Methods for in situ quantitative root biology

When dealing with plant roots, a multi-scale description of the functional root structure is needed. Since the beginning of XXI century, new devices like laser confocal microscopes have been accessible for coarse root structure measurements, including 3D reconstruction. Most re-searchers are familiar with using simple 2D geometry visualization that does not allow quantitatively determination of key morphological features from an organ-like perspective. We provide here a detailed description of the quantitative methods available for three-dimensional (3D) analysis of root features at single cell resolution, including root asymmetry, lateral root analysis, xylem and phloem structure, cell cycle kinetics, and chromatin determination. Quantitative maps of the distal and proximal root meristems are shown for different species, including Arabidopsis thaliana, Nicotiana tabacum and Medicago sativa. A 3D analysis of the primary root tip showed divergence in chromatin organization and cell volume distribution between cell types and precisely mapped root zonation for each cell file. Detailed protocols are also provided. Possible pitfalls in the usage of the marker lines are discussed. Therefore, researchers who need to improve their quantitative root biology portfolio can use them as a reference.

Intra-seasonal trends in phloem traits in Pinus spp. from drought-prone environments

Recent studies on the seasonal dynamics of secondary tissue formation in Mediterranean trees have shown that xylogenesis depends on species and site conditions, but many questions still remain open. On the other side of the cambium, even less information is available about phloem structure and timing of its formation. We analysed intra-annual phloem variation in width and cell traits in the conducting, non-collapsed phloem (CPH) of Pinus pinea and Pinus halepensis at Mediterranean sites in southern Italy and Spain. In all investigated trees, it was possible to differentiate among the non-conducting, collapsed phloem (NCPH), and the CPH. CPH showed no evident annual growth layers; no differences in radial dimensions of early- and late phloem sieve cells, and no cyclic patterns of axial parenchyma distribution. Since it was not possible to study the seasonality of the phloem growth, we analysed the entire CPH. CPH width showed seasonal fluctuations and was generally the widest during the maximum cambial activity and narrowest during summer and winter. The radial size of newly formed sieve cells varied in relation to seasonal dynamics of cambial activity and fluctuations in local weather conditions. The number of axial parenchyma cells in CPH increased during the summer. The observed intra-annual variations in CPH width and structure seemed to be correlated with seasonal weather conditions in order to ensure a sufficient amount of conducting phloem tissue for translocation of photosynthates and signalling molecules to the actively growing tissues along the stem of a tree growing in the harsh Mediterranean conditions.

Scaling of phloem structure and optimality of photoassimilate transport in conifer needles

The phloem vascular system facilitates transport of energy-rich sugar and signalling molecules in plants, thus permitting long-range communication within the organism and growth of non-photosynthesizing organs such as roots and fruits. The flow is driven by osmotic pressure, generated by differences in sugar concentration between distal parts of the plant. The phloem is an intricate distribution system, and many questions about its regulation and structural diversity remain unanswered. Here, we investigate the phloem structure in the simplest possible geometry: a linear leaf, found, for example, in the needles of conifer trees. We measure the phloem structure in four tree species representing a diverse set of habitats and needle sizes, from 1 ( Picea omorika ) to 35 cm ( Pinus palustris ). We show that the phloem shares common traits across these four species and find that the size of its conductive elements obeys a power law. We present a minimal model that accounts for these common traits and takes into account the transport strategy and natural constraints. This minimal model predicts a power law phloem distribution consistent with transport energy minimization, suggesting that energetics are more important than translocation speed at the leaf level.

Vegetative remains of the Magnoliaceae from the Princeton chert (Middle Eocene) of British Columbia

One wood block and many small twigs (up to 1.3 cm diam.) with little secondary growth and showing magnoliaceous characters were identified from the Princeton chert locality (Middle Eocene) of British Columbia, Canada. Specimens were studied with a modified cellulose acetate peel technique and hydrofluoric acid. Well-preserved primary tissues include a chambered pith that distinguishes these twigs from other woods in the chert. Secondary xylem has solitary vessels, radial multiples, and clusters, scalariform perforation plates with 8–27 bars, scalariform, transitional, and opposite intervascular pitting, and tyloses. Imperforate tracheary elements with circular bordered pits, heterocellular and homocellular rays, and marginal parenchyma characterize the twigs. Secondary phloem has dilated rays, alternating bands of fibers and thin-walled cells, and sclerified ray and axial cells. In older wood, opposite intervascular pitting and homocellular rays, suggest affinities with Liriodendron L.; however, the presence of opposite, scalariform, and transitional intervascular pitting and secondary phloem structure necessitate its inclusion in Liriodendroxylon Prakash et al. Liriodendroxylon princetonensis Cevallos-Ferriz et Stockey sp.nov. is distinguished from other species in this genus by the presence of homocellular rays, scalariform intervascular pitting, and well-preserved extraxylary tissues that are unknown for the other fossil species. Liriodendroxylon princetonensis adds to the diversity of extinct magnoliaceous plants during the Eocene and represents the oldest known species of this genus. These plants were probably part of the surrounding forest vegetation in the Princeton basin. Like most extant Magnoliales, L. princetonensis probably lived under subtropical to warm-temperate, moist conditions. Key words: Magnoliaceae, Liriodendroxylon, fossil woods, Eocene.