Greener Journal of Agricultural Sciences

Vol. 11(2), pp. 98-107, 2021

ISSN: 2276-7770

Copyright ©2021, the copyright of this article is retained by the author(s)

https://gjournals.org/GJAS

 

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The effect of Potyvirus resistance loci from the maize inbred line Oh1VI on development of maize lethal necrosis (MLN)

 

 

Victoria B. Bulegeya1*; Mark W. Jones2; Tryphone G. Muhamba3; Biswanath Das4; Peter R. Thomison5;  David M. Francis6; Margaret. G. Redinbaugh7

 

 

1-   Tanzania Agriculture Research Institute (TARI) – Dakawa Center, P.O.Box 1892, Morogoro, Tanzania.

2-   United States Department of Agriculture–Agricultural Research Service (USDA-ARS), Corn, Wheat and Soybean Research, Wooster, OH 44691, USA;

3-   Department of Crop Science and Horticulture, Sokoine University of Agriculture (SUA), P.O.Box 3005, Morogoro, Tanzania

4-   International Maize and Wheat Improvement Center (CIMMYT), P.O.Box 1041, Village Market, Nairobi 00621, Kenya

5-   Department of Horticulture and Crop Science, The Ohio State University, 2021 Coffey Rd, Columbus, OH 43210, USA.  

6-   Department of Horticulture and Crop Science, The Ohio State University-Ohio Agriculture Research and Development Center (OARDC), Wooster, OH 44691, USA 

7-   USDA-ARS, Corn, Wheat and Soybean Research, Department of Plant Pathology, The Ohio State University, Wooster, OH 44691, USA.

 

 

ARTICLE INFO

ABSTRACT

 

Article No.: 060421055

Type: Research

 

Maize lethal necrosis (MLN), a viral disease currently affecting corn in East and Central Africa is caused by a combined infection of Maize chlorotic mottle virus (MCMV) and any maize infecting potyvirus. Most of African maize germplasm is susceptible to the disease and there are no known sources of resistance. Recombinant inbred lines (RIL) derived from Oh1VI, a line known for multi-virus resistance with different QTL for potyvirus resistance on chromosome 3, 6 and 10 were selected and screened against MLN under artificial inoculation and natural infestation. Differences were observed among genotypes and QTL groups at P=0.05 in all experiments except under field inoculation. Genotypes with QTL combination of 3, 6 and 10 had at least 20% reduction in MLN symptoms compared to a susceptible check. These results provide useful baseline information on utilization of potyvirus resistance genes for MLN resistance and control in Sub Saharan Africa.

 

Accepted:  06/06/2021

Published: 31/07/2021

 

*Corresponding Author

Victoria Bulegeya

E-mail: victoriabulegeya@ rocketmail.com

 

Keywords: Maize; Maize lethal necrosis (MLN); Potyvirus; Genetic resistance; Sub Saharan Africa

 

 

 

 

 


1.      INTRODUCTION

 

Maize lethal necrosis (MLN) is a disease currently affecting corn (Zea mays) production in East and Central Africa (Mahuku et al., 2015a, 2015b; Wangai et al., 2012; Adams et al., 2013, 2014; Lukanda et al., 2014). MLN is caused by combined infection of Maize chlorotic mottle virus (MCMV) and any maize infecting virus in the Potyviridae family such as Wheat streak mosaic virus (WSMV), Maize dwarf mosaic virus (MDMV) and Sugarcane mosaic virus (SCMV) (Niblett & Claflin 1978; Uyemoto et al., 1980). In East Africa the primary cause of the disease is co- infection with Maize chlorotic mottle virus and Sugarcane mosaic virus (Wangai et al., 2012; Adams et al., 2014; Lukanda et al., 2014; Mahuku et al., 2015).

A survey carried out in East African countries to study the distribution of MLN causing viruses suggested up to 94% incidence in randomly selected symptomatic plants (Mahuku et al., 2015). Tanzanian samples collected at Arusha and Mwanza had 60% to 69% incidence and both viruses were detected (Mahuku et al., 2015). The survey indicated wide distribution and high prevalence of MLN viruses in East and Central Africa.

