Thigmotropism is the bending or turning of an organism due to the  stimulus of touch. Phototropism is the orientation of an organism  with respect to a light source. Thigmotropism is growth in response to touch.

Root hydrotropism and thigmotropism in Arabidopsis thaliana are differentially controlled by redox status ( JournalPlant Signaling & Behavior Volume 12, 2017 - Issue 4)

A BSTRACT : Factors that affect the direction of root growth in response to environmental signals influence crop productivity. We analyzed the root tropic responses of thioredoxin (trxs), thigmotropic (wav2-1), and hydrotropic (ahr1 and nhr1) Arabidopsis thaliana mutants treated with low concentrations of paraquat (PQ), which induces mild oxidative stress, and established a new method for evaluating root waviness (root bending effort, RBE). This method estimates root bending by measuring and summing local curvature over the whole length of the root, regardless of the asymmetry of the wavy pattern under thigmostimulation. In roots of the wav2-1 mutant, but not in those of the trxs and ahr1 mutants, RBE was significantly inhibited under mild oxidative stress. Thigmotropic stimulation of wav2-1 mutant roots, with or without PQ treatment, showed high levels of reactive oxygen species fluorescence, in contrast to roots of the ahr1 mutant. Furthermore, PQ inhibited root growth in all genotypes tested, except in the wav2-1mutant. In a hydrotropism assay of the trxs and wav2-1 mutants, root growth behavior was similar to the wild type with and without PQ, while the root growth of ahr1 and nhr1 mutants was diminished with PQ. These results indicate that hydrotropic and thigmotropic mutants respond differently to exogenous PQ, depending on the tropic stimulus perceived. Therefore, the mechanisms underlying hydrotropism and thigmotropism may differ.

Planta, July 1998, Volume 206, Issue 1, pp 1–6|

The tropic response of plant roots to oxygen: oxytropism in Pisum sativum L.

Abstract: Plant roots are known to orient growth through the soil by gravitropism, hydrotropism, and thigmotropism. Recent observations of plant roots that developed in a microgravity environment in space suggested that plant roots may also orient their growth toward oxygen (oxytropism). Using garden pea (Pisum sativum L. cv. Weibul's Apollo) and an agravitropic mutant (cv. Ageotropum), root oxytropism was studied in the controlled environment of a microrhizotron. A series of channels in the microrhizotron allowed establishment of an oxygen gradient of 0.8 mmol · mol−1 · mm−1. Curvature of seedling roots was determined prior to freezing the roots for subsequent spectrophotometric determinations of alcohol dehydrogenase activity. Oxytropic curvature was observed all along the gradient in both cultivars of pea. The normal gravitropic cultivar showed a maximal curvature of 45° after 48 h, while the agravitropic mutant curved to 90°. In each cultivar, the amount of curvature declined as the oxygen concentration decreased, and was linearly related to the root elongation rate. Since oxytropic curvature occurred in roots exposed to oxygen concentrations that were not low enough to induce the hypoxically responsive protein alcohol dehydrogenase, we suspect that the oxygen sensor associated with oxytropism does not control the induction of hypoxic metabolism. Our results indicate that oxygen can play a critical role in determining root orientation as well as impacting root metabolic status. Oxytropism allows roots to avoid oxygen-deprived soil strata and may also be the basis of an auto-avoidance mechanism, decreasing the competition between roots for water and nutrients as well as oxygen.

Key words: Alcohol dehydrogenase Diagravitropism Oxygen Oxytropism 

Dear Anoop Kumar Srivastava ,

thank your for your very interesting answer.

Best,

Łukasz

Please check the attached files.

Dear J.C. Tarafdar,

thank you for providing so many references. I will study them in details very soon.

Lukasz

Thigmo do not only refer to touch but also to transient mechanical stimuli and wind is often the main source of thigmomorphogenesis or maybe thigmotropism but this is more debated.

On the roots, there is an interesting paper on mechanical stimuli triggering organogenesis on the opposite side from the bending :

Richter, G.L., Monshausen, G.B., Krol, A., Gilroy, S., 2009. Mechanical Stimuli Modulate Lateral Root Organogenesis. Plant Physiol. 151, 1855–1866. https://doi.org/10.1104/pp.109.142448 That might interest you