Soil Fertility as a Contingent Rather than Inherent Characteristic - Principles of Sustainable Soil Management in Agroecosystems - page 159

• Some major proteins involved in both light and dark reactions of photosyn-

thesis, which suggests that there would be an overall increase of photosyn-
thetic efficiency.

• Some chloroplast membrane and envelope proteins, which increase nutrient

transportation and protein folding, protein translocation through biological
membranes, and signal transduction.

Some changes were also detected in the levels of proteins involved in plant hor-
mone metabolism. One auxin-induced protein was detected in the up-regulated set
of proteins, which is consistent with increased auxin content in inoculated plants
(Chi et al. 2005). At the same time, IAA-amino acid hydrolase, which hydrolyzes
IAA-l-amino acid conjugates into IAA, was detected as being down-regulated in
roots. This suggests that the increase in plant-synthesized IAA biosynthesis that is
induced by rhizobia might be through a tryptophan-independent route, even though
tryptophan and rice root exudates significantly enhance IAA synthesis in endophytic
rhizobia cultured ex planta (Yanni et al. 2001).
Defensive reactions in roots and shoots : Most of the up-regulated defense-
related proteins were found to be changed in root tissues, which are the primary
loci for rhizobial colonization and infection. These defense-related proteins could be
grouped into two categories based on their functional mechanisms. The first set of
up-regulated protein expression relating to bacterial killing includes the following:

• Exoglucanase, which affects important pathogenesis-related (PR) proteins;

these can antagonize external pathogens by lysing their cell walls.

• Subtilisin-like proteinase, which is a calcium-activated endopeptidase that

acts in the posttranslational modification of proteins that participate in the
defense response.

• Aminopeptidase N, which is an enzyme releasing the amino-terminal

amino acid residues from proteins and peptides, thereby inactivating their
function.

• A protein with sequence similarity to 30-N-debenzoyltaxol N-benzoyl-

transferase, which is involved in the synthesis of a phytoalexin that is an
important plant antipathogenic antibiotic.

• Peroxidase, which can detoxify reactive oxygen species (ROS), a common

strategy used by hosts to kill infective bacteria, and a cross-linking enzyme
that strengthens plant wall polymers that have been broken during bacterial
ingress (Salzwedel and Dazzo 1993).

The second group of up-regulated protein expression that relates to plant cell wall
synthesis to inhibit bacterial colonization includes

• Arabinoxylan arabinofuranohydrolase isoenzyme (AXAH-II), which par-

ticipates in the accumulation of arabinoxylan that thickens the secondary
plant host cell wall.

• Germin protein 4, which could improve plant resistance to pathogens by

strengthening the host cell wall structure.