Understanding the root sink.

Root development and growth is similar to shoot growth in that extension growth is initiated by an apical meristem and girth growth of mature roots is carried out by the vascular cambium. However, the initiation of lateral roots is entirely different than the initiation of lateral leaves or shoot meristems. This chapter deals with understanding the root sink in fruit trees by studying root growth, including the initiation of lateral roots, root classification according to size and function, factors affecting their growth, and rootstocks.

HAM Gene Family and Shoot Meristem Development

Land plants develop highly diversified shoot architectures, all of which are derived from the pluripotent stem cells in shoot apical meristems (SAMs). As sustainable resources for continuous organ formation in the aboveground tissues, SAMs play an important role in determining plant yield and biomass production. In this review, we summarize recent advances in understanding one group of key regulators – the HAIRY MERISTEM (HAM) family GRAS domain proteins – in shoot meristems. We highlight the functions of HAM family members in dictating shoot stem cell initiation and proliferation, the signaling cascade that shapes HAM expression domains in shoot meristems, and the conservation and diversification of HAM family members in land plants. We also discuss future directions that potentially lead to a more comprehensive view of the HAM gene family and stem cell homeostasis in land plants.

Stable establishment of organ polarity several plastochrons before primordium outgrowth in Arabidopsis

In many species, leaves are initiated at the flanks of shoot meristems. Usually, subsequent growth mainly occurs in the plane of the leaf blade, which leads to the formation of a bifacial leaf with dorso-ventral identities. In a classical set of surgical experiments in potato meristems, Sussex provided evidence that dorsoventrality depends on a signal emanating from the meristem centre. Although these results could be reproduced in tomato, this concept has been debated. We revisited these experiments in Arabidopsis where a range of markers are available to target the precise site of ablation. Using specific markers for organ founder cells and dorsoventral identity, we were unable to perturb the polarity of leaves and sepals long before organ outgrowth. While results in Solanaceae suggested that dorsoventral patterning was unstable during early development, we find that in Arabidopsis the local information contained within and around the primordium is able to withstand major invasive perturbations, long before polarity is fully established.

Plastid Transcriptomics: An Important Tool For Plastid Functional Genomics

Plastids in higher plants carry out specialized roles such as photosynthesis, nitrogen assimilation, biosynthesis of amino acids, fatty acids, isoprenoids, and various metabolites. Plastids arise from undifferentiated precursors known as proplastids, which are found in the root and shoot meristems. They are highly dynamic as they change their number, morphology, and physiology according to the tissue they are present. In addition to housing various metabolic activities, plastids also serve as a global sensor for both internal and external environmental cues including different stresses, and help plants to respond/adjust accordingly. They relay information to the nucleus, which then responds by changing the expression levels of specific genes. It has been shown that plants with impaired plastid functions exhibit abnormalities. One of the sources emanating these signals to the nucleus is plastid transcription. Normal plastid functioning is therefore critical for plant survival. Despite immense significance for plant acclimation, the plastid transcriptome is largely an unstudied research area. In this review, we discuss the importance of plastid transcriptomics for the acclimation of plants under changing environmental conditions and summarize the key literature published in this field.

Investigation on the direct shoot regeneration in date palm CV.Pyarum and possible genetics changes in induced shoots

The palm (Phoenix Dactylifera) is one of important trees, and is economically important in south of Iran. Date palm is propagated by the offshoots, number of which is limited. Therefore, adult Date palms produce shoot tips and axillary shoot meristems through the use of a tissue culture. This study was conducted to perform in -vitro tissue culture direct shoot regeneration and determine the best combination of plant regulators and other conditions. To achieve organogenesis and multiplication directly from shoot tips and axillary shoot meristems of Date palm (Phoenix Dactylifera L. Var Pyarum) was used without callus formation. Direct regeneration of vegetative buds minimizes the risk of somaclonal variation among plant regenerates. Results revealed that MS medium supplemented with 4mg/l of kinetin and 3 mg/l of IAA or 2mg/l of BA and 4 mg/l of NAA was the best formation from shoot tip after 16-20 weeks. Subculture per month was evaluated at following conditions: temperature for growth of 27±1°C during the lighted period and 22±1°C during the dark period.

Anisotropic growth is achieved through the additive mechanical effect of material anisotropy and elastic asymmetry

Fast directional growth is a necessity for the young seedling; after germination, it needs to quickly penetrate the soil to begin its autotrophic life. In most dicot plants, this rapid escape is due to the anisotropic elongation of the hypocotyl, the columnar organ between the root and the shoot meristems. Anisotropic growth is common in plant organs and is canonically attributed to cell wall anisotropy produced by oriented cellulose fibers. Recently, a mechanism based on asymmetric pectin-based cell wall elasticity has been proposed. Here we present a harmonizing model for anisotropic growth control in the dark-grown Arabidopsis thaliana hypocotyl: basic anisotropic information is provided by cellulose orientation) and additive anisotropic information is provided by pectin-based elastic asymmetry in the epidermis. We quantitatively show that hypocotyl elongation is anisotropic starting at germination. We present experimental evidence for pectin biochemical differences and wall mechanics providing important growth regulation in the hypocotyl. Lastly, our in silico modelling experiments indicate an additive collaboration between pectin biochemistry and cellulose orientation in promoting anisotropic growth.