MLN causes chlorotic mottling from the plant base, leaf necrosis from the margins to the midrib, stunted plant growth, premature death, male sterility and failure to tassel, malformed ears or lack of ear formation, and rotten or small cobs with little or no grain fill (Niblett & Claflin, 1978; Wangai et al., 2012). The magnitude of yield loss associated with the disease makes developing cultivars with disease resistance crucial. In Kenya, MLN caused an estimated loss of $187 million equivalent to $364/ton in 2012 (De Groote et al., 2016). Farmers in MLN areas have experienced a significant decrease in yield since MLN was first reported in 2010 (Makone et al., 2014).

Potyviruses are endemic to East Africa and were observed to cause crop loss of 18% to 46% (Louie, 1980). The introduction of MCMV and co-infection of maize with the endemic potyviruses to cause MLN represents a new threat to maize production in East African countries (Wangai et al., 2012).  There is a need for identification of MLN resistance sources, mapping of genomic regions with MLN resistance and intogression of resistance genes into widely used susceptible inbred lines and hybrids in East Africa (Semagn et al., 2015). The study evaluated Recombinant Inbred Lines (RIL) with potyvirus resistance QTL in disease hotspots in Tanzania and under high disease pressure through artificial inoculation in the growth chamber and field. The RIL population is derived from multi-virus resistant parent Oh1VI and a susceptible parent Oh 28. The population was genotypically analyzed for potyvirus resistance and QTL for potyvirus resistance were mapped to chromosome 3, 6 and 10 (Zambrano et al., 2014). Selected lines with combinations of the QTL were used to analyse the influence of potyvirus resistance in MLN control. The study aimed to fill the knowledge gap concerning the influence of potyvirus resistance QTL for the control of MLN and the suitability of temperate lines in managing MLN in Africa.

 

 

2.      MATERIALS AND METHODS

 

2.1   Plant materials

 

Inbred lines selected from a Recombinant Inbred line (RIL) population derived from a multi-virus resistant parent Oh1VI and susceptible parent Oh28 were used for the study.  The RIL population was generated by the Corn, Soybean and Wheat Quality Research Unit (CSWQRU) at the Ohio Agricultural Research and Development Centre (OARDC). The RIL population was previously genotyped with 768 single nucleotide polymorphism (SNP) markers and QTL for potyvirus resistance (Zambrano et al., 2014).  Selections were based on molecular markers flanking QTL for potyvirus resistance on chromosomes 3, 6 and 10 alone and in all possible combinations. Flanking markers PHM13823-7 and PZA00667-1 were used to select for chromosome 3 QTL, markers PHM15961-13 and PZA00540-3 selected chromosome 6 QTL and flanking markers PHM1812-32 and PHM15868-5 selected chromosome 10. Five independently chosen lines represented one individual QTL or a combination QTL from chromosome 3, 6 and 10. Lines 80231, 80229, 80209, 80221 and 80196 had allele for resistance on chromosome 3, 6 and 10 forming a treatment group of 3_ 6_10.

Genotypes were planted for evaluation in a growth chamber at the Department of Plant Pathology, Ohio State University/OARDC, Wooster, Ohio, May to July 2015 and at the CYMMIT – KALRO MLN Screening Facility, Naivasha, Kenya in December 2015 to March 2016. In natural infection trials treatments were planted for evaluation in fields at BabatiManyara (latitude: -4.20963602, longitude: 35.73990726, elevation: 1378m) and MlangaliniArusha (latitude: -3.3666700, longitude 36.6833300, elevation 1415m), Tanzania during the rain seasons of 2015 and 2016.

In all experiments except for the field inoculation and 1st natural infestation experiment, both parents were included as resistant and susceptible controls and to provide baseline information on disease incidence and severity on each experiment. Control lines 80066 and 80293 from Oh1VI RIL population which lack resistance alleles on all three chromosomes were included in a growth chamber experiment and CML444 and entry73 tropical lines from CIMMYT were included as controls in a field inoculation experiment as susceptible local controls and local checks CML144, CML197, CML442, CML395, KS23-6 and KS23-5 were used in a natural infection experiment.

