Recent developments in Lablab purpureus genomics: A focus on drought stress tolerance and use of genomic resources to develop stress-resilient varieties

Drought is a major climatic challenge that contributes significantly to the decline of food productivity. One of the strategies to overcome this challenge is the use of drought-tolerant crops with a wide range of benefits. Lablab is a leguminous crop that has been showing high promise to drought tolerance. It is reported to have higher drought resilience compared with the commonly cultivated legumes such as common beans and cowpeas. Because of its great genetic diversity, Lablab can withstand high temperature and low rainfall, unlike other related crops. On top of that, it is grown for multitudes of purposes including food, forages, conservation agriculture, and improved soil fertility. To enhance its production and benefits during the present effects of climate change, it is crucial to develop improved varieties that would overcome the challenge of drought stress. In the past years, there have been several reviews on Lablab based on origin, domestication, characterization, utilization, germplasm conservation, some cultivation constraints, and conventional breeding with limitations on the genomic exploitation of the crop for drought tolerance. physiological, biochemical, and molecular approaches would be required to develop drought-tolerant cultivars of Lablab. In this review, we discuss recent developments in Lablab genomics with a focus on drought stress tolerance and the use of genomic resources to develop stress-resilient varieties.

Maintaining production in drylands is most likely the major challenge in modern agriculture that deserves immediate intervention.
Because we have some crops that perform well in drought-prone environments, the solution can involve an identification of the best cultivars and the knowledge of their drought-tolerance capability (Zandalinas et al., 2018).
It is popularly regarded as grain legume, vegetable, and fodder which is rich in protein (comparable with soybean), nutrients, and vitamins (Minde et al., 2020). In the sustainability of conserved agriculture and enhanced soil fertility, farmers have been intercropping Lablab with their major crops or utilizing it as a cover crop and green manure (Chakoma et al., 2016;Mkonda & He, 2017). The crop is a good source of rare pharmaceuticals used to cure diseases in humans and animals. It has been established recently that a carbohydrate-binding protein from Lablab can efficiently block SARS-CoV-2 and influenza viruses, thus providing room for a cure of infections (Liu et al., 2020).
Insulin-like protein has also been isolated from the crop (Sachin et al., 2020). The crop also plays a great role in ensuring income security among smallholder farmers especially in dryland and semidryland ecosystems (Raghu et al., 2018).
The broad genetic diversity of Lablab has supported adaptation and distribution of the crop over a broad range of environmental and climatic conditions (Ewansiha et al., 2007;Venkatesha et al., 2013;Vidigal et al., 2018). It spreads along the tropical and subtropical region between 30 N and 30 S at an elevation of about 0-2000 m above sea level. Lablab also adapts to a wide range of temperature (18 C to 50 C) and annual rainfall (200-2500 mm). This is different from other related species whose favorable growth temperature ranges only between 18 C and 30 C while unable to survive in the little amount of rainfall compared with Lablab (Bhandari et al., 2017;Maass et al., 2010). Its ability to grow vigorously when rainfall resumes after drought has led to its greater resilience compared with other legumes such as common beans (Phaseolus vulgaris), soybeans (Glycine max), cowpeas (Vigna unguiculata), and pigeon peas (Cajanus cajan) (Ewansiha & Singh, 2006;Miller et al., 2018).
To enhance economic productivity and associated benefits of Lablab in the present era of frequent drought spells, there is a need of developing drought-tolerant varieties. In the past 10-15 years, detailed studies and reviews on Lablab origin, domestication, dispersal, utilization, germplasm conservation, characterization, cultivation, some production constraints, and conventional breeding have been written. The conventional breeding of many Lablab varieties was focused on improvement in soil fertility, forage, high yield, and photosensitivity while neglecting stress tolerance. The drought-tolerant traits are polygenic and possess complex nature of inheritance that would require integration of genomic, physiological, biochemical, and molecular approaches for their manipulation. Until presently, there is limited genomic information on Lablab (Rai et al., 2018b;Wang et al., 2018). In this review, we discuss recent developments in the genomics of the crop with a focus on drought stress tolerance and the use of genomic resources to develop stress-resilient varieties. This would help in improving the economic production of the crop and its associated benefits to the farming community.

