Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 221

Information about the response of the antioxidant defence system under condi-
tions of stress in mycorrhizal plants is contradictory: an increase, lack of change,
and even a decrease in SOD, CAT, APX and POD activity were reported in mycor-
rhizal soy (Porcel et al. 2003 ) subjected to hydric stress and tomatoes subjected to
salinity (He et al. 2007 ; Hajiboland et al. 2010 ).
Under conditions of hydric deficit, plants attempt to maintain their water content
by accumulating compatible, non-toxic solutes such as proline and glycine beta-
ine, which do not interfere with the normal physiological processes of the plant
(Ma et al. 2006 ; Zhang et al. 2008 ). The accumulation of these solutes is a sensi-
tive physiological index of plants in response to salt and other stresses (Peng et al.
2008 ). For plants to survive under conditions of salinity and water, adjusting the
leaf osmotic potential is important and requires intracellular osmotic balance. Thus,
under hydric and salinity stresses, plants accumulate some organic solutes (pro-
line, soluble sugars, glycine betaine, among others) and inorganic ions to maintain
greater osmotic adjustment (Yang et al. 2009 ). The presence of AMF in the roots
could modify the osmotic potential of the leaves and influence the carbohydrate
composition and proline level.
Proline is the most common compatible osmolyte in plants and plays an impor-
tant role in increasing the adaptation of plants to drought and salinity (Hasegawa
et al. 2000 ). In addition to osmotic adjustment, this molecule has other proposed
functions in osmotically stressed plant tissues: it maintains and protects the integ-
rity of the plasma membrane (Hincha and Hagemann 2004 ), acts as a source of
carbon and nitrogen, and eliminates hydroxyl radicals. Proline accumulation in my-
corrhizal plants subjected to drought has been reported, and the variable effects of
mycorrhizal colonisation on the levels of proline in plants under saline stress have
been observed. However, to date, there is little information concerning the influence
of colonisation by arbuscular mycorrhizal fungi on this accumulation (Sannazzaro
et al. 2007 ).
Despite the accumulation of proline induced in plants under stress (Andrade et al.
2009 ), evidence of the effects of mycorrhizal symbiosis on the levels of proline or
soluble amino acids are scarce or null under stress conditions. Andrade et al. ( 2010 )
observed that soluble amino acids and the proline content of the leaves of mycor-
rhizal and non-mycorrhizal bean plants increased in response to the addition of Cu
to soil, suggesting a stress response similar to the excess of this metal in the soil.
However, proline accumulation in the leaves of mycorrhizal plants showed a more
pronounced increase in response to Cu in the soil when compared to homologous
non-mycorrhizal plants, indicating a possible role of this amino acid in the response
to Cu toxicity in mycorrhizal plants, which exhibited greater biomass accumulation
than non-mycorrhizal plants.
Moreover, AMF increase the vigorousness of the root system and stimulate the
production of hormones by plants (Yao et al. 2005 ). Thus, the increase in plant tol-
erance to hydric and/or saline stress might be related to the increased expression of
genes in response to stress.
Glycine betaine acts as a protective non-toxic osmolyte during periods of drought
in many organisms, including algae, bacteria, large plants and animals (Treberg and