 

 

2.2   Viral inoculum sources

 

The isolates of SCMV and MCMV used for a growth chamber experiment were maintained by the USDA, CWSQRU.  The SCMV-OH isolate was collected from Ohio (Louie, 1986) and the MCMV-KS isolate was collected from Kansas (Niblett & Claflin, 1978). The sequence of MCMV-KS is 96%-97% identical to the East African isolate which is 98%-99% identical to isolates from China (Mahuku et al., 2015). The SCMV-OH isolate was maintained by serial mechanical transmission to a susceptible maize line, and the MCMV-KS isolate was stored frozen and in liquid nitrogen and transmitted to the susceptible line Oh28 as a source of inoculum.  Presence of the viruses in symptomatic plants was confirmed by tissue blot immunoassay as previously described (Jones et al., 2011).  Inoculum made from a mixture of infected leaf tissues for both viruses was prepared in a combination of 1:4 MCMV to SCMV to attain uniform MLN pressure. Inoculum was prepared by grinding symptomatic leaf tissues in a 0.1 M potassium phosphate in 1:10 dilution ratio (1 gram of tissue to 10 milliliters of the 0.1M, 7.0 pH potassium phosphate buffers) using mortar and pestle. Carborundum (0.02 g/ml) was added as an abrasive agent. The prepared inoculum was rub inoculated to leaves of 14 days old seedlings (Jones et al., 2007). There were two inoculations per experiment with the second inoculation applied two days after the first to ensure successful infection. Plants were transferred to a growth chamber with a 25-21oC (day-night), 75% relative humidity, 532 µmol light intensity (microeinsteins) and a 12 hr photoperiod.

In a field inoculation experiment the inoculum was prepared following the protocol used at the MLN screening facility at Naivasha under CIMMYT and KARLO using East African isolates of SCMV and MCMV maintained through serial transmission to susceptible maize (Gowda et al., 2015). The inoculum was made from a mixture of symptomatic tissues with individual infection of SCMV and MCMV in a combination of 4:1 ratio respectively. The inoculum was prepared by harvesting the plants infected with SCMV and MCMV separately, and then leaves were chopped, weighed and blended in 0.1M potassium phosphate buffer with 1:20 dilution ratio (leaf material: buffer) at a pH of 7.0 and sieved to remove plant debris. The inoculum was mixed in a larger tank and Carborundum 1g/liter was added. Field inoculation was done using a motorized mist blower (Solo423 MistBlower, 11 liter capacity). The inoculum was delivered at a pressure of 10 kg/cm2 with a 2-inch nozzle. Inoculation was carried out at the 4-6 leaf stage and repeated after one week.

 

2.3   Experimental design

 

All experiments were established following the alpha lattice design. Except for trial 4 under natural infection all experiments were arranged in an alpha lattice design of 42 treatments in 3 replications; each replication consisted of 6 blocks with 7 treatments each. Trial 4 had a total of 40 treatments and each replication had 4 blocks of 10 treatments. Each treatment was planted in a row of 5m with intra-row spacing of 25 cm and inter-row spacing 75 cm. All trials were planted under rainfed conditions; irrigation was supplementary in non-rain days and Diammonium phosphate (DAP) was added at planting and UREA as a top dressing to supplement nitrogen and phosphorus sources using local recommended rates.

 

2.4 Data collection

 

For the growth chamber experiment plants were evaluated for disease development beginning 7 days post second inoculation and rating continued every four days until 23 days post inoculation. For the field inoculation experiment disease rating was done 2 weeks post inoculation continuing every 7 days until 42 days post inoculation and for natural infection disease severity ratings were initially performed every seven days and then extended to 14 days covering a total of 56 days. Disease was scored on a scale of 1 to 5 as follows: 1 = no visible MLN symptoms, 2 = fine chlorotic streaks mostly on older leaves, 3 = chlorotic mottling throughout the plant, 4 = excessive chlorotic mottling on lower leaves and necrosis of newly emerging leaves (dead heart), and 5 = complete plant necrosis (Gowda et al, 2015). Severity scores collected were used to generate area under the disease progress curve (AUDPC) values. The equation for AUDPC is

  where; Yi is disease assessment (score), at the ith observation, ti is the time of observation (days) at the ith observation and n is the total number of observation. First scores, last scores mean scores and AUDPC values were used to test for differences among treatments.

