November 21, 2019 | Our Team, Perennial Sorghum

Interview with Pheonah Nabukalu

Pheonah Nabukalu, Research Associate, Perennial Sorghum

What brought you to The Land Institute (TLI)?

I came to The Land Institute in 2013 for a postdoctoral research position after graduating with my PhD in plant breeding from Louisiana State University (LSU).

Did you know much about TLI before you came here?

Yes. The first time I got to know of TLI was from Dr. Steven Harrison (a renowned wheat breeder) who taught plant breeding and genetics at LSU. In one of the lectures on plant domestication and interspecific hybridization, he mentioned TLI as one of the research centers in the U.S. developing new crops using these approaches. I gained interest in the work done by TLI from that point on.

Since you’ve been here, you’ve been actively involved in the breeding and putting some research papers together – do you and Stan each have your own projects or are you working together on projects he assigns?

Stan Cox oversees the perennial sorghum program and has been very supportive of my work. I started off with a project funded by the US Department of Agriculture in collaboration with the University of Georgia. The project mainly focused on studying the genetics of perenniality in grain sorghum and involved mapping QTL (quantitative trait loci) for specific traits in perennial sorghum (QTL or quantitative trait loci mapping identifies which molecular markers correlate with an observed trait). Through this whole time, I have also worked with Stan in the core perennial sorghum breeding program: making interspecific pollinations, collecting data, making selections in the field, harvesting, etc. In the past four years, I have conducted our cytological (microscope) work, distinguishing among 20, 30, and 40-chromosome plants.

I later got involved in perennial sorghum breeding trials in Uganda in collaboration with our international partners. We provide our research partners with diverse germplasm for trials in tropical conditions. The goal is to develop perennial sorghum lines that are adapted to local conditions in Uganda. Farmers would benefit from having a cost- friendly multiple harvest sorghum crop. We have published several papers in major journals, and are still submitting more. *

With your own projects, are there any specific challenges that you’ve had in breeding sorghum?

The main challenge has been the limited number of breeding lines that we can cross with the perennial sorghum germplasm. While most of the existing breeding lines are diploids (with 20 chromosomes), the perennial germplasm we use in our breeding work is tetraploid, with 40 chromosomes. Making crosses between diploid and tetraploid is possible, but tricky, and it is difficult to produce strains of tetraploid sorghum with stable expression of important traits like perenniality.

In addition, we had some challenges in publishing our work on the molecular genetics of perenniality in perennial grain sorghum that is based on a tetraploid system. Most of the molecular genetics studies on sorghum have been done on diploids. With our Georgia colleagues, we are kind of the pioneers in mapping QTL in a tetraploid system, and this required a more sophisticated approach. The core manuscript “Transmission genetics of a Sorghum bicolor and S. halepense backcross populations” describing the methodology used in developing the genetic linkage maps of these backcross populations has been accepted by Frontiers in Plant Science journal. We can now publish the remaining papers on QTL mapping of various traits in perennial grain sorghum. We could not publish this work until the methodology mapping paper was successfully published. This publication opens the door for other researchers to publish on this cutting-edge research.

Stan has talked about how the tetraploid to now perennial diploid breeding makes perennial sorghum easier to breed with regional annual varieties. Can you explain more about that?

Most of our collaborators in Africa and Asia are working with locally adapted diploid sorghum lines. Most of our perennial breeding germplasm at TLI is tetraploid. This presents a breeding challenge to some of our research partners in those regions. Starting three years ago, we discovered that we can produce diploid hybrids from crosses between diploid grain sorghum and tetraploid perennial sorghum. If, in these populations, we can select perennial diploid lines that can be easily crossed to normal diploid grain sorghum, that will make breeding and development of regional perennial grain sorghum easier. Our research partners will find it very easy to cross TLI diploid perennial germplasms with their locally adapted diploid germplasms.

About a year ago, we did produce a single diploid plant that showed strong growth of rhizomes—the underground stems that are essential to making a plant perennial. This year, we produced hundreds more siblings of that plant and thousands of its offspring. We expect those plants to vary widely for all sorts of traits, including rhizomes, perenniality, and grain production, bringing the possibility of breeding a diploid perennial sorghum closer to reality. New perennial diploid lines will make it much easier to cross with any other existing diploid sorghum breeding line. Generally, that will increase genetic diversity within the TLI breeding program and make it feasible for our colleagues in Africa to cross with their local varieties.

Could you talk some about your perennial sorghum research partners?

We currently have research partners in the U.S. (University of Georgia and University of Texas A&M), in India at The International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), and in several countries in Africa (universities and research institutes in Uganda, Kenya, Ethiopia, Mali, and South Africa).

Are there more research partners in other countries because here in the U.S. sorghum is used more for animal feed and in Africa and India it’s used for human consumption?

TLI is developing grain sorghum exclusively for human consumption, and it’s mainly in Africa and India where sorghum is an important human staple food crop. However, we also work on breeding perennial grain sorghum for food production in the United States. Sorghum grain is increasingly used in food products in this country.

Is the sorghum intercropping with pigeonpea in Africa similar to the Kernza/alfalfa intercropping being done at TLI, with the intercrop focused on the nitrogen-fixing properties of those legumes?

This is what our Swedish group of collaborators at Lund University are investigating. The nitrogen-fixing properties by way of intercropping sorghum with pigeonpea stand out as the major focus of that investigation: by how much would this crop combination reduce dependence on mineral-N fertilizers? Pigeonpea, a perennial legume, has been traditionally intercropped by farmers in Africa with short-season cereal crops such as sorghum, pearl millet, and maize to serve as a buffer against crop failures, provide a source of additional income, and also to harvest more food in time and space.

