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OBITUARIES
Tsvi
Sachs
With great sadness
we report that Tsvi Sachs passed away on January 9, 2007. Tsvi was professor
emeritus at the Hebrew University of Jerusalem and a world-class plant
biologist who had a huge impact on our understanding of the origin of
cellular patterns in plants. His creative experimentation and thoughtful
comments will be missed by many.
Born in 1936, Tsvi
grew up in a rural area near Tel Aviv. For his PhD, he moved to Harvard
University, where he graduated in 1965 with a thesis on mechanisms of
apical dominance under the supervision of Kenneth Thimann. Between 1966
and 1979 he was successively promoted from lecturer to full professor
at the Hebrew University in Jerusalem, but he found time for study abroad
at Yale University in 1973 and at the Medical Research Council in Cambridge,
England, in 1979. In 1986 he became Otto Warburg Professor of Botany
at the Hebrew University in Jerusalem. Tsvi received numerous awards for
his scientific work, such as the Jeanne Siron Pelton award from the Botanical
Society of America, but a primary emphasis of his work involved his various
activities to promote teaching. A dedicated and much-esteemed teacher
himself, he became a member of the National Committee for High School
Education in Israel (1985) and was chair of the Botany Department (1991–1995)
and Hebrew University (2000–2001) teaching committees.
Tsvi was a unique
scientist in both achievement and style. The words that characterize his
style best might be independence of thought. He was perfectly immune
to trends and fashions in science. Instead, he preferred to apply his
very abstract way of reasoning to diverse subjects of his personal choice,
each of which he then pursued over many years. One such subject was the
generation of plant tissue patterns and their relationship to the acquisition
of apical–basal cell polarity. What is often briefly tagged the auxin
canalization hypothesis is part of a larger context of observations,
which he published in a detailed paper in Advances in Botany in
1981. Here, and in many individual research contributions and later reviews
and two books, he expressed his opinion that plant cells are distinguished
with regard to the degree of their polarization and that this feature
can then feed back on the three-dimensional pattern of certain tissues,
most evidently in the pattern of vascular strands. In his studies on vascular
patterning in stems and leaves, he was adept at using surgical techniques
to alter the pathway of hormone and growth regulator flow through developing
tissues. These studies, which provided stunning examples of his ideas,
were both elegant and cost efficient, matching his preferred working style
with small groups of committed students. His experimental results revealed
that polar auxin fluxes were primary determinants of vascular strand initiation
and continuity, and also demonstrated how vascular networks formed in
developing organs.
In other studies at
about the same time, Tsvi examined patterning of stomata on leaf surfaces,
again a system that could be readily examined microscopically. In experimental
and theoretical analyses, he showed that there was a definite space around
each stoma from which new stomata were excluded. These studies on vascular
and stomatal patterns carried out in the 1970s and 1980s are still cited
extensively in research publications and formed the basis of his book
Pattern Formation in Plant Tissues (Cambridge, England: Cambridge
University Press, 1991).
Tsvi was well aware
that the concepts he developed from his early studies on vascular regeneration
applied to plant pattern formation in general. For his abstract way of
reasoning, the molecular nature of the flowing signal was not at all critical,
and he correctly described it as “experimentally not separable from
auxin.” Molecular geneticists are still in a somewhat similar situation,
except that they prefer concrete substances over abstract principles and
have finally reached an agreement to talk about auxin as the polarizing
substance. In recent years, detailed knowledge about auxin signal transduction
and transport has been linked to patterning processes and has convinced
many researchers that self-regulated signal flux could underlie patterning
in the most diverse locations in plants. Ten years ago or so, many researchers
excluded roles of plant hormones in the control of these processes, while
there is now an expanding field of mathematical modeling of hormone-based
self-regulatory mechanisms. This shift in perception demonstrates the
pioneering nature of Tsvi’s thoughts. In their mathematical description
by Mitchison, Tsvi’s postulated feedback mechanisms took the form
of an early systems biology description of plant biological patterning.
Flexibility and self-organization
are hallmarks of plant development as a whole and are reiterated at all
levels of plant organization. Of all these levels, Tsvi became fascinated
by the plasticity of plant architecture—for example, the arrangement
of branches on the trunk of a tree—which can be correlated to obvious
external influences such as weather conditions or human activity. Tsvi
applied the same basic principles that explained the variable orientation
and reproducible continuity of vascular tissues to better understand the
mechanism underlying the inherent plasticity of plant development. A growing
branch can be seen as a source of auxin that, through its orienting action
on the vascular tissues that connect the branch with the roots, diverts
substrates and signals toward itself and away from competing branches,
thus inhibiting their development. Further, if increased light conditions
enhance auxin supply, competition of different branches for the best environmental
space and for limiting nutrient supply from the roots could be integrated
by the same signal. Plant architecture would thus be shaped by a self-organizing
process in which branches inhibit or compete with one another.
Just as among individuals
in Darwinian selection, this “developmental selection” among
plant organs is based on environmentally influenced selection. As in the
case of vascular patterning, the final balanced state would still be constrained
by species-specific controls, but within this frame, developmental selection
could optimize adaptation. An interesting aspect of this model is that
the required interactions could be carried out by a single central signal,
and auxin would again be the most likely candidate. As a long-range coordinating
signal (on top of its role in local patterning), auxin could integrate
information from different sources, which in turn could be differentially
interpreted by response tissues. Although the idea of auxin as a messenger
in many processes did not immediately appear attractive, results from
recent studies suggest that the complexity of auxin signaling processes
could originate from tissue-specific signal interpretation rather than
from a diversity of signal substances.
Although Tsvi was
a quiet person, not inclined to dominate any discussion, he influenced
many people through the originality of his ideas over the decades. Once
approached, he usually offered lots of fresh thoughts and original insights.
He was very generous in sharing his time and thoughts on subjects of common
interest and was actively engaged in productive exchanges with molecular,
cell, and systems biologists as well as ecologists working on plant patterning.
Tsvi cared a lot for the people around him and was highly dedicated to
his family, students, and colleagues. His wife Laura and some of his former
students contributed to many of his manuscripts, and his biography documents
a strong commitment to the improvement of teaching at the university.
Tsvi was active beyond
retirement; most recently, he worked on a manuscript on leaf venation
patterning, a book on morphogenic implications of plant hormone signals,
and a popular book on botanical phenomena.
It is regrettable
that Tsvi will not be with us to witness the dramatic progress in the
analysis of the processes he was so influential in envisioning and unraveling.
The currently thriving molecular genetic analyses of plant patterning
owe a lot to the guiding influence of his combined experimental and theoretical
insights.
Thomas Berleth
University of Toronto
Enrico Scarpella
University of Alberta
Ian
Sussex
Yale University
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