334 G. McCOLLUM AND E. BALDWIN
2012). Water dikinase (WD) is the first enzyme of starch hydrolysis in
dicot leaves (Smith et al. 2005) and in addition to an increase in ADPG, a
decrease in WD could contribute to the CLas-induced increase in starch.
Fan et al. (2010) reported that the abundance of transcripts for dispro-
portioning enzyme 2 (DPE2) and maltose export protein 1 (MEX1) are
lower in CLas-infected leaves than in non-infected leaves. The combina-
tion of an increase in ADPG transcripts and a decrease in WD transcripts
suggests that both an increase in starch biosynthesis and a decrease in
starch degradation could contribute to the CLas-induced accumulation
of starch.
D. Hormonal Balance
HLB may also affect hormonal balances. Rawat et al. (2015) found
that most potential candidate metabolic network hub sets distinguish-
ing CLas-infected from non-infected citrus were co-expressed with gib-
berellin pathway probe sets. Gibberellins are required for plant growth.
Paparelli et al. (2013) showed that carbohydrate starvation result-
ing from inefficient starch metabolism at night strongly reduces the
expression ofent-kaurene synthase, a key regulatory enzyme for gib-
berellin synthesis, the following day.Ent-kaurene oxidase catalyzes
the three steps of gibberellin biosynthesis froment-kaurene toent-
kaurenoic acid. Two ent-kaurene oxidases were significantly sup-
pressed in response to CLas infection in mandarins (Xu et al. 2015),
which may indicate a decrease in the synthesis of gibberellic acid that
contributes to the stunting seen in CLas-infected plants.
Incomplete mobilization of starch in sink-limited conditions is at
least partly due to feedback inhibition of starch breakdown by the
sucrose signal trehalose 6-phosphate (Tre6P) (Martins et al. 2013; Lunn
et al. 2014). In a review of all published reports on transcriptomic anal-
yses of CLas infection, we could find no mention of any genes related
to Tre6P synthesis.
Although no direct measurements comparing photosynthetic rates
of CLas-infected with non-infected plants have been reported, there
is considerable evidence suggesting that photosynthesis is reduced in
leaves expressing HLB symptoms. Sagaram and Burns (2009) measured
leaf chlorophyll fluorescence parameters in mildly, moderately, and
severely HLB-symptomatic leaves of 20 citrus genotypes or relatives.
They found that mildly symptomatic genotypes were characterized by
increased photosystem II excitation pressure, unregulated heat dissipa-
tion, and decreased regulated heat dissipation, whereas genotypes with