The submergence tolerance regulator SUB1A differentially coordinates molecular adaptation to submergence in mature and growing leaves of rice (Oryza sativa L.).

A typical adaptive response to submergence regulated by SUB1A, the ethylene-responsive transcription factor gene, is the restricted elongation of the uppermost leaves. However, the molecular and physiological functions of SUB1A have been characterized using entire shoot tissues, most of which are mature leaves that do not elongate under submergence. We aimed to identify leaf-type-specific and overlapping adaptations coordinated in SUB1A-dependent and -independent manners. To this end, we compared the transcriptomic and hormonal responses to submergence between mature and growing leaves using rice genotypes with and without SUB1A. Monosaccharide, branched-chain amino acid, and nucleoside metabolism, associated with ATP synthesis, were commonly activated in both leaf types regardless of genotype. In both leaf types, pathways involved in carbohydrate and nitrogen metabolism were suppressed by SUB1A, with more severe restriction in growing leaves that have a greater energy demand if SUB1A is absent. In growing leaves, accumulation of and responsiveness to growth-regulating hormones were properly modulated by SUB1A, which correlated with restricted elongation. In mature leaves, submergence-induced auxin accumulation was suppressed by SUB1A. This study demonstrates that different sets of hormonal pathways, both of which are modulated by SUB1A, contribute to distinct adaptive responses to submergence in mature and growing rice leaves.

The Central Negative Regulator of Flooding Tolerance, the PROTEOLYSIS 6 Branch of the N-degron Pathway, Adversely Modulates Salinity Tolerance in Arabidopsis.

Seawater intrusion in coastal regions and waterlogging in salinized lands are serious constraints that reduce crop productivity under changing climate scenarios. Under these conditions, plants encounter flooding and salinity concurrently or sequentially. Identification and characterization of genes and pathways associated with both flooding and salinity adaptation are critical steps for the simultaneous improvement of plant tolerance to these stresses. The PROTEOLYSIS 6 (PRT6) branch of the N-degron pathway is a well-characterized process that negatively regulates flooding tolerance in plants. Here, we determined the role of the PRT6/N-degron pathway in salinity tolerance in Arabidopsis. This study demonstrates that the prt6 mutation enhances salinity tolerance at the germination, seedling, and adult plant stages. Maintenance of chlorophyll content and root growth under high salt in the prt6 mutant was linked with the restricted accumulation of sodium ions (Na+) in shoots and roots of the mutant genotype. The prt6 mutation also stimulated mRNA accumulation of key transcription factors in ABA-dependent and independent pathways of osmotic/salinity tolerance, accompanied by the prominent expression of their downstream genes. Furthermore, the prt6 mutant displayed increased sensitivity to ethylene and brassinosteroids, which can suppress Na+ uptake and promote the expression of stress-responsive genes. This study provides genetic evidence that both salinity and flooding tolerance is coordinated through a common regulatory pathway in Arabidopsis.

Physiological Effect of Cutting Height and High Temperature on Regrowth Vigor in Orchardgrass.

Producers of orchardgrass (Dactylis glomerata L.) hay in the Mid-Atlantic US have experienced a reduction in regrowth vigor and a decline in the persistence of their swards. The common management practice for the region is to harvest the first growth of hay by cutting at 2.5-7.5 cm height in May or June. We hypothesize that high temperature and low cutting height interact to limit the regrowth rate. To test this, orchardgrass plants were cut to either 2.5 or 7.5 cm and then placed into environmentally controlled chambers with a constant temperature of 20 or 35°C. Stubble was harvested on days 0, 1, 3, and 11 following cutting and subjected to metabolite analysis. Photosynthetic parameters were measured in the regrown leaves on days 3 and 11, and regrowth biomass was recorded on day 11. Under optimal growth temperature (20°C), vegetative regrowth upon defoliation was significantly enhanced when more stubble tissue remained. However, this advantage was not observed under heat stress. Defoliation generally decreases the abundance of carbohydrate reserves in stubble. Interestingly, high temperature stimulated the accumulation of starch and ethanol-soluble carbohydrates in plants cut to 7.5 cm. The similar trends were also observed in protein, amino acids, nitrate, and ammonium. These responses were not pronounced in plants cut to 2.5 cm, presumably due to inhibited photosynthesis and photosystem II photochemistry. Overall, we anticipated that heat-activated metabolite accumulation is part of adaptive response to the stress. However, modified allocation of carbohydrate and nitrogen reserves leads to reduced vegetative regrowth upon defoliation. These data suggest that cutting height management for orchardgrass may be more effective for its regrowth vigor and productivity in cool seasons or when cool weather follows hay harvest.