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Lateral organs are formed in plants by post embryonic developmental programs. Leaves, and flowers differentiate from the shoot apical meristem and lateral roots from the primary root pericycle meristem. Adventitious roots are roots formed from non-root lateral meristematic tissues, mostly the cambium, in many cases in response to stress signals. The ability of plants to regenerate adventitious roots is fundamental for selection and breading programs which are based on vegetative propagation of elite clones. Thus, recalcitrant plants, losing their rooting capability, may form a genuine commercial barrier in agricultural and forestry improvement programs. Some cellular mechanisms underlying adventitious root formation have been revealed, but much is yet to be clarified. The plant primary cell wall is a dynamic organ that can change its form, and perceive and relay molecular signals inward and outward during certain stages of development in particular cells. Therefore, before the secondary cell wall is deposited and plants become the wood from which walls and furniture are built, and the fibers from which cloths are woven, primary cell walls actively participate in plant cell differentiation and developmental programs. While auxin is a major regulator, cell walls are important in regulating coherent formative cell division and synchronized polar elongation of cell lineages that are necessary for lateral organ induction and formation, and collaborative cell functioning. Nevertheless, little is known of how cell wall changes are molecularly sensed and translated to intracellular signals during differentiation of adventitious roots. Here we summarize recent data linking, directly or indirectly, cell wall events to auxin signaling and to lateral or adventitious root induction and formation.

In: Israel Journal of Plant Sciences

Summer and autumn in Israel are highly arid with not enough plants in bloom offering nectar and pollen to support the local apiary. This leads to decline in colony health and honey production. To increase food sources for honeybees, we initiated a project to clone elite Eucalyptus trees exhibiting constant and rich blooming from late summer to early winter. We induced adventitious roots from cuttings of two mature Eucalyptus trees of which nectar production and honeybees’ attraction was measured: Eucalyptus brachyphylla and Eucalyptus x trabutii. During the rooting process, a high frequency of cylindrical callus formation instead of roots was obtained. To shed light on the inner anatomy of the callus chunks, we compared their cell organization and cell-wall composition to those of roots. Whereas in the root, cells were organized in circumferential symmetry, no symmetry was found in the callus. Instead, a more chaotic accumulation of meristematic-like cells with sporadic clusters of tracheary elements laid in different directions were observed. The outer cell layer of the callus often included swollen cells with thin cell walls. Most callus cells stained more strongly for cellulose and lignin than cells in the root meristem. In addition, specific antibodies to methylesterified and de-methylesterified pectin showed differential staining of callus vs. root cells indicating cell wall differences. Strikingly, roots were seen to differentiate from the chaotic cell organization of the callus, albeit at low rates. Further investigation of the cellular and molecular mechanisms underlying callus formation, are required.

In: Israel Journal of Plant Sciences

Forest trees possess high genetic diversity and high heterozygosity which allow adaptation to changing environmental conditions. There is a tendency to propagate successful and unique genotypes, which are identified at their mature stage in the forests, for future improvement programs and conservation purposes. However, vegetative propagation of mature forest trees is still a challenge in many conifers. In this study, we focused on improving the rooting of cuttings of mature and old Pinus halepensis and its hybrids. We observed that storage of cuttings before rooting at 4°C for 4 weeks and prolong immersion of cuttings in a solution containing 400 mg/l of indole-3-butyric acid, 5 mg/l of the auxin conjugate 2-(2,4-dichlorophenoxy)propanoic acid-glycine methyl ester, and 0.01% of Amistar fungicide significantly improved rooting of mature cuttings. The active ingredient in Amistar is azoxystrobin, an uncoupler of respiration, which seems to directly promote rooting. Rooted cuttings of selected clones demonstrated unique and uniform growth performance, most likely delivering the intrinsic growth parameters of the mother trees. It was also observed that trees growing under drought stress possess improved rooting ability. By using rooted cuttings, it will be possible to study the relationship between growth rate and adaptation to semi-arid climate conditions. The ability to clonal propagate mature and old P. halepensis trees not only enables vegetative propagation of elite trees for improvement programs, but also provides an opportunity to preserve unique naturally occurring old P. halepensis genotypes.

In: Israel Journal of Plant Sciences