| A BRIEF INTRODUCTION ON LABLAB GENOME
Lablab, which is also known as Dolichos Lablab (in English) and Fiwi or Ngwara (in Swahili), is a leguminous crop in the Fabaceae family. Its genome has recently been sequenced, assembled, and compared with related species (Chang et al., 2018;Iwata et al., 2013) (Table 1).
The comparison shows that Lablab with chromosome numbers 2n = 2x = 20, 22, and 24 is less complex, has a smaller genome size (367 Mb), scaffold assembly (395.47 Mb), and protein-coding genes (20,946) compared with other related species. However, these genes possess tremendous characteristics in functionality such as gene length and coding sequence which are longer compared with other species. Lablab has also longer exons and introns.
Lablab and Bambara nut were compared based on their plastid genome. While their genomes have a quadripartite structure with two inverted repeats (IRs), a large and a small single-copy region, the lengths of their plastomes are 151,753 and 152,015 bp, respectively.
Each of these plastomes has four rRNAs and 71 protein-coding genes.
However, their tRNA genes were not consistent in each plastome.
The plastome in Lablab has 32 tRNA genes, whereas that of Bambara nut has 33 tRNA genes. Phylogenetically, Lablab was noted to relate closely with common beans (Liao et al., 2019;Wang et al., 2017).
Sequenced and assembled transcriptome from Lablab was also compared with other three legumes, that is, Bambara nut, winged bean, and grass peas. The comparison revealed that the number of reads (16,190,774), transcripts (52,019), and assembled bases (51,997,858) in Lablab exceeded most of the legumes in the study.
N50 of all transcripts was also higher (1570 bp) in Lablab than other legumes and thus formed a more complete assembly. This corresponded also to the highest percentage of putative orthologs in both Lablab and Bambara nut (Chapman, 2015).

| Drought-adaptive mechanisms
Crops adapt three resistance mechanisms to cope with drought, that is, drought escape, drought avoidance, and drought tolerance. For plants to escape drought conditions, they have to opt for rapid growth and development which will lead to completion of the growth cycle before drought events (Shavrukov et al., 2017). Few numbers of seeds and reduced biomass are parameters associated with drought escape.
In drought avoidance, plants increase root growth while limiting their vegetative growth and transpiration rates.
The ability of the plant to produce abundantly even under optimal water conditions is known as drought tolerance (Abobatta, 2019;Basu et al., 2016). Early plant vigor, fast ground cover, large seed size, long and deep root system, high root biomass, small leaflets, and high leaf water potential are some of the attributes for drought tolerance (Yadav & Sharma, 2016). This type of drought-adaptive mechanism has been noted in Lablab (Robotham & Chapman, 2015) through early or late maturing varieties. For instance, early maturing varieties can escape terminal drought, but if they are exposed to intermittent stress, they perform very poorly (Mai-Kodomi et al., 1999;Shavrukov et al., 2017). For late-maturing varieties, the sensitivity of the crop to drought stress is more during the flowering stage (Nadeem et al., 2019). These challenges can be taken care of, first by introgression of drought-tolerant attributes to the early maturing varieties, second by identifying late-maturing cultivars with drought tolerance, and third by the use of a computational model to resolve various drought scenarios influenced by climate change. Because some agroecological zones are not well defined in many places (Batieno, 2016), USDA, 2012). There have been some "traditional (conventional)" and "improved (modern)" ways of evaluating the phenotyping effect of drought stress on Lablab (Guretzki & Papenbrock, 2013). The traditional method quantifies the effects on few accessions by analyzing their easily measurable parameters such as root parameters (e.g., length, width, and density), leaf parameters (e.g., size, number, greenish, and waxiness), plant height, stem size, and weight of fresh and dry biomass through destructive methods. The improved method can screen many accessions very efficiently without destruction. It computes the effects based on physiological processes. Some parameters that are easily computed through this method are stomatal conductivity (Grant et al., 2006), transpiration rate (Chaerle et al., 2009), and chlorophyll content (Sperdouli & Moustakas, 2012). As it demands more time and labor, the traditional method is thus regarded as less effective compared with the improved method (Golzarian et al., 2011;Honsdorf et al., 2014). BhGRP1 drought-tolerant gene (Yao et al., 2013). A similar library was also developed by Wang et al. (2018) where 2792 unigenes were gathered from 4064 drought-induced ESTs. Two drought-tolerant microRNAs (miRNAs), that is, miRNA 156 and miRNA 172, were isolated from Lablab (Thilagavathy & Devaraj, 2016). As part of transcriptomic regulation for drought tolerance, the γECS gene was noted to influence the free radical system and antioxidant activities during fruit ripening in Lablab (Rai et al., 2017).
There have been also some genes, E, Dt1, GmFT2, GmGIa, PvTFLY1, and GmPhyA3, studied to relate high temperature and photoperiodic sensitivity in Lablab (Ramtekey et al., 2019). As temperature increases in many areas, flower dropping is becoming a major problem faced by farmers growing Lablab in Tanzania

