Most plants are essentially rooted to a spot and immobile. While the factors they need for growth – e.g. sunlight, minerals, and water – are often present in the environment, they aren’t always close enough to the plant to be used fully. Nature has overcome this problem by giving plants the remarkably ability for some of their parts – e.g. roots and shoots – to grow in response to those abiotic factors. In this way, shoots tend to grow towards the light, thereby promoting photosynthesis (the phenomenon of phototropism), and roots tend to grow downwards, which helps anchor the plant in the soil (the behavioural response known as geotropism (or, alternatively, gravitropism)). Of the many factors in the soil exploited by roots, water is probably the most important. Indeed, so important is water that it should come as no surprise to know that roots have a hydrotropic response whereby roots grow towards water sources, a behaviour which is distinct from geotropism.*
Although both geo- and hydrotropism are similar in their reliance upon differences in growth between the two ‘sides’ of the root – that closest to the gravity/water source grows slower relative to that furthest away – they also have differences. A major difference is that geotropism involves the plant hormone auxin, whereas hydrotropism utilises ABA (abscisic acid).
In view of the importance of plants getting adequate water to grow properly – and relevance of that to a future world’s food security where water scarcity is likely to limit crop growth – Daniela Dietrich et al. have further dissected the root hydrotropic response. Their work emphasises even more its distinctiveness from geotropism. In particular, they demonstrate that hydrotropism still occurs in roots whose meristem and root cap have been destroyed by laser treatment, but is inhibited if differential cell-length increase in the cortex tissue is prevented.
Their elegant study leads to the conclusion that the elongation zone of roots performs a dual function in hydrotropism, in both sensing a water potential gradient and subsequently undergoing differential growth. This is in marked contrast to geotropism (where stimulus-perception and growth response are spatially separated). Now, the big question remains – which part of the root is responsible for perception of the sound of water, as revealed by Monica Gagliano et al. in their study of the bio-acoustic response of roots?[Ed. – lest our more geo-fixated audience feel that their own rhizobehavioural interests are being diluted by all this talk of hydrotropism, we’re happy to alert readers to the open-access paper by Oliver Pouliquen et al. entitled ‘A new scenario for gravity detection in plants: the position sensor hypothesis’ in which they propose that a plant’s gravity sensor detects an inclination and not a force… This is one of many articles in that journal’s Special Issue on the biophysics of plant development.
* Having long-recognised a geotropic response of roots, identification of an additional hydrotropic response was going to be difficult to establish. But, its elucidation was aided in great part by the discovery of a plant that didn’t respond to gravity. Use of this ageotropicum mutant of pea (Pisum sativum) thereby allowed separation of a gravity response from a non-gravity response, such as hydrotropism (e.g. this). As is so often found, for those with the right frame of mind to recognise it, Nature gives a clue to help humans explore and understand her manifold wise and wonderful biological ways.