The endodermis in roots acts as a selectivity filter for nutrient and water transport essential for growth and development. This selectivity is enabled by the formation of lignin-based Casparian strips. Casparian strip formation is initiated by the localization of the Casparian strip domain proteins (CASPs) in the plasma membrane, at the site where the Casparian strip will form. Localized CASPs recruit Peroxidase 64 (PER64), a Respiratory Burst Oxidase Homolog F, and Enhanced Suberin 1 (ESB1), a dirigent-like protein, to assemble the lignin polymerization machinery. However, the factors that control both expression of the genes encoding this biosynthetic machinery and its localization to the Casparian strip formation site remain unknown. Here, we identify the transcription factor, MYB36, essential for Casparian strip formation. MYB36 directly and positively regulates the expression of the Casparian strip genes CASP1, PER64, and ESB1. Casparian strips are absent in plants lacking a functional MYB36 and are replaced by ectopic lignin-like material in the corners of endodermal cells. The barrier function of Casparian strips in these plants is also disrupted. Significantly, ectopic expression of MYB36 in the cortex is sufficient to reprogram these cells to start expressing CASP1–GFP, correctly localize the CASP1–GFP protein to form a Casparian strip domain, and deposit a Casparian strip-like structure in the cell wall at this location. These results demonstrate that MYB36 is controlling expression of the machinery required to locally polymerize lignin in a fine band in the cell wall for the formation of the Casparian strip.

The contention that quantitative profiles of biomolecules contain information about the physiological state of the organism has motivated a variety of high-throughput molecular profiling experiments. However, unbiased discovery and validation of biomolecular signatures from these experiments remains a challenge. Here we show that the Arabidopsis thaliana (Arabidopsis) leaf ionome, or elemental composition, contains such signatures, and we establish statistical models that connect these multivariable signatures to defined physiological responses, such as iron (Fe) Arabidopsis ͉ biomarker ͉ ionomics ͉ multivariable signature

Primary metabolism in flowers sustains a plenitude of physiological and ecological functions related to floral development and plant reproduction. Carbohydrates and amino acids provide energy and precursors for the reactions of floral secondary metabolism, such as the molecules for color and scent, and constitute an important resource of food for the pollinators. Recent discoveries have advanced our understanding of the cycles of carbohydrate hydrolysis and resynthesis that regulate pollen development, pollen tube growth, and pollination as well as the composition of nectar. Pathways of de novo amino acid biosynthesis have been described in flowers, and the proteins that regulate pollen tube guidance and ultimately control fertilization are being progressively characterized. Finally, a novel field of research is emerging that investigates the chemical modification of sugars and amino acids by colonizing microorganisms and how these affect the pollinators' preferences for flowers. In this Update article, we provide an overview of the new discoveries and future directions concerning the study of the primary metabolism of flowers.The chemistry of flowers is unique, as it sustains very diverse physiological functions such as flower development, the transition from flower to fruit, and the initial phases of seed set (O'Neill, 1997;Pélabon et al., 2015). In addition, floral metabolites fulfill relevant ecological roles, such as acting as signaling molecules in the chemical communication with animal pollinators (Borghi et al., 2017), providing protection against pests and colonizing microorganisms (Kessler and Baldwin, 2007;Nepi, 2014). Stunning displays of the metabolic resources of flowers are, for example, their colors and fragrance, which occur following the accumulation and emission of tinted and scented secondary metabolites, respectively (Grotewold, 2006;Tanaka et al., 2008;Khan and Giridhar, 2015). Perhaps less sensational, but equally as important, are the reactions of primary metabolism. Indeed, these sustain the physiology of flowers, provide the chemical precursors, and meet the energy demand of floral secondary metabolism (Muhlemann et al., 2014). Moreover, sugars and amino acids also serve as a nutritional reward for the pollinators that feed on nectar and pollen (Pacini et al., 2006;Heil, 2011;Roy et al., 2017). Therefore, knowing how primary metabolites are imported or de novo synthesized in flowers, and how they are secreted and catabolized, is at the heart of floral biology, being relevant to understand the physiology of plant reproduction and the role that flowers play in the ecosystem. In this article, we aim to provide an overview of floral central metabolism. How primary metabolism sustains flower development, and fruit and seed set, also will be addressed. Finally, given the considerable proportion of primary metabolites that are channeled into nectar, we will discuss the chemical composition of nectar, the modifications that arise from fermentation processes by colonizing yeasts and bacteria, an...

We report the results of a genome-wide analysis of transcription in Arabidopsis thaliana after treatment with Pseudomonas syringae pathovar tomato. Our time course RNA-Seq experiment uses over 500 million read pairs to provide a detailed characterization of the response to infection in both susceptible and resistant hosts. The set of observed differentially expressed genes is consistent with previous studies, confirming and extending existing findings about genes likely to play an important role in the defense response to Pseudomonas syringae. The high coverage of the Arabidopsis transcriptome resulted in the discovery of a surprisingly large number of alternative splicing (AS) events – more than 44% of multi-exon genes showed evidence for novel AS in at least one of the probed conditions. This demonstrates that the Arabidopsis transcriptome annotation is still highly incomplete, and that AS events are more abundant than expected. To further refine our predictions, we identified genes with statistically significant changes in the ratios of alternative isoforms between treatments. This set includes several genes previously known to be alternatively spliced or expressed during the defense response, and it may serve as a pool of candidate genes for regulated alternative splicing with possible biological relevance for the defense response against invasive pathogens.

The relative importance that biomechanical and biochemical leaf traits have on photosynthetic capacity would depend on a complex interaction of internal architecture and physiological differences. Changes in photosynthetic capacity on a leaf area basis and anatomical properties during leaf development were studied in a deciduous tree, Prunus persica, and an evergreen shrub, Olea europaea. Photosynthetic capacity increased as leaves approached full expansion. Internal CO 2 transfer conductance (g i ) correlated with photosynthetic capacity, although, differences between species were only partially explained through structural and anatomical traits of leaves. Expanding leaves preserved a close functional balance in the allocation of resources of photosynthetic component processes. Stomata developed more rapidly in olive than in peach. Mesophyll thickness doubled from initial through final stages of development when it was twice as thick in olive as in peach. The surface area of mesophyll cells exposed to intercellular air spaces per unit leaf area tended to decrease with increasing leaf expansion, whereas, the fraction of mesophyll volume occupied by the intercellular air spaces increased strongly. In the sclerophyllous olive, structural protection of mesophyll cells had priority over efficiency of photochemical mechanisms with respect to the broad-leaved peach. The photosynthetic capacity of these woody plants during leaf development relied greatly on mesophyll properties, more than on leaf mass per area ratio (LMA) or nitrogen (N) allocation. Age-dependent changes in diffusion conductance and photosynthetic capacity affected photosynthetic relationships of peach versus olive foliage, evergreen leaves maturing functionally and structurally a bit earlier than deciduous leaves in the course of adaptation for xeromorphy.