Zebra Chip Disease and Potato Biochemistry: Tuber Physiological Changes in Response to ‘Candidatus Liberibacter solanacearum’ Infection Over Time

Abstract

Zebra chip disease, putatively caused by the bacterium 'Candidatus Liberibacter solanacearum', is of increasing concern to potato production in Mexico, the United States, and New Zealand. However, little is known about the etiology of this disease and changes that occur within host tubers that result in its symptoms. Previous studies found that increased levels of phenolics, amino acids, defense proteins, and carbohydrates in 'Ca. L. solanacearum'-infected tubers are associated with symptoms of zebra chip. This study was conducted to quantify variations in levels of these biochemical components in relation to the time of infestation, symptom severity, and 'Ca. L. solanacearum' titer. Levels of phenolics, peroxidases, polyphenol oxidases, and reducing sugars (glucose and, to some extent, fructose) changed during infection, with higher levels occurring in tubers infected at least 5 weeks before harvest than in those infected only a week before harvest and those of controls. Compared with the apical tuber ends, greater levels of phenolics, peroxidases, and sucrose occurred at the basal (stolon attachment) end of infected tubers. With the exception of phenolics, concentrations of the evaluated compounds were not associated with 'Ca. L. solanacearum' titer. However, there were significant associations between biochemical responses and symptom severity. The lack of a linear correlation between most plant biochemical responses and 'Ca. L. solanacearum' titer suggests that shifts in metabolic profiles are independent of variations in 'Ca. L. solanacearum' levels.

Zebra chip disease decreases tuber (Solanum tuberosum L.) protein content by attenuating protease inhibitor levels and increasing protease activities

“…The difference in concentrations of amino acids between healthy and ZC tubers were greatest for proline (?301 %), tyrosine (?129 %), histidine (?97 %), tryptophan (?70 %), isoleucine (?59 %) and leucine (?57 %). Similar increases in amino acids have been reported for other cultivars infected by CLso and depending on cultivar, were linked to up-regulation of phenylpropanoid metabolism, leading to increases in chlorogenic acid, flavonoids, total phenolics, and defense-related enzyme activities (Navarre et al 2009;Wallis et al 2012;Rashed et al 2013;Wallis et al 2014).…”

“…ZC-afflicted tubers have reduced dry matter and significantly elevated levels of sucrose, glucose and fructose (Buchman et al 2011;Rashed et al 2013); a consequence of global changes in tuber carbohydrate metabolism that have yet to be fully characterized at metabolic/molecular levels. The respiration rate of ZC tubers is higher than healthy tubers and this correlates with higher activities of enzymes involved in modulating oxidative stress (Kumar et al 2015).…”

“…CLso-induced increases in oxidative stress (Kumar et al 2015), perturbations in osmotic balance caused by sugar buildup, and protein catabolism (this paper) likely stimulate proline accumulation. The higher level of tyrosine in ZC-afflicted tubers undoubtedly contributes to the increased polyphenol oxidase (PPO)-mediated browning reported for freshly cut tissue (Navarre et al 2009;Rashed et al 2013).…”

“…Liberibacter psyllaurous) (Crosslin et al 2010), is transmitted from infected to healthy plants by the psyllid vector, Bactericera cockerelli (Munyaneza et al 2007a, b). Tubers from infected plants exhibit characteristic brown discoloration of vascular tissue and contain high levels of glucose, fructose (reducing sugars) and free amino acids (Wallis et al 2012;Rashed et al 2013). The reaction of free amino acids with reducing sugars during frying (Maillard reaction) produces unacceptable dark-colored chips and fries along with increased acrylamide formation (Ohara-Takada et al 2005).…”

“…The enhanced accumulation of free amino acids and reducing sugars in ZC afflicted tubers (Wallis et al 2012;Rashed et al 2013;Wallis et al 2014) indicates that infection alters protein and carbohydrate turnover and metabolism. Reduced protein synthesis and/or enhanced proteolysis may contribute to the extensive accumulation of free amino acids in ZC afflicted tubers.…”