Abstract
The effects of 2 mM silicon (Si) and 10 mM KNO3 (N)—prime signals for plant resistance to pathogens—were analyzed in healthy and Cowpea chlorotic mottle virus (CCMV) or Cowpea mild mottle virus (CMMV)-infected Bradyrhizobium-nodulated cowpea, yardlong bean and mung bean plants. In healthy plants of the three Vigna taxa, nodulation and growth were promoted in the order of Si + N > N > Si > controls. In the case of healthy cowpea and yardlong bean, the addition of Si and N decreased ureide and α-amino acids (AA) contents in the nodules and leaves in the order of Si + N> N > Si > controls. On the other hand, the addition of N arrested the deleterious effects of CCMV or CMMV infections on growth and nodulation in the three Vigna taxa. However, the addition of Si or Si + N hindered growth and nodulation in the CCMV- or CMMV-infected cowpea and yardlong bean, causing a massive accumulation of ureides in the leaves and nodules. Nevertheless, the AA content in leaves and nodules of CCMV- or CMMV-infected cowpea and yardlong bean was promoted by Si but reduced to minimum by Si + N. These results contrasted to the counteracting effects of Si or Si + N in the CCMV- and CMMV-infected mung bean via enhanced growth, nodulation and levels of ureide and AA in the leaves and nodules. Together, these observations suggest the fertilization with Si + N exclusively in virus-free cowpea and yardlong bean crops. However, Si + N fertilization must be encouraged in virus-endangered mung bean crops to enhance growth, nodulation and N-metabolism. It is noteworthy to see the enhanced nodulation of the three Vigna taxa in the presence of 10 mM KNO3.
Highlights
Cowpea (Vigna unguiculata (L.) Walp.), yardlong bean (Vigna unguiculata subsp. sesquipedalis (L.)Verdc.) and mung bean (Vigna radiata (L.) Wilczek) are important grain and fodder legume crops grown across tropical and sub-tropical agro-ecological zones worldwide [1]
In cowpea and yardlong bean, the systemic symptoms elicited by chlorotic mottle virus (CCMV) included leaf mottling, yellow mosaic, bright vein clearing and deformations in all trifoliolated leaves (Supplemental Figure S1(1))
The infections by CCMV or Cowpea mild mottle virus (CMMV) caused the appearance of a mild mosaic (Supplementary Figure S1(1))
Summary
Verdc.) and mung bean (Vigna radiata (L.) Wilczek) are important grain and fodder legume crops grown across tropical and sub-tropical agro-ecological zones worldwide [1]. Cowpea is preferred by the farmers due to the high nutritional quality of the leaves and seeds [2], as well as the plant tolerance to drought [3] and salinity [4]. Plants 2017, 6, 40 iron-protein-rich seeds, antioxidant capacity and tolerance to mild drought [6,7]. These three grain legumes have in common the ability to fix atmospheric N2 via an efficient symbiosis with compatible rhizobial strains belonging mainly to the genus Bradyrhizobium [8]. Ureides (allantoin and allantoate) are the final products of the symbiotic N2 fixation to be exported to the leaves for their catabolism [12]
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