In a nutshell
Recent advances in adventitious
rooting have discovered inhibitors (Rasmussen et al 2012 Plant Phys, Rasmussen et al 2012 Plant Signalling and Behavior; Rasmussen et al 2013 Molecular Plant) and their interactions with other phytohormones for regulating adventitious rooting (Rasmussen et al (to be submitted)). In addition increased age is well known to inhibit adventitious rooting (Rasmussen and Hunt 2010). However, little is known about the asymmetric cell division which is the first step to adventitious rooting. My current research aims to learn molecular techniques for investigating adventitious root initiation using the better studied lateral root system for future application to adventitious rooting.
Adventitious root formation: definition
Adventitious roots are roots that form from non-root tissues such as stems. Adventitious root formation is critical to all industries using cutting propagation and adventitious roots are the main roots that form in important food species like rice and maize.
Initiation of adventitious roots in stems occurs with cell divisions from the cambial region (and often close to ray parenchyma) leading to callus formation. Tracheids then differentiate within the callus and elongate towards existing vascular tissue and in the opposite direction towards the outer cell layers. By the time the tracheids reach the epidermis a root primordia has formed with all the normal root tissues (Rasmussen and Hunt 2010). In the hypocotyl (such as in Arabidopsis) adventitious roots form from asymmetric cell divisions in the pericycle (Boerjan et al 1995). Examples of adventitious root primordia in Arabidopsis
hypocotyls. A stage 2; B and C stage 4; D and E greater
than stage 4.
Maturational Changes in Pine with Respect to Rooting Ability of Cuttings
In forestry cuttings are taken from a range of superior trees and planted in plantations. Unfortunately as the trees get older root formation declines in the cuttings. Since it can take longer than 8 years to identify superior trees this poses a significant problem for forestry. This also occurs in other tree-crops such as fruit and nut trees.
 |
| Left cuttings are from 7 year old plants, cuttings on the right are from 6 month old plants |
While working for Queensland Department of Primary Industries and Fisheries we were investigating the optimal conditions for improving rooting in pine cuttings (Rasmussen et al 2009; Hunt et al 2011). This work involved trying different hormone treatments and different temperature conditions.
Although we could find ways to improve rooting, it was clear that ultimately we needed to understand the mechanisms controlling adventitious roots before we could predict ideal conditions. And so began my PhD at The University of Queensland....
Strigolactone Control of Adventitious Root Formation (Post-Doc and PhD)
Initially I set out to use pea mutants with different auxin levels to study adventitious rooting but strangely the phenotype didn't match the auxin levels....
Strigolactones are a group of hormones that were identified first for the role in promoting germination of parasitic weed species (Matusova et al. 2004). Soon after they were discovered to stimulate mycorrhyzal branching and hence facilitate this beneficial symbiotic association (Akiyama et al., 2005). In 2008 it was discovered that strigolactones were the unknown hormone controlling axillary bud outgrowth (Gomez-Roldan et al 2008; Umehara et al 2008). At the same time we discovered that strigolactones inhibit adventitious root formation. During my PhD I investigated this regulation in two model species (pea and Arabidopsis) and looked at the interaction between strigolactones and cytokinins and auxins (Rasmussen et al., 2012a). In addition I used general inhibitors to demonstrate that inhibiting strigolactones could improve adventitious root formation (Rasmussen et al., 2012b).
Post-doc 1: I continued my research on adventitious rooting by studying whether strigolactones could be part of the decline in adventitious rooting with age. This work was funded by a Marie Curie International Incoming Fellowship at the Universiteit Gent. In addition to using the pea system to study aging, we also used the Arabidopsis system that I adapted during my PhD (based on Sorin et al 2005) to study the interaction between strigolactones and other hormones in regulating adventitious rooting. In addition we have used this bioassay to test new strigolactone analogues (Rasmussen et al 2013) and inhibitors and to develop new markers for strigolactone action.
Post-doc 2: Adventitious root initiation begins with an asymmetric cell division. In the hypocotyl this is followed by cell divisions alternating in orientation leading to the formation of a primordium with normal root tissues. This process is very similar in morphology to the well-studied process of lateral root initiation. Relatively few studies have been done at the molecular level for adventitious root formation. There are many molecular tools available for lateral root initiation making it ideal for learning techniques valuable to adventitious root initiation. What causes the asymmetric cell division that leads to a lateral (or possibly adventitious) root and how those daughter cells are regulated is still poorly understood. The intention of the current project which is funded by a Newton International Fellowship is to learn molecular techniques while investigating asymmetric cell division and daughter cell regulation. These techniques will then be applied or adapted to adventitious root formation and phenotypes.
