Friday, December 12, 2014

On how plants keep Time.

Sun is THE MASTER CLOCK  for all  earthly beings, no doubt about that. (Of course having a clock and being punctual are two entirely different stories, we won't get into that now). We take a brief look at how living beings carry out their cellular functions punctually.  Mammals have a built-in super synchronizer  suprachiasmatic nucleus, (SCN) which coordinates and maintains a standard time for all cellular functions and set in motion the circadian rhythmHow do plants keep time? They directly receive their daily bread (via photosynsthesis) from the sun. But  apart from that   plants also have  short term and long term ( daily and seasonal ) rhythms. Some  flowers bloom with the early rays of Sun while others are literally  late bloomers; there are even those which spread their fragrance exclusively during the night. Mangoes and litchies arrive only in summer, while ripe oranges flood the markets  winter . So the question is do plants have a synchronizer? Or in other words, what is the SCN equivalent  in plants.?  Till now the general assumption has been that plants do have several built in  clocks but they all keep their own local time and are not exactly   synchronized. Sounds  a bit chaotic, but then there were no evidence to prove otherwise. And without proof scientists don't take decide one way or the other. 

20th November issue of Nature carries an interesting report by Endo et al  on the circadian clocks operative in plants. Endo and team proceeded to take a closer look at the  plant Arabidopsis thaliana. With short life span and  a  small genome (135mbp), arabidopsis thaliana is roughly  the vegetarian  equivalent of the guinea pig  in the lab. 
  

Endo et al sought answers at  molecular and genetic level on the circadian rhythms in arabidopsis thaliana.   The team  separately analysed three main cell lines in the leaf:  the epidermal cells on the surface ,  the   mesophyll cells underneath , which run  the photosynthesis factory and then the   vasculature cells  responsible for the plumbing system. This is important because, even when the solar factory is closed for the day, the plumbing system continues to do the logistics work  Endo and team  monitored the gene expression and  level of specific marker proteins  in vivo and also   in vitro for  these three types of tissues. It was  found that the mesophyll and vasculature clocks function independently but are not altogether uncoupled. Plants have two loops of cellular activity, the day loop and the night loop. The genes responsible for rhythmic behaviour in the mesophyll tissue are active during day time and those in the vasculature  tissues  report for duty during the evening hours. The day partollers must be leaving telltale signs for the night patrollers. The team also found that the vasculature clock has the upper hand and controls the physiological responses of the  plant as a whole. They suggest that the vasculature and mesophyll clocks are akin to the central and peripheral clocks in mammals  or evening and morning cells in drosophila
Tailpiece  
Acharya Jagadish Chandra Bose was one of the first to suggest  that plants have life and that they responded to various stimuli including light and dark cycles through  electrical impulses. In 1902  Bose compiled his  experiments, observations and conclusions   and published   Response in the living and non-living . This book  (currently  available  an e-book  in public domain, thanks to the Project Gutenberg),  carries complete details of the simple,  elegant and extensive  studies.
 References:

1. Response in the Living and the non-Living : J.C. Bose
2. Time specific clocks in Arabidopsis show assymmetric coupling.  Endo et al Nature,       20th Nov. 2014 p419-422.
3. Spontaneous spaciotemporal waves of gene expression from biological clocks in the leaf : Wenden B et al  Proc. Natl. Acad. Sci.(USA),109, 6757-6762 (2012)
4. The circadian clock in Arabidopsis is a simplified slave version of the clock in shoots.James, A.B. et al  Science 322, 1832-1835(2008)