 

2.4   Data analysis

 

Analysis was done using the R package version 3.1.1(R Development Core team, 2014). The Agricolae package version 1.2-3 (de Mendiburu, 2010) was used to test for differences among treatments in measured parameters. The experimental model for the alpha lattice was Yij = µ (Mean effect) + Ɍi (Replicate) +Ƭj (Treatment effect) + βi (Incomplete Block effect) +Ɛij (Intra –block error effect). The PIBI.test function was used for the partial incomplete block design to correct for incomplete block effects (de Mendiburu, 2010). A two-tiered analysis was conducted in which the adjusted means from the alpha-lattice were then used to test the null hypothesis that there are no differences between higher order QTL treatments when comparing 3, 6, and 10 alone; 3 and 6, 3 and 10, 6 and 10, in combinations; and 3, 6 and 10 together. The later model was then tested using a general linear model in the R core package version 3.1.1(R Development Core team, 2014). Since different checks were used in different experiments, each experiment was analysed differently and all the treatments were normalized to a susceptible check Oh28.

 

 

3.      RESULTS

 

3.1   Response of genotypes to natural and artificial infestation of MLN

 

In the growth chamber, there was significant variation among genotypes and among different combinations of potyvirus resistance QTL to MLN inoculation. Genotypes with combinations of resistance QTL groups from chromosome 3, 6 and 10 developed less disease symptoms compared to genotypes with single resistance QTL (Table 1).

Under field inoculation no significant differences in MLN symptoms expression was observed between individual RIL genotypes and controls or for QTL groups. Field ratings were conducted over an extended 42 days period, which may have affected our ability to discern differences.  The analysis based on QTL groups indicates that genotypes with QTL combinations of 3 + 6 + 10 has significantly lower means and AUDPC scores compared to other genotype groups. However, the adjusted means for first scores and last scores were not significantly different in the field environment (Table 1).

There was no significant variation among genotypes with different QTL combinations in trial 1 set at Mlangalini, Arusha, Tanzania presumably due to low incidence. Significant variation (P = 0.05) among QTL groups was observed in 3 experiments (trials 2 through 4) set at Krishna seed farm and KIRU-6 village at Babati, Manyara, Tanzania in the first scores, last scores, mean scores and AUDPC (Table 1).


 

Table 1. Importance of specific QTL and QTL combinations in response to MLN infection under natural infestation and artificial inoculation.

Environment h

QTL groups i

Severity score j

FIRST SCORE

LAST SCORE

MEAN SEVERITY

AUDPCk

Growth chamber

Oh28l

3.06a

4.97a

4.23a

52.74a

(OHIO- US)

80066m

3.21a

4.70ab

4.08a

46.88ab

80293n

2.76a

4.33abc

3.62ab

43.16abc

10

1.95b

4.04abc

3.09bc

36.63bcd

6

1.91b

4.00abc

3.05bc

34.75bcd

10_6

1.76bc

3.45bc

2.80bc

34.32cd

3

1.72bc

3.26bc

2.66cd

29.58de

3_6

1.43cd

2.90c

2.28d

26.51e

3_6_10

1.21d

3.08c

2.27d

25.61ef

3_10

1.27d

3.16c

2.17de

25.57ef

Oh1V1o

1.23d

1.38d

1.35e

13.32f

P-values

 

2.2x10-8***

0.001***

3.1x10-6***

1.173x10-3***

 

Field inoculation

 

Oh28l

 

3.18a

 

4.33ab

 

4.46a

 

96.20a

(KENYA)

Entry73p

2.82a

5.00a

4.36a

94.64a

3_10

2.86a

4.68a

4.15a

89.11a

83649q

2.80a

4.50ab

4.14ab

88.74ab

6

2.90a

4.60a

4.09ab

88.32ab

CML444r

2.80a

5.00a

4.11ab

87.98ab

3_6

2.86a

4.89a

4.00ab

85.83ab

3

2.96a

4.50ab

4.00ab

85.74ab

10_6

2.76a

4.63a

3.97ab

85.31ab

10

2.86a

4.90a

3.99ab

84.81ab

 

