CHLOROPLAST UNUSUAL POSITIONING 1 is a plant-specific actin polymerization factor regulating chloroplast movement

Evolutionary diversification of C2 photosynthesis in the grass genus Homolepis (Arthropogoninae)

BACKGROUND AND AIMS

To better understand C4 evolution in monocots, we characterized C3-C4 intermediate phenotypes in the grass genus Homolepis (subtribe Arthropogoninae).

METHODS

Carbon isotope ratio (δ13C), leaf gas exchange, mesophyll (M) and bundle sheath (BS) tissue characteristics, organelle size and numbers in M and BS tissue, and tissue distribution of the P-subunit of glycine decarboxylase (GLDP) were determined for five Homolepis species and the C4 grass Mesosetum loliiforme from a phylogenetic sister clade. We generated a transcriptome-based phylogeny for Homolepis and Mesosetum species to interpret physiological and anatomical patterns in an evolutionary context, and to test for hybridization.

KEY RESULTS

Homolepis contains two C3 species (H. glutinosa, H. villaricensis), one species with a weaker form of C2 termed sub-C2 (H. isocalycia), and two C2 species (H. longispicula, H. aturensis). Homolepis longispicula and H. aturensis express over 85 % of leaf glycine in centripetal mitochondria within the BS, and have increased fractions of leaf chloroplasts, mitochondria and peroxisomes within the BS relative to H. glutinosa. Analysis of leaf gas exchange, cell ultrastructure and transcript expression show M. loliiforme is a C4 plant of the NADP-malic enzyme subtype. Homolepis comprises two sister clades, one containing H. glutinosa and H. villaricensis and the second H. longispicula and H. aturensis. Homolepis isocalycia is of hybrid origin, its parents being H. aturensis and a common ancestor of the C3  Homolepis clade and H. longispicula.

CONCLUSIONS

Photosynthetic activation of BS tissue in the sub-C2 and C2 species of Homolepis is similar to patterns observed in C3-C4 intermediate eudicots, indicating common evolutionary pathways from C3 to C4 photosynthesis in these disparate clades. Hybridization can diversify the C3-C4 intermediate character state and should be considered in reconstructing putative ancestral states using phylogenetic analyses.

Translational photobiology: towards dynamic lighting in indoor horticulture

Crop productivity depends on the ability of plants to thrive across different growth environments. In nature, light conditions fluctuate due to diurnal and seasonal changes in direction, duration, intensity, and spectrum. Laboratory studies, predominantly conducted with arabidopsis (Arabidopsis thaliana), have provided valuable insights into the metabolic and regulatory strategies that plants employ to cope with varying light intensities. However, there has been less focus on how horticultural crops tolerate dynamically changing light conditions during the photoperiod. In this review we connect insights from photobiology in model plants to the application of dynamic lighting in indoor horticulture. We explore how model species respond to fluctuating light intensities and discuss how this knowledge could be translated for new lighting solutions in controlled environment agriculture.

Comprehensive characterization of poplar HSP20 gene family: genome-wide identification, stress-induced expression profiling, and protein interaction verifications

BACKGROUND

Heat shock proteins (HSP20s) are crucial components in plant stress responses, acting as molecular chaperones to safeguard cellular integrity and prevent abnormal protein aggregation. While extensive research has been conducted on HSP20s in various plant species, limited information is available regarding the HSP20 protein family in poplar (Populus yunnanensis), a species of significant ecological and economic importance native to southwestern China.

RESULTS

To elucidate the distribution, structural features, and functional characteristics of HSP20 proteins in P. yunnanensis, a combination of bioinformatics tools and experimental validation was utilized. A total of 53 PyHSP20s were identified within the P. yunnanensis genome and classified into 12 subfamilies: CI, CII, CIII, CIV, CV, CVI, CVII, MI, MII, ER, CP, and Px containing 24, 1, 1, 1, 2, 2, 14, 3, 1, 1, 2, and 1 HSP20 proteins, respectively. Classification was based on subcellular localization and phylogenetic relationships, revealing subfamilies with varying exon-intron structures and conserved motifs. The 3D structures analysis showed significant differentiation, with the CI subfamily PyHSP20s exhibiting 8 β-sheets, compared to 7 β-sheets in other subfamilies. Additionally, the N-terminal arms displayed heterogeneity in length and sequence. The 53 PyHSP20s were unevenly distributed across 15 chromosomes, with tandem segmental duplications explaining the expansion of subfamilies, particularly CI, CV, CVI, and CVII. The analysis of cis-elements associated with stress response and hormone regulation underscored the critical role of PyHSP20 in stress adaptation. Expression profiling via database analysis and qRT-PCR confirmed the responsiveness of PyHSP20s to multiple stressors, including salt, mannitol, drought, heat, and abscisic acid (ABA). Furthermore, Yeast Two-Hybrid (Y2H) assays demonstrated potential regulatory interactions between PyHSP20s and other functional proteins involved in stress responses.

CONCLUSIONS

These findings provide a comprehensive understanding of the classification, structural differentiation, and functional roles of PyHSP20s in P. yunnanensis, thereby establishing a foundation for future functional investigations into this protein family.

