Integrated stress responses in okra plants (cv. ''Meya']: unravelling the mechanisms underlying drought and nematode co-occurrence

Abstract

Background: Climate change threatens sub-Saharan Africa's agricultural production, causing abiotic and biotic stressors. The study of plant responses to joint stressors is crucial for understanding molecular processes and identifying resilient crops for global food security. This study aimed to explore the shared and tailored responses of okra plants (cv. ''Meya'), at the biochemical and molecular levels, subjected to combined stresses of drought and Meloidogyne incognita infection.

Design: The study involved 240 okra plants in a completely randomized design, with six treatments replicated 20 times. Okra plants were adequately irrigated at the end of every 10-days water deficit that lasted for 66 days (D). Also, the plants were infected with M. incognita for 66 days and irrigated at 2-days intervals (R). The stresses were done independently, in sequential combination (D before R and R before D) and concurrently (R and D). All biochemical and antioxidant enzyme assays were carried out following standard procedures.

Results: Significant reductions in leaf relative water content were recorded in all stressed plants, especially in leaves of plants under individual drought stress (D) (41.6%) and plants stressed with root-knot nematode infection before drought stress (RBD) (41.4%). Malondialdehyde contents in leaf tissues from plants in D, nematode-only stress (RKN), drought stress before root-knot nematode infection (DBR), RBD, and concurrent drought-nematode stress (RAD) significantly increased by 320.2%, 152.9%, 186.5%, 283.7%, and 109.6%, respectively. Plants in D exhibited the highest superoxide dismutase activities in leaf (147.1% increase) and root (105.8% increase) tissues. Catalase (CAT) activities were significantly increased only in leaves of plants in D (90.8%) and RBD (88.9%), while only roots of plants in D exhibited a substantially higher CAT activity (139.3% increase) in comparison to controlled plants. Okra plants over-expressed NCED3 and under-expressed Me3 genes in leaf tissues. The NCED3 gene was overexpressed in roots from all treatments, while CYP707A3 was under-expressed only in roots of plants in RBD and RKN. CYP707A3 and NCED3 were grouped as closely related genes, while members of the Me3 genes were clustered into a separate group.

Conclusion: The biochemical and molecular responses observed in okra plants (cv. ''Meya') subjected to combined stresses of drought and Meloidogyne incognita infection provide valuable insights into enhancing crop resilience under multifaceted stress conditions, particularly relevant for agricultural practices in sub-Saharan Africa facing increasing climatic challenges.

Keywords: Meloidogyne incognita; Biotic and abiotic stress; Drought; Gene regulation; Okra.

Fig. 1

Different stages of M. incognita development at ×40 magnification. A: Initial phase of M. incognita egg development, B: Advanced phase of M. incognita egg development, C: first stage juvenile inside egg, D: freshly hatched second stage juvenile

Fig. 2

Effect of individual, sequentially and concurrently occurring stresses of drought and nematode infection on the antioxidants level in okra plant leaves [A: MDA content (F- values: 22.542-leaf; 66.088-root; p- values: 0.000), B: SOD enzyme activity (F- values: 11.127-leaf; 7.185-root; p- values: 0.000), C: CAT enzyme activity (F- values: 83.875-leaf; 23.570-root; p- values: 0.000), D: APx enzyme activity in leaf and root tissues (F- values: 32.861-leaf; 22.230-root; p- values: 0.000)]

Fig. 3

Effect of individual, sequentially, and concurrently occurring stresses of drought and nematode infection on the expression of genes in okra plants (A: leaf and B: root. Error bars represent the standard deviation of the means)

Fig. 4

Fold changes in CYP707A3, NCED3, SCAR_N, SCAR_PM6A, and SCAR_PM6B in A: leaf and B: root tissues of okra plants under individual drought stress conditions

Fig. 5

Fold changes in CYP707A3, NCED3, SCAR_N, SCAR_PM6A, and SCAR_PM6B in A: leaf and B: root tissues of okra plants under sequentially occurring combined stresses of drought and nematode infection (DBR)

Fig. 6

Fold changes in CYP707A3, NCED3, SCAR_N, SCAR_PM6A, and SCAR_PM6B in A: leaf and B: root tissues of okra plants under concurrent drought and nematode infection

Fig. 7

Fold changes in CYP707A3, NCED3, SCAR_N, SCAR_PM6A, and SCAR_PM6B in A: leaf and B: root tissues of okra plants under sequentially occurring combined stresses of nematode infection and drought stress

Fig. 8

Fold changes in CYP707A3, NCED3, SCAR_N, SCAR_PM6A, and SCAR_PM6B in A: leaf and B: root tissues of okra plants challenged with nematode-only infection (RKN)

Fig. 9

Hierarchical clustering heatmap representation of changes in relative gene expression levels measured in the leaf and root tissues of okra plants under individual and combined stresses of drought and Meloidogyne infection (Clustering was applied to columns and rows using the Euclidean correlation coefficient. Red or light green indicates either an increased or decreased level of gene expression compared to that of the control, respectively. All values are presented as the means of three replicates and are shown in false-colour code)