Abstract
Little is known about how salt impacts primary metabolic pathways of C4 plants, particularly related to kernel development and seed set. Osmotic stress was applied to maize (Zea mays) B73 by irrigation with increasing concentrations of NaCl from the initiation of floral organs until 3 d after pollination. At silking, photosynthesis was reduced to only 2% of control plants. Salt treatment was found to reduce spikelet growth, silk growth, and kernel set. Osmotic stress resulted in higher concentrations of sucrose (Suc) and hexose sugars in leaf, cob, and kernels at silking, pollination, and 3 d after pollination. Citric acid cycle intermediates were lower in salt-treated tissues, indicating that these sugars were unavailable for use in respiration. The sugar-signaling metabolite trehalose-6-phosphate was elevated in leaf, cob, and kernels at silking as a consequence of salt treatment but decreased thereafter even as Suc levels continued to rise. Interestingly, the transcripts of trehalose pathway genes were most affected by salt treatment in leaf tissue. On the other hand, transcripts of the SUCROSE NONFERMENTING-RELATED KINASE1 (SnRK1) marker genes were most affected in reproductive tissue. Overall, both source and sink strength are reduced by salt, and the data indicate that trehalose-6-phosphate and SnRK1 may have different roles in source and sink tissues. Kernel abortion resulting from osmotic stress is not from a lack of carbohydrate reserves but from the inability to utilize these energy reserves.
Highlights
Little is known about how salt impacts primary metabolic pathways of C4 plants, related to kernel development and seed set
Greenhouse-grown B73 maize plants were subjected to a gradual increase of salt concentration from 12.5 to 75 mM starting at stage V7 until 3 d after pollination (DAP)
We showed that salt-induced osmotic stress impacts photosynthesis, cellular respiration, and growth despite the presence of sufficient sugars in the source and sink tissues
Summary
Little is known about how salt impacts primary metabolic pathways of C4 plants, related to kernel development and seed set. Osmotic stress resulting from drought or salinized soils can be disastrous for crops such as maize, primarily because of its impact on early kernel development (Westgate and Boyer, 1985). Abortion caused by osmotic stress correlates with depleted Suc and reduced sugar levels, reduced Suc-degrading enzyme activity and transcript levels, and depletion of starch in the kernels These events occur in a short period of time around pollination and can be partially prevented by stem Suc feeding (Zinselmeier et al, 1995, 1999; Andersen et al, 2002; McLaughlin and Boyer, 2004; Boyer, 2010). As a result of impaired photosynthesis and sink strength, sugar allocation to the reproductive organs is disrupted and the young embryo rapidly starves and aborts
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