3_6_10

2.67a

4.07ab

3.62b

78.00b

P- values

 

0.601ns

0.601ns

0.663ns

0.651ns

 

Natural infestation

 

Oh28l

 

2.33a

 

2.33a

 

2.08a

 

41.73a

TRIAL 1(TANZANIA)

3_10

1.75ab

1.40b

1.63ab

34.44ab

 

83649q

1.62ab

1.67ab

1.90ab

41.80a

 

10

1.54ab

1.67ab

1.71ab

36.74a

 

3_6_10

1.51ab

1.60ab

1.74ab

37.87a

 

3_6

1.50ab

1.44ab

1.61ab

34.78ab

 

6

1.50ab

1.38b

1.63ab

35.51a

 

10_6

1.39b

1.27b

1.57ab

34.79ab

 

3

1.32b

1.12b

1.36b

29.17b

 

Pannars

1.00b

1.00b

1.17b

25.67b

 

sc-627t

1.00b

1.00b

1.17b

25.67b

P-values

 

0.03*

0.12ns

0.002**

0.009**

 

Natural infestation

 

Oh28 d

 

1.67a

 

3.5a

 

2.46a

 

101.5a

TRIAL 2 (TANZANIA)

CML197

1.50ab

3.33ab

2.25ab

92.17ab

 

6

1.41ab

3.07bc

2.20b

91.73ab

 

10

1.43ab

3.03bcd

2.19b

91.35ab

 

3

1.34b

2.93bcd

2.12bc

89.04b

 

10_6

1.40ab

3.03bcd

2.13b

88.55b

 

3_6

1.33b

2.81de

2.09bc

88.17b

 

3_10

1.37ab

2.87cd

2.09bc

87.42b

 

CML144

1.50ab

2.83cde

2.08bc

86.33b

 

3_6_10

1.31bc

2.60e

1.99c

84.22b

 

Oh1V1b

1.00c

2.17f

1.58d

66.5b

P-values

 

0.02**

0.001***

2.07x10-5***

9.8x10-5***

 

Natural infestation

 

Oh28l

 

2.00a

 

4.00a

 

3.04a

 

128.3a

TRIAL 3 (TANZANIA)

10

1.97a

3.77a

2.92a

123.3a

 

10_6

1.79ab

3.68ab

2.83a

120.0a

 

6

1.74ab

3.64ab

2.78a

117.8a

 

CML197y

1.34ab

4.00a

2.83a

117.8ab

 

3

1.80ab

3.58b

2.74a

116.0ab

 

CML144z

1.50bc

3.00cd

2.42bc

103.8bc

 

3_6

1.58bc

3.18cd

2.43b

102.6c

 

3_10

1.40c

3.29c

2.38bc

100.6c

 

3_6_10

1.39c

2.98d

1.22c

93.86c

 

Oh1V1o

1.33c

2.83d

1.17c

92.17c

P – values

 

1.12x10-5***

2.43x10-5***

8.3x10-10***

3.87x10-9***

 

Natural infestation

 

Oh28l

 

2.34a

 

3.83a

 

2.91a

 

120.0a

TRIAL 4 (TANZANIA)

CML442u

2.35a

3.50bc

2.88ab

120.2a

 

CML395v

2.16ab

3.67bc

2.83abc

117.7ab

 

10

2.06ab

3.50bc

2.77abc

116.0ab

 

6_10

2.07ab

3.50bc

2.74bcd

114.5abc

 

6

1.99ab

3.43cd

2.73cd

114.9ab

 

3

2.05ab

3.43cd

2.70cd

112.6bc

 

3_6

1.86bcd

3.47bcd

2.66de

111.5cd

 

3_10

1.95abc

3.33de

2.50e

108.8de

 

3_6_10

1.73cd

3.23e

2.50f

105.3ef

 

KS523-6w

1.66cd

3.33de

2.41fg

100.2fg

 