| PROMOTING EXPLOITATION OF GENOMIC RESOURCES IN LABLAB FOR STRESS TOLERANCE
Lablab is increasingly becoming a popular crop in the community due to its multitude of values. The crop has therefore been engaged in several research programs, many of them taking place in Asia rather than Africa where the crop originates (Maass, 2016;Maass et al., 2010).
Among research happening in Africa, it is only little or none that has been directed to the genomic development affecting the release of varieties for commercial purposes. So far, many of the world-known commercial varieties in Lablab, for example, Koala, HA3, and HA4 for grains and Rongai, Endurance, and Highworth for forage, come from Asia and Australia (Gopalakrishnan, 2007;Maass et al., 2010;Ramesh & Byre Gowda, 2016). In Africa, only Kenya has commercialized its varieties: Eldo-KT Black 1 and 2 (Eldoret Kirkhouse Trust black), Eldo-KT cream, and Eldo-KT Maridadi (KEPHIS, 2017;Kirkhouse Trust, 2015). Evaluation performance of the promising cultivars has also been taking place in Northern Tanzania towards their commercialization (Miller et al., 2018;Nord et al., 2020). The reason behind these few recommended varieties was research focusing more on morphological characterization, forage, and soil properties especially in Africa.
Making genomic information of Lablab easily available such as the AOCC (Hendre et al., 2019) will provide inputs for translational F I G U R E 1 (a-f) Drought-tolerance evaluation of about 300 Lablab accessions at seedling stage is being carried out by the Nelson Mandela African Institution of Science and Technology (NM-AIST), Arusha, Tanzania, in a two repeating experiment. Polythene cover noted in the picture is used to induce high temperature inside the house research on its sustainable development. However, we need further bioinformatics training among researchers for the efficient use of genomic databases. Moreover, the adoption of high-throughput technologies such as next-generation, genome-wide association studies (GWAS), transcript profiling, and gene and genomic editing (CRISPR/ cas9) will bring discovery of more drought-tolerant genes and their expressed quantitative loci (eQTLs). Increased deployment of these genomic tools with an increase of research collaboration will also bring a new revolution in farming systems of drylands (Njarui & Mureithi, 2010;Sennhenn et al., 2017). One of the benefits of using new drought-tolerant varieties is to protect their productivity in dry environments against increased aridity and semiaridity conditions especially in SSA where desertification is highly concerned. As already predicted that desertification will increase as noted in the Saharan  Xu et al. (2015) Note: Some QTLs and molecular markers from some commonly grown legumes that could provide useful knowledge in their transferability in Lablab. Abbreviations: AFLP, amplified fragment length polymorphism; QTLs, quantitative trait loci; RAPD, random amplification of polymorphic DNA; SNP, single nucleotide polymorphism; SSR, simple sequence repeat.
desert that keeps spreading to the south, sensitization of Lablab production to the region would be an important opportunity to minimize the effects of drought stress on the region. In return, production will get improved to make it more commercialized.

Genomic exploitations of Lablab cannot become successful if
there is a limitation of genetic resources. This is because, useful resources for exploitation come from a wide range of genetic materials (Azeez et al., 2018). interest of moving to towns and cities for small jobs and business. To handle this challenge, we need a strong collaboration among all stakeholders; farmers, researchers, government, and international agencies that will efficiently control in situ and ex situ conservation of the resources.

| CONCLUSION
Lablab is exhibiting an increased research interest due to its wide range of benefits. It has shown a great ability to withstand drought stress compared with other related species. Despite this advantage, there has been very little effort in exploitation of genomic resources in Lablab for drought tolerance. As a result, the crop has been underutilized in many areas. However, because of this genomic potential, the development of the crop through an application of "omics" technology is proposed so that we can convert it into a commercialized crop. The challenge behind this mission is the high cost for most of the tools in "omics technology." Additionally, the methods are time consuming, requiring very expensive consumables, and not feasible for a quick response. Reducing their running cost while deploying cheap and simple tools such as Nanopore MinION field sequencer would lead to the best findings. With sustainable utilization of genomic resources in Lablab, the crop can be transformed from an orphan legume into an industrial crop.

CONFLICT OF INTEREST
None.

ETHICS STATEMENT
This manuscript does not contain any studies with human or animal subjects.

DATA AVAILABILITY STATEMENT
No new data were created or analyzed in this study.