Sustainable resource preservation of Santalum species that yield commercially important forest products is needed. This review provides an understanding of their basic biology, propagation, hemi-parasitic nature, reproductive biology, and biotechnology. Many species of the genus Santalum (Santalaceae) have been exploited unremittingly for centuries, resulting in the extinction of one and the threatened status of three other species. This reduction in biodiversity of sandalwood has resulted from the commercial exploitation of its oil-rich fragrant heartwood. In a bid to conserve the remaining germplasm, biotechnology provides a feasible, and effective, means of propagating members of this genus. This review provides a detailed understanding of the biological mechanisms underlying the success or failure of traditional propagation, including a synopsis of the process of hemi-parasitism in S. album, and of the suitability of host plants to sustain the growth of seedlings and plants under forestry production. For the mass production of economically important metabolites, and to improve uniformity of essential oils, the use of clonal material of similar genetic background for cultivation is important. This review summarizes traditional methods of sandalwood production with complementary and more advanced in vitro technologies to provide a basis for researchers, conservationists and industry to implement sustainable programs of research and development for this revered genus.

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Main conclusion Shoot tip necrosis is a physiological condition that negatively impacts the growth and development of in vitro plant shoot cultures across a wide range of species. Abstract Shoot tip necrosis is a physiological condition and disorder that can arise in plantlets or shoots in vitro that results in death of the shoot tip. This condition, which can spread basipetally and affect the emergence of axillary shoots from buds lower down the stem, is due to the cessation of apical dominance. STN can occur at both shoot multiplication and rooting stages. One of the most common factors that cause STN is nutrient deficiency or imbalance. Moreover, the presence or absence of plant growth regulators (auxins or cytokinins) at specific developmental stages may impact STN. The cytokinin to auxin ratio within an in vitro plant can be modified by varying the concentration of cytokinins used in the culture medium. The supply of nutrients to in vitro shoots or plantlets might also affect their hormonal balance, thus modifying the occurrence of STN. High relative humidity within culture vessels and hyperhydricity are associated with STN. An adequate supply of calcium as the divalent cation (Ca2+) can hinder STN by inhibiting the accumulation of phenolic compounds and thus programmed cell death. Moreover, the level of Ca2+ affects auxin transport and ethylene production, and higher ethylene production, which can occur as a result of high relative humidity in or poor ventilation of the in vitro culture vessel, induces STN. High relative humidity can decrease the mobility of Ca2+ within a plant, resulting in Ca2+ deficiency and STN. STN of in vitro shoots or plantlets can be halted or reversed by altering the basal medium, mainly the concentration of Ca2+, adjusting the levels of auxins or cytokinins, or modifying culture conditions. This review examines the literature related to STN, seeks to discover the associated factors and relations between them, proposes practical solutions, and attempts to better understand the mechanism(s) underlying this condition in vitro.

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Sweet potato (Ipomoea batatas [L.] Lam.) is an important food crop that is cultivated and consumed worldwide, and its nutritional value makes it especially important in developing countries. However, sweet potato plants produced using conventional vegetative propagation can be prone to several diseases, thereby hindering crop yield. Therefore, micropropagation techniques are useful for producing higher quality plant material. The aim of the present study was to determine the optimal medium formulation for the axillary shoot multiplication of single-node 'Abees' explants. The influences of cytokinins (6-benzyladenine [BA], kinetin, thidiazuron, and zeatin), gibberellic acid, phenylacetic acid, indole-3-butyric acid, silver nitrate, and oxalic acid, culture duration, and medium salt strength were investigated. Optimal proliferation was observed when shoots were cultured on half-strength MS medium that was supplemented with 2 mg L-1 BA and 100 mg L-1 oxalic acid for 2 wk, followed by culture on half-strength MS medium without plant growth regulators for 4 wk. In this medium, the greatest number of shoots (3.1) and total number of nodes (16.1) per explant were observed. All (100%) of the regenerated shoots were rooted and acclimatized, with a survival rate of 96% when transferred to ex vitro conditions and no morphological abnormalities.

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Vitex is a large genus consisting of 230 species of trees and shrubs with multiple (ornamental, ethnobotanic and pharmacological) uses. Despite this, micropropagation has only been used to effectively propagate and preserve germplasm a limited number (six) of Vitex species (V. agnus-castus, V. doniana, V. glabrata, V. negundo, V. rotundifolia, V. trifolia). This review on Vitex provides details of published micropropagation protocols and perspectives on their application to germplasm preservation and in vitro conservation. Such details serve as a practically useful user manual for Vitex researchers. The importance of micropropagation and its application to synthetic seed production, in vitro flowering, production of secondary metabolites, and the use of molecular markers to detect somaclonal variation in vitro, are also highlighted.

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