Oh1V1o

1.50de

3.33de

2.34g

97.05gh

 

KS523-5x

0.98e

2.83f

2.08h

89.73h

P-values

 

2.02x10-5***

0.001***

1.2x10-8***

1.92x10-7***

 

h Location with different mode of infection where maize genotypes were tested for resistance to MLN

i Groups of maize genotypes with Potyvirus resistance QTL on chromosome 3, 6 and 10 alone or in a combination of 2 and 3 QTL group.

j Severity scores collected at different time points under artificial inoculation and natural infestation.

k Area under disease progress curve (AUDPC) values calculate from disease severity scores at different time points.

l A susceptible parent

m Susceptible checks from Oh1VI RIL population with no resistance QTL from 3, 6 and 10

n Susceptible checks from Oh1VI RIL population with no resistance QTL from 3, 6 and 10

o A resistant parent

pA tropical line from CYMMIT susceptible to MLN

q A susceptible checks from a Oh1VI RIL population with no resistance QTL from 3, 6 and 10

r A tropical line from CYMMIT susceptible to MLN

s A local check, commercial hybrids used by farmers in Tanzania

t A local check, commercial hybrids used by farmers in Tanzania

u A tropical line from CYMMIT susceptible to MLN

v A tropical line from CYMMIT susceptible to MLN

w A Kansas line with resistance to MLN

x A Kansas line with resistance to MLN

y A tropical line from CYMMIT susceptible to MLN

z A tropical line from CYMMIT susceptible to MLN

 

 


3.2   Importance of potyvirus resistance QTL interaction for MLN control

 

The analysis indicated differences in disease development for germplasm with potyvirus resistance QTL compared to a susceptible control Oh28. In general, genotypes with a combination of three QTL from chromosomes 3, 6, and 10 performed the best across experiments, reducing disease severity by an average of 20% (Table 2).  Also, combinations of 2 QTL (3 + 10 and 3 + 6) developed less MLN symptoms compared to genotypes with a single resistance QTL sources. These results indicate a role for QTL interaction in MLN control.


 

 

Table 2. Response of genotypes with specific QTL and QTL combinations to MLN infections normalized to a susceptible parent

QTL groupq

MEANr

 LSD GROUPs

CML 442t

1.002

a

Oh 28u

1

a

CML395v

0.9808

a

10

0.927

ab

CML197w

0.9131

abc

6

0.9076

abc

6_10

0.8989

abc

3_6

0.8577

bcd

3

0.8547

bcd

3_10

0.8443

cd

3_6_10

0.8366

cd

KS523-6x

0.835

cde

KS523-5y

0.7478

def

Oh1V1z

0.7274

ef

P- value = 6.148e-10***

 

 

Alpha level = 0.05

 

 

Critical value = 2.04

 

 

 

q Groups of maize genotypes with Potyvirus resistance QTL on chromosome 3, 6 and 10 alone or in a combination of 2 and 3 QTL group.

r Average severity scores collected at different time points under artificial inoculation and natural infestation.

s Least significant difference group in response average severity scores

t A tropical line from CYMMIT susceptible to MLN

u A susceptible parent

v A tropical line from CYMMIT susceptible to MLN

w A tropical line from CYMMIT susceptible to MLN

x A Kansas line with resistance to MLN

y A Kansas line with resistance to MLN

z A susceptible parent

.

 


3.3   Agronomic performance of genotypes under natural infection of MLN

 

Data on agronomic performance among genotypes shows a clear difference between RIL, QTL groups and local checks adapted to a tropical environment. Agronomic data were not collected from trials 3 and 4 because these experiments did not reach reproductive maturity. In parameters such as emergency%, days to flowering and yield there is a significant difference between RIL genotype and genotype groups in trial 1 and trial 2 (Table 3). In both trials the difference is seen with treatments and local checks since local checks were adapted hence they outweigh genotypes under study. 

 

 


Table 3. Agronomic performance of genotypes with potyvirus resistance under natural MLN infection at MLN hotspot in Arusha and Babati.

