Box 1.

Prelude to the story of phytochrome, essentially in SCM’s words

“To my mind one of the greatest achievements in botanical science is the isolation of the red-far red absorbing pigment called phytochrome, back in 1959. It is a spellbinding story that began in the 1920s, perhaps even earlier – if besides the work in USA, we also consider some work done in Europe. In my personal view, the discovery of “phytochrome” was deserving of a Nobel Prize (https://www.nobelprize.org/). Nobel Prizes have been awarded to scientists working in photosynthesis, often in Chemistry, or Physics or Physiology or Medicine. However, since Alfred Nobel’s ‘Will’ did not include botany and agriculture, many discoveries have not received the recognition they deserve. (There is, however, the Crafoord Prize, which includes Biosciences: https://www.crafoordprize.se/startsida.)

While the role of light in photosynthesis is well known [see e.g., Shevela et al. (2019)], light has also a very significant role in breaking dormancy in seeds and buds as well as in inducing flowering in plants – and these latter effects are controlled by very weak light [See Björn (2015) for a glimpse of the area of Photobiology]. Plants are now known to have several photoreceptors, but perhaps phytochrome is the most important and distributed in all parts of a plant. Recent work has unraveled the extraordinary mode of its action as a light-induced kinase. Further in its active, FR (far-red) absorbing, form it also enters the nucleus and allows light to turn on key genes. The story of the discovery and isolation of phytochrome is a long one but started with a critical finding by James Bonner, who in [the] 1930s taught Plant Biology at Caltech (California Institute of Technology, Pasadena, California) in USA. Nirmala, my wife, and I spent a year with him, discovering new things about [RNA and] RNA polymerases in plants [see e.g. Nirmala Maheshwari’s paper: Huang et al. (1960)]. Bonner, after finishing his Ph.D. with Kenneth V. Thimann and Fritz Went, decided to work on the mechanism of photoperiodism. In the 1930s, he found (jointly with K.C. Hamner of Chicago) that a very brief exposure to red light in the middle of a long night (otherwise inductive of flowering) completely abolished flowering [See e.g., Hamner and Bonner (1938)].

Photoperiodism in plants was originally discovered at USDA (United States Department of Agriculture) in Beltsville, Maryland, just north of Washington, DC. Usually most fundamental discoveries have come from universities, but many discoveries in plant biology and agriculture have been made at USDA; here, active work was done and a discovery was made that FR (far red) light (given as flashes, but immediately after R (red) light) completely negated the effect of an earlier treatment. By the 1950s, two botanists M.W. Parker and H. Borthwick were leading plant research at USDA. But already around the middle of the 1940s, Borthwick had approached Hendricks, the first Ph.D. student of the great Caltech chemist Linus Pauling, then in USDA’s Soils and Mineral Division [Hendricks was a pioneer in unraveling the X-ray structure of silicates and other soil and clay components]. Hendricks threw himself whole-heartedly into Borthwick’s project. To get an idea of the photoreceptors involved, the two together set out to determine the action spectrum using live plants – often coming back to the lab at night, switching on the light and flashing monochromatic beams through a giant spectrograph that they had specially built, using two prisms (each nearly a foot long) and “throwing” spectrum [light of different wavelengths] that was 8 feet wide on the back of a large hall. Then, Borthwick and Hendricks deduced the existence of a proteinaceous pigment with a chromophore that had two isomeric forms and interconverted with R and FR light. Using a special custom-built difference spectrophotometer and live tissue, they had already demonstrated that pigment in the cells could be converted into a R or FR absorbing forms by beams penetrating such tissues. Everything pointed to the existence of a pigment; however, by the end of an International Botanical Congress, at that time, it still remained to be isolated in a tube or a cuvette! [To get a scientific picture at that time, see Borthwick et al. (1952)]”