Yes, plants react to touch and not only adjust their biochemical reactions but also adapt their size, shape and safety.
Of the five senses of touch (tactile), smell (olfactory), sight (visual), taste (gustatory) and hearing (aural), what are the ones that plants possess or respond to? This question has attracted scientists and gardeners alike. Of course, plants are masters in the art of capturing light and using it to grow and to make food. When it comes to their response to sounds (hearing), amateur gardeners swear ‘by personal experience’ that plants respond to your talking to them or to music. Scientists dismiss this as romantic and not real. But do plants respond and react to smells or to taste? This question is wide open and science is yet to provide definite answers.
But the response and reaction of plants to touch is an issue that is becoming clearer. Yes, plants react to touch and not only adjust their biochemical reactions but also adapt their size, shape and safety. This response is referred in scientific literature by the tongue-twister thigmorphogenesis. Trees subjected to winds tend to reduce their length and thicken their trunks. Plants respond to touch — the famous examples are the shy and reclusive “touch-me-not” (mimosa pudica) and the rogue meat eater ‘Venus fly trap’ (dionaea muscipula), but beside these, just every plant responds to touch or vibrations — be if from rainfall, wind or touch. And the model plant that plant scientists prefer to use is Arabidopsis thaliana (a member of the mustard family with a six-week life cycle, prolific in seeds, easily cultivated in pots, with plenty of mutants, and whose genomic DNA has been sequenced; essentially the botanist’s e. coli or drosophila the fruitfly). When its leaves are gently cradled back and forth during its developmental or growth periods, it tends to flower late, shorten its stem and produce shorter petioles.
The research group of Dr Janet Braam of Rice University in Houston, Texas, U.S., has been studying the mechanism of the thigmorphogenesis of arabidopsis for quite some time. In their latest paper (Current Biology, 22, 701–706, 2012), they show that much of this is effected by the molecule called jasmonate. As the name reveals, jasmonate is abundant in the oil obtained from the jasmine plant. Besides its pleasant smell, jasmonate is found to be a plant hormone that triggers a whole host of processes in the biochemistry, cell biology and defence (or protection, immunity if you will) of the plant. In the above paper, the Braam group shows that jasmonate protects the plant against pests.
The experiments involved gently touching and rocking the plant for a while. The more they touched the plants in the experiments, the more jasmonate the plant produced. The plant itself delayed flowering, with shorter flower stems and leaf clusters. These results suggested that when a pest lands on the plant in order to eat it, the “host” plant reacts to resist or defends itself by jasmonate — induced thigmophogenes. The plant hormone sets off to turn on a whole host of genes in the plant that not only adjust its growth pattern but also its resistance to pest attack. This latter effect became clear when they tested the susceptibility of a fungus invader that they introduced from outside. Plants that were touched often showed higher resistance to the fungus and to another pest called cabbage looper.
Braam concludes saying that mechanically stimulated plants may by primed for defence and have greater resistance to plant invaders. “Perhaps wind, a critical mechanism for fungal spore dispersal, prepares plants for potential infection, and the mechanical perturbation caused by alighting insects or passing larger animals triggers jasmonate production to activate anti-herbivore defences in case the interaction becomes an attack. Despite the apparent quiescence of their life style, plants are well- equipped to mount defences to withstand the often violent environments in which they live” write the authors in their above Current Biology paper.
Enthusiasts will of course ask whether visual and tactile senses are the only two senses that plants possess or respond to; and some may even claim that they respond to sounds. Recall the report in the April 12, 2012 issue of The Hindu (http://www.thehindu.com/sci-tech/article3304060.ece), which refers to the work by Dr. Monica Gagliano and others from the University of Western Australia. These researchers established that young roots of corn made regular clicking sounds. They also found that young corn roots suspended in water leaned toward the source of a continuous sound emitted in the region of 220Hz, which is within the frequency range that the same roots emitted themselves (called plant bioacoustics, this paper was published in Trends Plant Sci. 2012 Jun;17: 323-5).
Don’t rush in immediately to state that we knew this all along, our musicians have done this, and have even chosen specific ragams to elicit chosen responses. There is a lot of stuff of this kind on the web — that needs to be proven by rigorous study, analysis of the mechanisms, independent repeatability of the experiments and results, and other scientific studies. Until then, the jury is out!