 

 

QTL groups

 

Emergence

(%)

 

Flowering date (days)

 

 

Yield/ear

(Kg)

Ear rot

 

 

Anthesis

Silking

 

Trial 1

Pannart

93.33

69.00b

72.00b

0.1a

0.18b

83649u

83.29

76.94a

80.00a

0.08a

0.05c

3_6

73.45

74.11a

78.86a

0.05b

0.05c

10

72.83

75.90a

80.00a

0.05b

0.05c

3_10

68.58

74.43a

78.80a

0.05b

0.05c

6

66.81

73.63ab

78.87a

0.05b

0.04c

3_6_10

62.51

74.31a

79.17a

0.05b

0.04c

3

62.51

74.14a

78.39a

0.04b

0.04c

Oh28v

60.98

75.37a

79.56a

0.04b

0.05c

10_6

52.02

74.18a

79.17a

0.03b

0.04c

 

Sc-627w

28.33

73.33ab

77.33ab

0.03b

0.30a

P-values

 

2.87x10-8***

7.58x10-3***

7.88x10-6***

2.2x10-16***

0.05*

S

 

Trial 2

 

CML197x

 

18.33c

 

64.33a

 

67.67a

 

0.1a

 

2.00c

CML144y

45.00abc

62.00ab

65.67ab

0.08a

1.67c

3_6

57.49a

57.20c

61.73bc

0.05b

3.83ab

10_6

58.16a

57.04c

61.03c

0.05b

4.60a

3_10

40.44bc

57.45c

61.42bc

0.05b

4.60a

3_6_10

41.41bc

56.84c

61.24bc

0.05b

2.06a

10

49.93ab

57.99c

61.78bc

0.05b

3.87ab

6

47.76ab

58.42bc

62.67bc

0.04b

2.86bc

3

42.54bc

56.84c

60.73c

0.04b

4.00ab

Oh1VIz

25.00bc

58.33bc

61.67bc

0.03b

0.67c

 

Oh28v

51.67ab

59.33abc

61.00c

0.03b

6.33a

P-values

 

0.002***

8.5x10-3***

0.002***

9.9x10-6***

0.027***

 

s Groups of maize genotypes with Potyvirus resistance QTL on chromosome 3, 6 and 10 alone or in a combination of 2 and 3 QTL group.

t A local check, commercial hybrids used by farmers in Tanzania

u Susceptible checks from Oh1VI RIL population with no resistance QTL from 3, 6 and 10

v A susceptible parent

w A local check, commercial hybrids used by farmers in Tanzania

x A tropical line from CYMMIT susceptible to MLN

y A tropical line from CYMMIT susceptible to MLN

z A resistant parent

 

 

 


4.      DISCUSSION

 

The study aimed to determine which of the three potyvirus resistance QTL on chromosome 3, 6 and 10 might provide protection against MLN. No genotypes were unaffected by MLN, signifying that the QTL under study were not providing immunity. The best performing genotypes had a combination of potyvirus resistance QTL on chromosomes 3, 6 and 10. These three QTL were previously shown to be important in providing protection against SCMV (Zambrano et al., 2014), MDMV (Jones et al., 2007) and WSMV (Stewart et al., 2012). The potential role of two QTL interactions cannot be disregarded, as combinations of QTL 3 + 6 and 3 + 10 were also significantly better than controls.

Resistance to potyvirus is clustered in the maize genome (Redinbaugh & Pratt, 2009). Loci on the short arm of chromosome 6 and near the centromere of chromosome 3 have major effect on potyvirus resistance (Jones et al., 2007; Redinbaugh et al., 2004; Xia et al., 1999; Wang et al., 2003, Zhang et al., 2003; Zambrano et al., 2014).

The locus on chromosome 3 near the centromere at bin 3.04/3.05 in combination with other QTL confers resistance to many viruses including WSMV, SCMV, MMV and MCDV (Redinbaugh & Zambrano, 2014). The locus overlaps the position of translation factor eIF4e (Zambrano et al., 2014), involved in conferring virus resistance by producing proteins, which fail to interact with the virus (Gomez et al., 2009). Current studies on MLN resistance found other candidate genes for resistance to MLN on the same region (Gowda et al., 2015; 2018). Other candidate genes include those with a function predicted to restrict virus movement within the plant as demonstrated in arabidopsis by Chrisholm et al (2000). A locus on chromosome 3.05 is known to be responsible in plant defense against pathogens encodes nucleotide-binding site leucine-rich repeat (NBS-LRR) protein (Xiao et al., 2007).

