The role of alanine synthesis and nitrate‐induced nitric oxide production during hypoxia stress in Cucurbita pepo nectaries

Abstract

SUMMARY Floral nectar is a sugary solution produced by nectaries to attract and reward pollinators. Nectar metabolites, such as sugars, are synthesized within the nectary during secretion from both pre‐stored and direct phloem‐derived precursors. In addition to sugars, nectars contain nitrogenous compounds such as amino acids; however, little is known about the role(s) of nitrogen (N) compounds in nectary function. In this study, we investigated N metabolism in Cucurbita pepo (squash) floral nectaries in order to understand how various N‐containing compounds are produced and determine the role of N metabolism in nectar secretion. The expression and activity of key enzymes involved in primary N assimilation, including nitrate reductase (NR) and alanine aminotransferase (AlaAT), were induced during secretion in C. pepo nectaries. Alanine (Ala) accumulated to about 35% of total amino acids in nectaries and nectar during peak secretion; however, alteration of vascular nitrate supply had no impact on Ala accumulation during secretion, suggesting that nectar(y) amino acids are produced by precursors other than nitrate. In addition, nitric oxide (NO) is produced from nitrate and nitrite, at least partially by NR, in nectaries and nectar. Hypoxia‐related processes are induced in nectaries during secretion, including lactic acid and ethanolic fermentation. Finally, treatments that alter nitrate supply affect levels of hypoxic metabolites, nectar volume and nectar sugar composition. The induction of N metabolism in C. pepo nectaries thus plays an important role in the synthesis and secretion of nectar sugar.

Nitric Oxide Turnover Under Hypoxia Results in the Rapid Increased Expression of the Plastid-Localized Phosphorylated Pathway of Serine Biosynthesis

“…In plants and green algae, amino acid metabolism can be supportive of energy metabolism when oxygen is limiting or absent (Reggiani et al, 1988;Rocha et al, 2010;Narsai et al, 2011;Mustroph et al, 2014;Shingaki-Wells et al, 2014;António et al, 2016;Nakamura and Noguchi, 2020;Solhaug et al, 2021). Such amino acid metabolism may reduce lactate and ethanol accumulation, in this way lessening cytosolic acidification (in the case of lactate) and tissue carbon loss (in the case of ethanol).…”

Comparative analyses of the metabolite and ion concentrations in nectar, nectaries, and leaves of 36 bromeliads with different photosynthesis and pollinator types

“…However, in floral nectaries of cotton, several genes associated with amino acid biosynthesis were expressed, which may indicate that nectaries of cotton have the potential capacity for the de novo biosynthesis of different amino acids (Chatt et al, 2021). In the nectaries of male flowers of Cucurbita pepo, also an increased expression and activity of several enzymes involved in the biosynthesis of amino acids at the beginning of nectar secretion was detected (Solhaug et al, 2021). However, the floral nectar of cotton is exceptional with respect to the amino acid composition, as one amino acid-aspartate-is dominant with a proportion of about 90% (Chatt et al, 2021).…”

“…Most analyses of the sugar production and secretion were done on Arabidopsis, Brassica or Nicotiana ssp., whereas the knowledge about other plant species, such as monocots or species with CAM photosynthesis, remains incomplete. Even less is known about the origin, production and secretion of non-carbohydrate compounds in nectar, such as amino acids or inorganic ions, or about the corresponding concentrations of such compounds in nectaries (Solhaug et al, 2021). Moreover, studies focused on the nectaries only, which means that knowledge about the influence of the pollinator type or the photosynthesis type on this nectar producing tissue is scarce (Tiedge and Lohaus, 2018).…”

On the horizon for nectar‐related research

“…A number of research avenues that are on the horizon will provide a framework for identifying the cellular and physiological dynamics of nectar production. A key direction will be visualizing sugar and metabolite processing and transport during active nectar production and secretion using techniques such as microscopic imaging coupled with cell permeable dyes and molecular probes (e.g., Solhaug et al, 2021) (Figure 1L). Along similar lines, experiments utilizing carbon/nitrogen isotopes will shed light on how amino acids and sugars are trafficked into nectar, whether they are synthesized in nectaries de novo, derived from vasculature sources, or a mixture of both (e.g., Solhaug et al, 2021).…”

“…A key direction will be visualizing sugar and metabolite processing and transport during active nectar production and secretion using techniques such as microscopic imaging coupled with cell permeable dyes and molecular probes (e.g., Solhaug et al, 2021) (Figure 1L). Along similar lines, experiments utilizing carbon/nitrogen isotopes will shed light on how amino acids and sugars are trafficked into nectar, whether they are synthesized in nectaries de novo, derived from vasculature sources, or a mixture of both (e.g., Solhaug et al, 2021). Plant hormones such as auxin, gibberellins, and jasmonic acid play critical roles in nectar production (reviewed by Roy et al, 2017).…”