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K. daigremontiana as a Model Plant for the Study of Auxin Effects in Plant Morphology
A large number of studies have been conducted in order to understand the mechanisms by which shoots, plantlets and vegetative propagules are produced. According to the first studies reported by Yarbrough [2,3], in the fern species Camptosorus rhizophyllus (which has two different kinds of leaves), shoots are produced at the tip of long acuminate leaves, while in the species Tolmiea menziesii shoots are originated in a notch near the junction of the petiole and the leaf blade; in both cases, the shoots are produced from meristematic tissue and once they are mature are naturally detached from the leaf and fall to the ground originating a new plant. Also, in the orchid Malaxis paludosa, cells at the tip of mature leaves have an intense meristematic activity which originates embryo-like structures that once the mature leaf is wilted the small shoot detaches originating a new plantlet [4]. This phenomenon has been widely studied in plants of the Crassulaceae family, mainly in the genus Kalanchoë. According to the capacity to asexually produce such plants, species of this genus have been classified in four categories: a) plants which do not produce plantlets (K. marmorata, K. rhombopilosa, K. tomentosa and K. thyrsiflora), b) plants that spontaneously produce plantlets (K. daigremontiana), c) plants that produce plantlets by the action of an environmental stress (K. pinnata, K. fedstchenkoi, K. strepthantha, K prolifera and K. crenata) and d) plants that produce spontaneously plantlets by the action of stress and/or (K. gastonis-bonnieri) [5]. According to studies conducted by Batygina et al. [6] in the species Kalanchoë daigremontiana (formerly called Bryophyllum daigremontianum) and Kalanchoë pinnata (formerly called Bryophyllum calycinum) the development of the propagules is through somatic embryogenesis where the globular, heart and torpedo stages can be recognized as they occur in zygotic embryos, however, González-Hernández [7] found that the plantlets produced in the leaf margins of K. daigremontiana are produced by two mechanisms: organogenic, in which the vascular tissue of the shoot is linked to the vascular strands of the leaf, and somatic embryogenesis where no vascular connections are present between the embryo and the mother leaf (Figure1). Moreover, molecular studies suggest that both organogenesis and embryogenesis programs are involved in plantlet formation in the leaf notches of this species [8].
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