The subject of this paper, sun leaves are thicker and show higher photosynthetic rates than the shade leaves, is approached in two ways. The first seeks to answer the question: why are sun leaves thicker than shade leaves? To do this, CO2 diffusion within a leaf is examined first. Because affinity of Rubisco for CO2 is low, the carboxylation of ribulose 1,5-bisphosphate is competitively inhibited by O2, and the oxygenation of ribulose 1,5-bisphosphate leads to energy-consuming photorespiration, it is essential for C3 plants to maintain the CO2 concentration in the chloroplast as high as possible. Since the internal conductance for CO2 diffusion from the intercellular space to the chloroplast stroma is finite and relatively small, C3 leaves should have sufficient mesophyll surfaces occupied by chloroplasts to secure the area for CO2 dissolution and transport. This explains why sun leaves are thicker. The second approach is mechanistic or 'how-oriented'. Mechanisms are discussed as to how sun leaves become thicker than shade leaves, in particular, the long-distance signal transduction from mature leaves to leaf primordia inducing the periclinal division of the palisade tissue cells. To increase the mesophyll surface area, the leaf can either be thicker or have smaller cells. Issues of cell size are discussed to understand plasticity in leaf thickness.

In multicellular organisms, the coordination of cell proliferation and expansion is fundamental for proper organogenesis, yet the molecular mechanisms involved in this coordination are largely unexplored. In plant leaves, the existence of this coordination is suggested by compensation, in which a decrease in cell number triggers an increase in mature cell size. To elucidate the mechanisms of compensation, we isolated five new Arabidopsis (Arabidopsis thaliana) mutants (fugu1-fugu5) that exhibit compensation. These mutants were characterized together with angustifolia3 (an3), erecta (er), and a KIP-RELATED PROTEIN2 (KRP2) overexpressor, which were previously reported to exhibit compensation. Time-course analyses of leaf development revealed that enhanced cell expansion in fugu2-1, fugu5-1, an3-4, and er-102 mutants is induced postmitotically, indicating that cell enlargement is not caused by the uncoupling of cell division from cell growth. In each of the mutants, either the rate or duration of cell expansion was selectively enhanced. In contrast, we found that enhanced cell expansion in KRP2 overexpressor occurs during cell proliferation. We further demonstrated that enhanced cell expansion occurs in cotyledons with dynamics similar to that in leaves. In contrast, cell expansion was not enhanced in roots even though they exhibit decreased cell numbers. Thus, compensation was confirmed to occur preferentially in determinate organs. Flow cytometric analyses revealed that increases in ploidy level are not always required to trigger compensation, suggesting that compensation is only partially mediated by ploidy-dependent processes. Our results suggest that compensation reflects an organ-wide coordination of cell proliferation and expansion in determinate organs, and involves at least three different expansion pathways.

Regulation of cell number and cell size is essential for controlling the shape and size of leaves. Here, we report a novel class of Arabidopsis thaliana mutants, more and smaller cells 1 (msc1)-msc3, which have increased cell number and decreased cell size in leaves. msc1 has a miR156-resistant mutation in the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 15 (SPL15) gene. msc2 and msc3 are new alleles of paused and squint mutants, respectively. All msc mutants showed accelerated heteroblasty, a phenomenon in which several morphological traits of leaves change along with phase change. Consistent with this finding, in the wild type, leaves at higher nodes (adult leaves) also have increased cell number and reduced cell size compared with those at lower nodes (juvenile leaves). These facts indicate that precocious acquisition of adult leaf characteristics in the msc mutants may cause alterations in the number and size of cells, and that heteroblasty plays an important role in the regulation of cell number and size. In agreement with this suggestion, such heteroblasty-associated changes in cell number and size are severely inhibited by the constitutive overexpression of miR156 and are promoted by the elevated expression of miR156-insensitive forms of SPL genes. By contrast, rdr6, sgs3, zip, arf3 and arf4 mutations, which affect progression of heteroblasty, had little or no effect on number or size of cells. These results suggest that cell number and size are mainly regulated by an SPL-dependent pathway rather than by a tasiR-ARF-dependent pathway.

Physiological and ecological characteristics of sun and shade leaves have been compared in detail, but their developmental processes, in particular their light sensory mechanisms, are still unknown. This study compares the development of sun and shade leaves of Chenopodium album L., paying special attention to the light sensory site. We hypothesized that mature leaves sense the light environment, and that this information determines anatomy of new leaves. To examine this hypothesis, we shaded plants partially. In the low-light apex treatment (LA), the shoot apex with developing leaves was covered by a cap made of a shading screen and received photosynthetically active photon flux density (PPFD) of 60 micromol m(-2 )s(-1), while the remaining mature leaves were exposed to 360 micromol m(-2 )s(-1). In the high-light apex treatment (HA), the apex was exposed while the mature leaves were covered by a shade screen. After these treatments for 6 d, we analyzed leaf anatomy and chloroplast ultrastructure. The anatomy of LA leaves with a two-layered palisade tissue was similar to that of sun leaves, while their chloroplasts were shade-type with thick grana. The anatomy of HA leaves and shade leaves was similar and both had one-layered palisade tissue, while chloroplasts of HA leaves were sun-type having thin grana. These results clearly demonstrate that new leaves differentiate depending on the light environment of mature leaves, while chloroplasts differentiate depending on the local light environment.

ObjectiveThe objective of this study was to identify new causes of Charcot–Marie–Tooth (CMT) disease in patients with autosomal‐recessive (AR) CMT.MethodsTo efficiently identify novel causative genes for AR‐CMT, we analyzed 303 unrelated Japanese patients with CMT using whole‐exome sequencing and extracted recessive variants/genes shared among multiple patients. We performed mutation screening of the newly identified membrane metalloendopeptidase (MME) gene in 354 additional patients with CMT. We clinically, genetically, pathologically, and radiologically examined 10 patients with the MME mutation.ResultsWe identified recessive mutations in MME in 10 patients. The MME gene encodes neprilysin (NEP), which is well known to be one of the most prominent beta‐amyloid (Aβ)‐degrading enzymes. All patients had a similar phenotype consistent with late‐onset axonal neuropathy. They showed muscle weakness, atrophy, and sensory disturbance in the lower extremities. All the MME mutations could be loss‐of‐function mutations, and we confirmed a lack/decrease of NEP protein expression in a peripheral nerve. No patients showed symptoms of dementia, and 1 patient showed no excess Aβ in Pittsburgh compound‐B positron emission tomography imaging.InterpretationOur results indicate that loss‐of‐function MME mutations are the most frequent cause of adult‐onset AR‐CMT2 in Japan, and we propose that this new disease should be termed AR‐CMT2T. A loss‐of‐function MME mutation did not cause early‐onset Alzheimer's disease. Identifying the MME mutation responsible for AR‐CMT could improve the rate of molecular diagnosis and the understanding of the molecular mechanisms of CMT. Ann Neurol 2016;79:659–672