Alan C Beverly
The reproduction of Aloe polyphylla (A.p.) the spiral aloe has confounded many nurserymen because in the genus Aloe, asexual reproduction is only possible with stem cuttings. Leaf have no meristem cells to develop a new plant. The acaulescent habit offers no help, and the occasional offshoot is not enough to satisfy a market need for thousands. The species is self incompatible, with dichogamous anthers and stigmas maturing differently in time and space. There is also a biochemical incompatibility at the stigmatic surface, guaranteeing that only pollen from another genotype will germinate on the ripe stigma and fertilize the ovum below. This is a strategy of reproduction which favors hybrids, a somewhat risky way for any species to proceed,but which develops each new generation with genetic recombinants with differing abilities to survive. I have successfully created viable hybrid seeds of A.p. for the last 25 years and grown these hybrids into adult plants. There are many "gates" to open before even a few hundred plants can be marketed. Adult plants only flower at about 90 leaf,and then may not repeat flower for years.
The nursery industry has for many years utilized tissue culture science in their quest to produce desirable forms and varieties for ornamental and agricultural commodities. The science is well developed, but the results can be variable. In the majority of species, plantlets may require more development time but yield the desired product without any long term downside. In the last decade Abrie & van Staden (1) published their method for micropropagation of A.p. Soon nurserymen were in possession of lining out stock. This article reviews new discoveries about 7 year old A.p. clones' phyllotaxic abnormalities, biologists' knowledge and efforts to further understand how plants build leaves and the reasons why hybrid seed grown plants are better performers.
Clones obtained by a local grower, Robin Stockwell, were cultivated for several years and planted out in the landscape in a sandy, well drained soil. The plants are about 15" diameter and at the 75-90 leaf stage. With 5 rows of sequenced leaf oriented in either a Right hand or Left hand spiral a hybrid seedling plant will show a striking 5 row spiral at this stage of growth, about half the size of an adult plant (photo A). The clones reveal abnormalities in leaf sequence which are difficult to describe, and are disappointing to the the eye (photo B,C,D).
Photo A: A hybrid seedling Right adult plant showing 5 row spiral of correctly sequenced leaf.This plant has few marginal teeth on leaf, a primary variable.
Photo B: Clone side view shows confused phyllotaxy, leaf sequence.
Photo C: above, and D clones with abnormal leaf sequence
These plants have not been able to form a perfect 5 row spiral leaf sequence because they lack a key element in the apical meristem cell, which must divide in a specific way in space and time to produce the correct spiral sequence of leaf. I have seen adult plants in the wild and in my own nursery grow a second meristem and proceed to divide over 2-3 years, and some plants may even change the growth spiral from R to L or L to R. There is no genetic prescription which locks any individual plant into a R or L spiral. These plants look strange for about 2-3 years, the life cycle of any given leaf which arises in the center and gradually moves to the outside row position to find itself appressed to the soil. A shaded leaf in this position is of no use to the plant,so it is abandoned by resorption of the "goo". The paper thin skeleton may then be pulled off. The developmental dynamics of the apical meristem cell are fickle and vulnerable to perturbations in the route towards a new plant, and in the life of a hybrid plant. I've never seen such abnormalities of leaf sequence in any wild population I studied nor in any plant in my nursery.
Spiral phyllotaxy is common in the plant kingdom. There are animal species which have found the same biophysical solution to packing a maxi- mum number of units into a limited space. There is a mathematical model which describes this, the Fibonacci series. Other species may therefore be vulnerable to perturbation of apical meristem function if cloned. No species other than A.p. has demonstrated such a striking deviation potential. The explanation lies in the embryogeny of a newly fertilized egg cell deep in the ovule, and the exacting sequence of cell division required to produce the 5 row spiral of leaf. Siobhan and Kuhlemeier (2) reviewed how plants develop leaf and have offered insight into the A.p. clone dilemma. The first cell division of a newly fertilized plant egg cell is to establish polarity of future shoot/root cells. Then a sequence of cell divisions establishes the proto-tissue of each,and they become organized in space and time. A plantlet from tissue culture does not have any such early directive to develop. This may be why clones have longer development time. That they eventually grow into functional plants is amazing, and is one of the greatest unsolved mysteries of biology.
"How this early embryo forms itself into an organized entity has long been considered by many to be taxonomically significant" (Lersten 2007) (3)
Many researchers have wondered how to discover the answers but there is little to report. We know that genes are a blueprint for synthesis of specific proteins, but do not act as director of organogenesis. This is achieved by macromolecular self assembly from the "bottom up" rather from the "top down" . This gap in embryology is so profound that Dawkins (4) has underscored it in a discussion of how organogenesis occurs. In plant cells, Siobhan and Kuhlemeier discuss gradients of hormone signals in a ratio to accomplish specific cell function and leaf development. Other researchers have focused on identifying genetic mutants (5)(6) to describe malformed flower and leaf tissue using Arabidopsis.
Whatever plant biologists decide regarding organogenesis I am sure that A.p. hybrid seeds have an advantage over clones; a preformed embryo with tissues organized to produce a correct leaf sequence faster and without any mistakes. I pledge to continue the effort to produce genetic recombinants to help save this wonderful endangered species. Here's two pictures of plants which illustrate one primary variant of leaf morphology, the number of marginal teeth, and a one of a kind partial variegation.
Photo E. An adult Right spiral with many marginal teeth.
Photo F : An adult Left spiral with partial variegated leaf.