332 G. McCOLLUM AND E. BALDWIN
There is consensus that starch accumulation follows the appearance
of callose deposition around sieve pores (Achor et al. 2010; Fan et al.
2010). Sucrose and glucose were elevated in midveins of CLas-infected,
asymptomatic and symptomatic leaves, compared with midveins from
non-infected plants, whereas fructose concentration was higher in mid-
veins from infected but asymptomatic leaves than from non-infected or
infected symptomatic leaves, which were not different (Fan et al. 2012).
These results lend further support to the phloem blockage hypothesis.
However, although attributing the accumulation of starch exclusively to
the inhibition of phloem transport is an attractive hypothesis, there is
little solid data to substantiate that this inhibition is of sufficient mag-
nitude to be the cause of increased starch accumulation.
Recent reports of an involvement of sieve element occlusion related
(SEOR) proteins, a class of P-proteins involved in the sealing of injured
sieve tubes are inconclusive; various lies of evidence suggest that, in
neither intact nor injured plants there are SEORs that are determinative
of translocation stoppage (Knoblauch et al. 2014). The downward flow
of radiotracer stopped immediately after the partial girdling of potted
oak trees; however, it recovered after only 1 d. This fast recovery indi-
cates that the downward flow of assimilates probably found its way to
the roots by changing its normal position toward undamaged phloem
tissue (De Schepper et al. 2013). The phloem collapse seen in CLas-
infected trees is not equivalent in all parts of the tree suggesting that
translocation of sucrose from shoots to subtending sink tissues may
be rerouted around areas where phloem translocation is disrupted. In
addition, regrowth of damaged phloem is known to occur and may be
contributing to transport from source leaves to sinks (Brodersen et al.
2014). Other girdling studies with citrus rootstock seedlings demon-
strated that the symptoms of increased leaf starch, decreased root starch,
and blotchy mottle leaf symptoms were duplicated by girdling in the
absence of HLB, and thus, may be symptoms related to starch accumu-
lation and nutrient deficiencies in leaves (Cimo et al. 2013).`
Some evidence suggests that mechanisms other than phloem dys-
function may be involved in the CLas-induced accumulation of starch.
Bark and symptomless leaves in CLas-infected trees contain higher lev-
els of starch than do their counterparts in non-infected trees, even
prior to visible phloem blockage (Etxeberria et al. 2009). It is possible
that CLas-infectionper secan cause an increase in starch even with-
out phloem plugging. Ezquer et al. (2010) found that volatiles emitted
from a variety of microbes, including Gram-negative and Gram-positive
bacteria and fungi stimulated excessive starch accumulation in both
monocots and dicots. This response occurred in the absence of direct