activity, therefore, repressing chloroplast biogenesis in the etiolated cotyledons of Arabidopsis ^[35][48]. Interestingly, OR is present in the nucleus only in etiolated tissue in darkness. When plants are exposed to light the protein relocates into fully developed chloroplasts ^[35][48].