by Nicole Mihelich
Turfgrasses are diverse species that are of interest to researchers around the world. However, turfgrasses are not as well-studied compared to major world food crops such as rice. Like many of the turfgrasses grown in the northern US, rice is a cool-season grass. Rice is a globally important crop as well as a model species for many biological phenomena of grasses, including rhizome development. Rhizomes are horizontally underground stems that are characteristic of some cool-season grasses, notably Kentucky bluegrass, strong creeping red fescue, and slender creeping red fescue. The formation and interlocking of rhizomes are both thought to be helpful when establishing, harvesting, and transplanting thick mats of vegetation in sod production. Domesticated rice, Oryza sativa, is an annual crop that does not produce rhizomes; however, a closely related interfertile perennial wild relative called Oryza longistaminata does produce rhizomes, and is of interest for improvement as a sustainable perennial rice crop. Rhizome research in rice species is applicable due to the close relationship of rice with cool-season turfgrass species (Figure 1). Although rice and turfgrasses ultimately have different agricultural uses, research in O. longistaminata rhizome development can reveal insights for cool-season turfgrass sod improvement.
One major question that is crucial to both rice perennialization and sodded turf is: how are rhizomes different from aerial shoots? Both rhizomes and aerial shoots originate from axillary buds on the main stem/crown of the plant, yet their morphologies are distinct (Figure 2). An example of O. longistaminata research as a useful comparison to cool-season turf is in the paper by Yoshida et al. (2016) entitled “Analysis of Rhizome Development in Oryza longistaminata, a Wild Rice Species”, published in the journal Plant and Cell Physiology. Yoshida and coauthors used various observational methods to document the morphology, growth, and development of rhizomes in O. longistaminata as well as to compare this perennial species to the shoot structure of cultivated O. sativa. Presented here is an overview of the authors’ significant findings and their implications to cool-season turfgrass rhizomes.
In rice, rhizomes elongate in the vegetative phase, while aerial shoots remain unelongated in the vegetative phase until transition to the reproductive phase when a panicle begins to form. To support these morphological and developmental observations, the authors measured expression of two genes in O. longistaminata and O. sativa related to the synthesis of gibberellins, a group of hormones associated with stem elongation. Both genes had higher expression (i.e., were being used more) in aerial shoots in the reproductive phase verses the vegetative phase, implying that these genes are important for aerial shoot elongation. However, only one of the genes had higher expression in rhizomes, implying that only one of the two genes is important for below-ground stem elongation. Overall, this suggests that the elongation of above- and below-ground stems are independently regulated.
A second major focus of the paper was the observation that the direction of growth of O. longistaminata rhizome buds rapidly changes after initiation from parallel to the shoot to almost perpendicular (Figure 3; see my previous blog for an explanation of these terms). In contrast, buds on an aerial shoot remain relatively parallel after initiation. This axillary bud patterning is different between rice and cool-season turf grasses. For instance, in my research of strong creeping red fescue and slender creeping red fescue, both tillers and rhizomes can form at a more perpendicular angle from the parent shoot (extravaginal buds; Figure 4), though only tillers can form and remain at a parallel angle from the parent shoot (intravaginal buds/tillers).
Another interesting observation made by Yoshida and coauthors is that the fate of a rhizome bud (whether it becomes a rhizome branch or an aerial tiller) is dependent on light. Rhizomes and their axillary buds will transition from looking pale with internodes (typical for below-ground stems) to green with formed tillers (typical for above-ground stems; Figure 5). This shows that even though morphology and expression/regulation are different, rhizomes can transition both their shoots and axillary buds.
An additional morphological observation was that rhizome leaf scales look very similar to the youngest leaves on tillers of O. longistaminata and O. sativa due to their predominant leaf sheath and lack of leaf blade (Figure 6). This shows that rhizomes are maintained in a more “juvenile phase” of shoot development compared to aerial shoots, and again shows that tillers and rhizomes have the same origin but differentiate upon further development. This differentiation is possible because the developmental phases of below-ground and above-ground shoots are controlled independently.
With these observations of O. longistaminata, let’s return to the main question I posed: how are rhizomes different from aerial shoots? The authors found many commonalities in axillary buds regardless of its fate as a rhizome or aerial shoot. Distinct patterns of development do result in obviously unique structures, though often the common origin and similar structures are observed both above and below. These commonalities and modifications during development found in the aerial shoots and rhizomes of O. longistaminata suggest that rhizomes possess a substantial degree of developmental plasticity. This developmental plasticity of rhizomes can be translated to the turfgrass context to enhance sod production. By drawing knowledge from well-studied model species, such as rice, we can also learn about potential homologous structures, genes, and mechanisms for rhizome production in cool-season turfgrasses. By observing different hormone and light responses in rice, we can develop treatments to regulate rhizome development. As with all comparative research, a major caveat is that comparison between species is not perfectly transferable. Rice species differ from cool-season turf species in terms of morphology, overall plant size, and genome structure and complexity. However, learning from model species that are turfgrass relatives is crucial in advancing knowledge for breeding efforts of turfgrass and potential sod-specific traits.
Yoshida, A., Terada, Y., Toriba, T., Kose, K., Ashikari, M., & Kyozuka, J. (2016). Analysis of rhizome development in Oryza longistaminata, a wild rice species. Plant and Cell Physiology, 57(10), 2213-2220.
Grass Phylogeny Working Group II. (2012). New grass phylogeny resolves deep evolutionary relationships and discovers C4 origins. New Phytologist, 193(2), 304-312.