Multiple Mechanisms to Regulate Actin Functions: "Fundamental" Versus Lineage-Specific Mechanisms and Hierarchical Relationships

Eukaryotic actin filaments play a central role in numerous cellular functions, with each function relying on the interaction of actin filaments with specific actin-binding proteins. Understanding the mechanisms that regulate these interactions is key to uncovering how actin filaments perform diverse roles at different cellular locations. Several distinct classes of actin regulatory mechanisms have been proposed and experimentally supported. However, these mechanisms vary in their nature and hierarchy. For instance, some operate under the control of others, highlighting hierarchical relationships. Additionally, while certain mechanisms are fundamental and ubiquitous across eukaryotes, others are lineage-specific. Here, we emphasize the fundamental importance and functional significance of the following actin regulatory mechanisms: the biochemical regulation of actin nucleators, the ATP hydrolysis-dependent aging of actin filaments, thermal fluctuation- and mechanical strain-dependent conformational changes of actin filaments, and cooperative conformational changes induced by actin-binding proteins.

A Kinesin-Like Protein, KAC, is Required for Light-Induced and Actin-Based Chloroplast Movement in Marchantia polymorpha

Chloroplasts accumulate on the cell surface under weak light conditions to efficiently capture light but avoid strong light to minimize photodamage. The blue light receptor phototropin regulates the chloroplast movement in various plant species. In Arabidopsis thaliana, phototropin mediates the light-induced chloroplast movement and positioning via specialized actin filaments on the chloroplasts, chloroplast-actin filaments. KINESIN-LIKE PROTEIN FOR ACTIN-BASED CHLOROPLAST MOVEMENT (KAC) and CHLOROPLAST UNUSUAL POSITIONING 1 (CHUP1) are pivotal for actin-based chloroplast movement and positioning in land plants. However, the mechanisms by which KAC and CHUP1 regulate chloroplast movement and positioning remain unclear. In this study, we characterized KAC and CHUP1 orthologs in the liverwort Marchantia polymorpha, MpKAC and MpCHUP1, respectively. Their knockout mutants, Mpkacko and Mpchup1ko, impaired the light-induced chloroplast movement. Although Mpchup1ko showed mild chloroplast aggregation, Mpkacko displayed severe chloroplast aggregation, suggesting the greater contribution of MpKAC to the chloroplast anchorage to the plasma membrane. Analysis of the subcellular localization of the functional MpKAC-Citrine indicated that MpKAC-Citrine formed a punctate structure on the plasma membrane. Structure-function analysis of MpKAC revealed that the deletion of the conserved C-terminal domain abrogates its targeting to the plasma membrane and its function. The deletion of the N-terminal motor domain retains the plasma membrane targeting but abrogates the formation of punctate structure and shows a severe defect in the light-induced chloroplast movement. Our findings suggest that the formation of the punctate structure on the plasma membrane of MpKAC is essential for chloroplast movement.

CHUP1 restricts chloroplast movement and effector-triggered immunity in epidermal cells

Chloroplast Unusual Positioning 1 (CHUP1) plays an important role in the chloroplast avoidance and accumulation responses in mesophyll cells. In epidermal cells, prior research showed silencing CHUP1-induced chloroplast stromules and amplified effector-triggered immunity (ETI); however, the underlying mechanisms remain largely unknown. CHUP1 has a dual function in anchoring chloroplasts and recruiting chloroplast-associated actin (cp-actin) filaments for blue light-induced movement. To determine which function is critical for ETI, we developed an approach to quantify chloroplast anchoring and movement in epidermal cells. Our data show that silencing NbCHUP1 in Nicotiana benthamiana plants increased epidermal chloroplast de-anchoring and basal movement but did not fully disrupt blue light-induced chloroplast movement. Silencing NbCHUP1 auto-activated epidermal chloroplast defense (ECD) responses including stromule formation, perinuclear chloroplast clustering, the epidermal chloroplast response (ECR), and the chloroplast reactive oxygen species (ROS), hydrogen peroxide (H2O2). These findings show chloroplast anchoring restricts a multifaceted ECD response. Our results also show that the accumulated chloroplastic H2O2 in NbCHUP1-silenced plants was not required for the increased basal epidermal chloroplast movement but was essential for increased stromules and enhanced ETI. This finding indicates that chloroplast de-anchoring and H2O2 play separate but essential roles during ETI.

Chloroplast-actin filaments decide the direction of chloroplast avoidance movement under strong light in Arabidopsis thaliana

Chloroplast-actin (cp-actin) filaments are crucial for light-induced chloroplast movement, and appear in the front region of moving chloroplasts when visualized using GFP-mouse Talin. They are short and thick, exist between a chloroplast and the plasma membrane, and move actively and rapidly compared to cytoplasmic long actin filaments that run through a cell. The average period during which a cp-actin filament was observed at the same position was less than 0.5 s. The average lengths of the cp-actin filaments calculated from those at the front region of the moving chloroplast and those around the chloroplast periphery after stopping the movement were almost the same, approximately 0.8 µm. Each cp-actin filament is shown as a dotted line consisting of 4-5 dots. The vector sum of cp-actin filaments in a moving chloroplast is parallel to the moving direction of the chloroplast, suggesting that the direction of chloroplast movement is regulated by the vector sum of cp-actin filaments. However, once the chloroplasts stopped moving, the vector sum of the cp-actin filaments around the chloroplast periphery was close to zero, indicating that the direction of movement was undecided. To determine the precise structure of cp-actin filaments under electron microscopy, Arabidopsis leaves and fern Adiantum capillus-veneris gametophytes were frozen using a high-pressure freezer, and observed under electron microscopy. However, no bundled microfilaments were found, suggesting that the cp-actin filaments were unstable even under high-pressure freezing.