Plant & Cell Biology
by Rupert Sheldrake
Research in Developmental Biology and Plant Physiology - (1964-1985)
From 1964 to 1974, I worked in the Department of Biochemistry at the University of Cambridge, at first as a graduate student, then as a Research Fellow of Clare College Cambridge and as a Research Fellow of the Royal Society. In 1968, and again in 1971, I did research on tropical plants in Malaysia, based in the Botany Department of the University of Malaya, Kuala Lumpur, and also at the Rubber Research Institute of Malaya. From 1974 to 1985, I worked at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) in Hyderabad, India, on the physiology of tropical legume crops, first as Principal Plant Physiologist, and later as Consultant Physiologist.
Hormone production in plants
The first problem I investigated in my research career was how the hormone auxin (indole-3-acetic acid) is produced in plants.
See Hormone Production in Plants
When I started research on this subject, nobody knew how it was made, and years of efforts had failed to clarify the situation. As I reflected on the biochemical mechanisms by which auxin can be produced I realised that it could be a non-specific breakdown product of the amino-acid tryptophan and that it was likely to be produced in dying cells, as proteins break down, releasing tryptophan. When I started this research, in the 1960’s, little attention was paid to dying cells in plants or animals. Nevertheless programmed cell death – also known as apoptosis - is now a very fashionable topic of research in cell biology.
I showed dying cells could produce auxin as a by-product of their autolysis or self-digestion.
See
The Production of Auxin by Autolysing Tissues
There was nothing specific about the way it was produced in plants. Auxin was also produced, for example, by autolysing yeast cells, and also by autolysing rat liver. Many of the places in which auxin is known to be produced in plants are places where cells die, for example in germinating seeds, as storage tissues breakdown. Auxin is known to be produced in developing leaves and buds, and its formation is roughly proportionate to the development of veins within the leaf. Veins contain xylem or wood cells, and when wood cells develop, the walls thicken up and the cell contains break down and are dissolved away, so cell death occurs in all young growing tissues. Perhaps the differentiating xylem cells were a source of auxin, released as they died.
I studied this question in stems in which new xylem cells were being formed as a result of cambial activity, and found that these thickening stems did indeed produce auxin.
See The Production of Auxin by Tobacco Internode Tissues
I also looked at auxin production in senescent leaves. As they go yellow, the cells breakdown and sure enough I found the auxin levels increased dramatically.
See Production of Auxin by Detached Leaves
There was only one supposed site of auxin production in plants in which dying cells were not present, namely the tips of coleoptiles in serial seedlings. These sheathing structures around the seedling shoot were some of the first organs in which auxin was studied and were of particular importance in the classical literature on auxin production. However, although auxin was present in coleoptile tips, I found there was no persuasive evidence that it was made there, and found that in fact it was probably accumulating there having been carried up from the seed in the sap.
See Do Coleoptile Tips Produce Auxin?
The formation of auxin in developing xylem cells in the trunks of trees as they grow would mean that a gradient of auxin would be set up across the cambium, the region of dividing cells that separates the wood from the bark. I directly measured auxin levels in the xylem cambium and young phloem cells, from the inside of the bark and showed that there was indeed such a gradient. This was one of the first chemical gradients to be characterised in either animals or plants of a chemical known to have morphogenetic effects.
See Auxin in the Cambium and its Differentiating Derivatives
Since dying cells produces auxin, and since dying cells occur within all higher plants as a result of xylem differentiation this raised an evolutionary question. Had the responsiveness of plants to this cell-breakdown product, acting as a chemical signal of cell death, evolved only after cell death became an integral part of plant grown with the evolution of a vascular system? Or have plants already become sensitive to auxin before the vascular system evolved? In fact it was already none that non-vascular land plants, like mosses and liverworts are sensitive to low concentrations of auxin in the environment. They react by producing root hairs, or rhizoids. This sensitivity had developed in response to dying cells, it would enable mosses and liverworts to produce rhizoids which increase the surface area for absorption of nutrients, in places where there was decaying organic matter, in other words when nutrients were most likely to be abundant. Is auxin really present in such situations? I examined the humus on which mosses and liverworts were growing both in the tropics and in temperate countries and found that it did in fact contain auxin in quantities sufficient to produce rhizoid formation. This suggested an evolutionary origin for the auxin responses in higher plants. First, plants evolved sensitivity to auxin as a signal of organic decay in the external environment. Later, as cell death became an integral part of plant growth the evolution of the vascular system, this hormonal-response system became internalised and auxin evolved the wide range of signalling roles that it has today.
See The Occurrence and Significance of Auxin in the Substrata of Bryophytes
At the end of my time at Cambridge, I published a comprehensive review of research on production of auxin and other hormones in plants, summarising the dying-cell hypothesis.
See The Production of Hormones in Higher Plants
Cell differentiation in plants
As cells in plants turn into wood cells, called xylem cells, they thicken up their walls, and then the cell contents die and dissolve. The developing xylem cells also dissolve away their end walls, so that the dead, empty cells form tiny tubes through which the sap flow from the roots to the shoots. It seemed to me very likely that as new xylem cells formed and became part of the water-conducting system, the contents of the self-digested cells would be flushed away with the sap, and be carried upwards in it. I analysed the sap from several species of plants to see if it did contain breakdown products and enzymes of the kind likely to be involved in the autolysis, or self-digestion, of the differentiating xylem cells, and found that indeed it did.
See Some Constituents of Xylem Sap and their Possible Relationship to Xylem Differentiation
The Complete Collection
Rupert's papers on Research in Developmental Biology and Plant Physiology are collected as follows:
Hormone Production in Plants
Auxin Transport in Plants
Cell Differentiation
The Ageing and Death of Cells
Crop Physiology
Top of Page