Recently, the locus on chromosome 3 was identified among for QTL responsible for MCMV resistance in the Oh1VI RIL population others being the loci on chromosome 1, 2, and 10 (Jones et al., 2018). The study also denoted that the locus on chromosome 3 was near marker S3_37246834 which had the LOD score of 4.3 explaining 16% of the phenotypic variation and the locus on chromosome 10, which was centred at marker S10_134058628, had the LOD score of 9.0 explaining 11% of the phenotypic variation. These loci overlap the same region responsible for resistance to Potyvirus and other multiple virus families as explained by Zambrano et al. (2014). The identified locus on chromosome 2 was unique to the Oh1VI population centered on marker S2 _ 163825081 with a LOD score of 10 explaining 18% of phenotypic variance (Jones et al., 2018). Other studies have also mapped the region on chromosome 3 and 6 as potential candidate for marker assisted MLN resistance breeding (Gowda et al., 2018). Other recent studies have also suggested the need to focus improve resistance to both viruses causing MLN than focusing on the disease itself (Karanja et al., 2018).

Generally, results indicate the role of  potyvirus  resistance  in  MLN  control.  Although  none  of  the  genotype  were immune  to  MLN  there  is  differences  in  response  of  genotypes  and  QTL  to  MLN infection. Genotypes with all three potyvirus resistance QTL on chromosome 3, 6 and 10 had more resistance to MLN than genotypes with one of the above QTL. This  lead  to  a  conclusion  that,  there  is  a  role played by potyvirus  resistance  in  MLN  control  especially  in  reducing  MLN  effects.  More studies are needed to know the exact role played by potyvirus resistance and how much MLN effects are reduced with the presence of potivirus resistance QTL. This will provide the basis for introgressing potyvirus resistance in East African maize germplasm and pave way for a holistic approach of controlling MLN in Sub Saharan Africa.

In carrying out future studies, especially in field conditions in East Africa, materials used should   be   adapted to tropical environment. The RIL populations used  for  the  study  were derived  from  Oh1VI  and Oh28  which originate from  temperate  environment  hence  did  not  perform  well  in  tropical environment. Agronomically, genotypes performed poorly compared to tropical controls in parameter measured such as plant height, ear height, yield and days to anthesis and silking. The gap between anthesis and silking was also big indicating materials were under physiological  stress which  could  hinder reproduction and the plants were  attacked  by  a lot  of  endemic  disease  such  as  maize  streak virus and a variety of insects and vectors. This could affect the results and quality of the study especially when disease scoring is done until plants have reached maturity.

 

Acknowledgement

 

We deeply appreciate the USAID feed the future program under iAGRI-Tanzania for funding our research work in US and Tanzania and the Borlaug LEAP fellowship for funding the research work in Kenya. Our sincere thanks also go to the USDA, ARS Corn, Soybean and Wheat Quality Research Unit (CSWQRU) at Selby hall and Dr. Francis’ lab at Williams’s hall, OARDC, Wooster for supporting lab and green house activities. Many thanks also go to ARI-SELIAN in Arusha, Tanzania for their supporting field trials in MLN hotspots at Babati and to CIMMYT Kenya for their support in carrying out field inoculation experiments at the MLN screening facility in Naivasha, Kenya.

 

Funding

 

This work was funded by the Innovative Agriculture Research Initiative [iAGRI] project, 2014 - 2016 and Norman Borlaug Leadership Enhancement in Agriculture Program [Borlaug LEAP] project, 2015 -2016.

 

 

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Cite this Article: Bulegeya VB; Jones MW; Muhamba TG; Das B; Thomison PR;  Francis DM; Redinbaugh MG (2021). The effect of Potyvirus resistance loci from the maize inbred line Oh1VI on development of maize lethal necrosis (MLN). Greener Journal of Agricultural Sciences 11(2): 98-107.