**1. Introduction**

Potatoes are the fourth most consumed food crop worldwide and are an efficient source of energy, vitamins and minerals in the human diet [1]. High consumption rates and moderate concentrations of dietary antioxidants have led potatoes to be the third-largest source of total phenolics in the American diet [2]. Diets rich in phenolics have been implicated in the prevention of an array of degenerative diseases and concentrations of these compounds vary greatly based on the cultivar, highlighting the potential for the targeted breeding of potato (*S. tuberosum* L.) to enhance global human health [3]. In potato tubers, the primary polyphenol is chlorogenic acid with the remaining components comprised of carotenoids, anthocyanins, and flavonoids. While the phenolic content of tubers is largely genotype-dependent, the phenolic profiles are driven by the environmental conditions present during growth, tuber bulking, and throughout storage. In general, the production and accumulation of these compounds are favored at lower temperatures, with potatoes grown in warm, dry regions producing lower amounts of phenolics [4]. Environmental parameters could be manipulated to manage the concentrations of desired phytonutrients [4–6].

Substrate entry into the general phenylpropanoid pathway in eudicots is driven by phenylalanine ammonia-lyase (PAL), an enzyme that regulates the deamination of phenylalanine to yield the cinnamic acid from which monolignols, flavonoids, and anthocyanins are produced [7]. PAL activity responds to a variety of developmental and environmental cues, with transcriptional regulation occurring by way of MYB, LIM, and KNOX transcription factors [8]. Furthermore, independent MYB transcription factors play a prominent role in the regulation of anthocyanin and flavonoid biosynthesis genes such as flavonol synthase (FLS), flavanone 3-hydroxylase (F3H) and flavonoid 3–hydroxylase (F3H), while expression of genes such as dihydroflavonol reductase (DFR) require MYB transcriptional complexes. MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) have notable functions in the regulation of phenylpropanoid biosynthesis in eudicots through the targeting of MYBs, the most notable being miR858, miR828 and TAS4 [7,9]. Causal miRNA and the target MYB transcription factors have been previously identified in potato leaves under drought [10], however, the regulatory cascades present in potato tubers are still unknown.

In addition to phenolics, concentrations of available essential amino acids a ffect the nutritional value of potato tubers. Up to 50% of the amino acids in tubers are aspartic acid and glutamic acid, with the remaining portion made up of leucine, valine, alanine, lysine, and arginine with the total protein nutritional value of a potato being comparable to an egg white [11]. Genotypes for improving protein quality have been identified among non-traditional potato cultivars [12]. Essential amino acids function both as substrates for secondary metabolism and as a source of energy [13]. As a result, concentrations fluctuate in response to environmental stressors due to concurrent protein degradation and *de novo* synthesis. The transcriptional regulation of amino acid biosynthesis is highly complex and their function during stress response is still unclear [13].

Drought stress is one of the primary concerns for potato production given the projected increases in aridity. Potato is adapted to temperate climates with optimal tuber growth occurring at temperatures between 15 and 20 ◦C. Temperatures above this range, coupled with periodic drought, have resulted in reduced yields and increased incidences of tuber physiological defects [14]. Symptoms of drought in potato include reduced leaf size, increased chlorophyll content, reduced stomatal conductance, and wilting. However, rooting depth and plant recovery have been shown as the best indicators of plant susceptibility to drought [15,16]. Through comparisons between genotypes with di ffering tolerance to drought, novel potato drought-responsive genes and transcript markers for drought tolerance in potato leaves have been identified [17,18]. Gene responses in developing potato tubers to drought conditions are not well-documented outside of targeted metabolic pathways [3].

This study aimed at identifying drought-associated changes in developing potato tubers (i.e., tuber bulking phase) and their impacts on nutritional quality. The assessment of transcriptional changes in genes with metabolic functions and quantification of amino acid concentrations aims to guide production and harvest practices when optimizing the nutritional value of the crop. Analysis of small RNAs seeks to identify components of the drought regulatory cascade in potato tubers which, to our knowledge, has ye<sup>t</sup> to be explored.