By accident we had discovered that the very newly identified hormone called strigolactone, controls adventitious root formation.
 |
| Adventitious roots on wildtype (left) and strigolactone deficient (rms5, right) pea cuttings. |
 |
| Example of a pea cutting. Bases kept in the dark |
Post-doc 1: I continued my research on adventitious rooting by studying whether strigolactones could be part of the decline in adventitious rooting with age. This work was funded by a Marie Curie International Incoming Fellowship at the Universiteit Gent. In addition to using the pea system to study aging, we also used the Arabidopsis system that I adapted during my PhD (based on Sorin et al 2005) to study the interaction between strigolactones and other hormones in regulating adventitious rooting. In addition we have used this bioassay to test new strigolactone analogues (Rasmussen et al 2013) and inhibitors and to develop new markers for strigolactone action.
Post-doc 2: Adventitious root initiation begins with an asymmetric cell division. In the hypocotyl this is followed by cell divisions alternating in orientation leading to the formation of a primordium with normal root tissues. This process is very similar in morphology to the well-studied process of lateral root initiation. Relatively few studies have been done at the molecular level for adventitious root formation. There are many molecular tools available for lateral root initiation making it ideal for learning techniques valuable to adventitious root initiation. What causes the asymmetric cell division that leads to a lateral (or possibly adventitious) root and how those daughter cells are regulated is still poorly understood. The intention of the current project which is funded by a Newton International Fellowship is to learn molecular techniques while investigating asymmetric cell division and daughter cell regulation. These techniques will then be applied or adapted to adventitious root formation and phenotypes.
References
Akiyama K, Matsuzaki K, Hayashi H (2005) Plant sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi. Nature 435: 824–827
Boerjan W, Cervera MT, Delarue M, Beeckman T, Dewitte W, Bellini C, Caboche M, Van Onckelen H, Van Montagu M, Inze´ D (1995) Superroot, a recessive mutation in Arabidopsis, confers auxin overproduction. Plant Cell 7: 1405–1419
Gomez-Roldan V, Fermas S, Brewer PB, Puech-Page`sV, Dun EA, Pillot JP, Letisse F, Matusova R, Danoun S, Portais JC, et al (2008) Strigolactone inhibition of shoot branching. Nature 455: 189–194
Hunt MA, Trueman S & Rasmussen A (2011) Indole-3-butyric acid accelerates adventitious root formation and impedes shoot growth of Pinus elliottii var. elliottii 3P. caribaea var. hondurensis cuttings. New Forests 41:349-360
Matusova R, Rani K, Verstappen FWA, Franssen MCR, Beale MH, Bouwmeester HJ (2005) The strigolactone germination stimulants of the plant-parasitic Striga and Orobanche spp. are derived from the carotenoid pathway. Plant Physiol 139: 920–934
Rasmussen A, Smith TE, & Hunt MA (2009) Cellular stages of root formation, root system quality and survival of Pinus elliottii var. elliottii x P. caribaea varhondurensis cuttings in different temperature environments, New Forests 38:285-294 DOI 10.1007/s11056-009-9147-6
Rasmussen A, & Hunt MA (2010) Maturation delays the cellular stages of adventitious root formation in pine. Australian Journal of Forest Research 73:(1) 41-46.
Rasmussen A, Mason MG, De Cuyper C, Brewer PB, Herold S, Agusti J, Geelen D, Goormachtig S, Beeckman T & Beveridge CA (2012) Strigolactones suppress adventitious rooting in Arabidopsis and pea. Plant Physiology 158:1976-1987.
Rasmussen A, Beveridge CA & Geelen D (2012) Inhibition of strigolactones promotes adventitious root formation Plant Signaling and Behavior 7:(6)1-4.
Rasmussen A, Heugebaert T, Matthys C, Van Deun R, Boyer F-D, Goormachtig S, Stevens C & Geelen D (2013) A Fluorescent alternative to the synthetic strigolactone GR24. Molecular Plant 6: (1) 100-112.
Sorin C, Bussell JD, Camus I, Ljung K, KowalczykM, Geiss G, McKhann H, Garcion C, Vaucheret H, Sandberg G, et al (2005) Auxin and light control of adventitious rooting in Arabidopsis require ARGONAUTE1.
Plant Cell 17: 1343–1359
Umehara M, Hanada A, Yoshida S, Akiyama K, Arite T, Takeda-Kamiya N, Magome H, Kamiya Y, Shirasu K, Yoneyama K, et al (2008) Inhibition of shoot branching by new terpenoid plant hormones. Nature 455: 195–200.