Would we ever intercrop pigeonpea with perennial sorghum here?

Kansas does not provide the right climate for growing pigeonpea, which grows well only in tropical environments. Alfalfa and other legumes would do better here.

In the plant-breeding world, which is a traditionally more male-dominated field of science, are there any challenges unique to being a woman?

I’ve gotten used to few women being in this career, and I have learnt to cope with it. There are no unique challenges that any woman with an interest in the plant breeding career won’t be able to overcome. When Stan was starting his career, there were almost no women working in plant breeding. Today, that number is increasing.

What are the sorghum “seasons” at TLI for planting, harvesting, etc.?

We have different tasks we focus on at different times of the year.

Generally planting preparation starts off around April, when we are making experimental field designs and organizing seed for planting.
In late April or early May, perennial lines that survived the winter are emerging. Rhizomes of selected survivors are dug up and transplanted in the pollination block. These serve as the perennial parents in the next breeding cycle. Around this same time, we germinate seeds of the grain sorghum parental lines in the greenhouse; these also serve as parents in the pollination nursery.
In late May and early June, we transplant seedlings from the greenhouse and rhizomes from overwintering perennial lines in our pollination nursery. Then, with a mechanical plot planter and tractor, we direct-seed our much larger field nurseries.
In July – September we have the crop in the field. We start taking notes on germination rates and identifying existing pests and diseases. We also start making crosses (inter-pollinating) the perennial parents with various inbred lines in the pollination nursery.
Around August – October, we start taking detailed notes on the characteristics of the breeding lines we have in the field, assessing plant traits such as flowering, plant height, tillering, panicle compactness, rhizome development, etc.
Around late September – October, we start harvesting.
October through February, we are threshing seed heads, and collecting and analyzing data. We are also growing a “bonus” generation of the most crucial populations in the greenhouse.

And for the past few years, in October, we have sent just-harvested seed of important lines to be sown in a nursery in Puerto Rico near the small city of Salinas. Seed from those lines is harvested in February, well in time to sow in the field back here in Salina. The greenhouse and the Puerto Rico nursery allow us to get through two generations in a single year.

Is there a “season” for chinch bugs or other sorghum insect pests?

Chinch bug damage is often prevalent in dry weather conditions. For example, 2018 was a dry year, so from the time we planted, we had problems with chinch bugs. We are grateful to Ebony Murrell and Edy Cheremond who monitor and prevent the chinch bug population from reaching economic injury level. Then last year, 2019, was a wet year which chinch bugs don’t like, so damage was undetectable. There are other years in the recent past where our fields have been devastated by pests such as yellow sugarcane aphid (2016-17) and sorghum midge (2018) which drastically reduced grain yield. It really depends on the prevailing weather and the stage the crop is at when the pests arrive. In both 2017 and 2018, we had yield loss in most of our fields mainly because the pests showed up early in the season, before or when plants were starting to flower. Most of the heads had some yield loss.

It is important to note that we are not alone; farmers in this area also were hit hard by these pests recently. Sugarcane aphid and midge are tropical species and were virtually unknown around here until 2016 and 2017, respectively. Chinch bug, on the other hand, is native and has been an intermittent problem ever since farmers started growing sorghum in the Central Plains.

Are they less susceptible to insects once the seeds have set?

It depends on the pest; some pests can be devastating at all stages of the crop. For example, chinch bugs can damage the sorghum crop early in the season by sucking plant sap, making the plants wilt and die. If damage occurs in later stages, after seed set, the plants may lodge (the stems bend over near ground level making it difficult to harvest). The sorghum midge will attack the floral parts where seed is produced, and if they attack at flowering time, seed-set will be severely reduced. For sugarcane aphids, the pest feeds on the plant, sucking sap from its leaves and leaving a sticky substance that makes it hard to use a combined harvester or to mechanically thresh the seed. Then it’s hard to get the grain.

Do you have any 5-10 year goals for perennial sorghum?

The breeding program has a promising future. In addition to perenniality, we hope TLI can develop and release diploid perennial lines that are high yielding under Kansas conditions. We have sent some of the diploid perennial sorghum seed to Kenya (ICRISAT breeder) and Uganda to cross with their locally adapted sorghum lines. Developing new varieties will take some time, but there is a high chance of success. We hope our perennial grain sorghum can go beyond experimental research fields to farmers’ fields. So far, we have only engaged farmers in Uganda, but we will need to involve farmers here in the U.S. and other countries. We also hope TLI can breed new strains of perennial sorghum for a range of end uses, from human food to forage.

I think the work done at The Land Institute is on the right course – it’s very necessary – especially at this time and for the future.

*Among the papers that are already in print are:

Nabukalu P, Cox TS (2016) Response to selection in the initial stages of a perennial sorghum breeding program. Euphytica 209 (1):103-111

Cox S, Nabukalu P, Paterson AH, Kong W, Auckland S, Rainville L, Cox S, Wang S (2017) High proportion of diploid hybrids produced by interspecific diploid × tetraploid Sorghum hybridization. Genetic Resources and Crop Evolution. doi:10.1007/s10722-017-0580-7.

Nakasagga S, Biruma M, Tusiime G, Nabukalu P, Cox S (2018). Rhizome Development in Sorghum bicolor × S. halepense Families in the Tropical Ecosystem of Uganda. African Journal of Agricultural Research, 13 (4). pp. 151-166.

Cox S, Nabukalu P, Paterson AH, Kong W, Nakasagga S (2018). Development of Perennial Grain Sorghum. Sustainability 10 (1):172