Recent preliminary data have suggested that microbe-to-plant signaIs, and plant
internaI signaIs elicited by microbial signaIs, affect aspects of plant physiology,
development and growth. The reported research investigated the responses of plants to
signal compounds of microbial and plant origin, such as lipo-chitooligosaccharides
(LCOs - signal molecules in rhizobia-Iegume associations), chitin and chitosan (present
in fungal cell walls), and phenolic compounds (salicylic acid, acetylsalicylic acid and
gentisic acid - internaI signaIs in plants, often affected by signaIs from microbes).
Phenylalanine ammonia-Iyase (PAL) and tyrosine ammonia-Iyase (TAL) are key
enzymes of the phenylpropanoid pathway. Oligomers of chitin and chitosan increased the
activities ofboth PAL and TAL in soybean leaves. The degree ofincrease was
dependent on oligomer chain length and time after treatment. LCO [Nod Bj V (C lS:"
MeFuc)] was isolated fromBradyrhizobiumjaponicum strain 532C. WhenArabidopsis
thaliana plants were grown for two weeks on agar containing this LCO (IO-sM) or chitin
pentamer (10-4 M), they had greater root length, root diameter, root surface area and
number ofroot tips than control plants. Chitosan (tetramer and pentamer) did not have
this effect. Chitin and chitosan were also tested for effects on corn and soybean
photosynthetic rates and growth. High molecular weight chitosan generally reduced
photosynthetic rates, but did not reduce the growth of corn or soybean. However, foliar
application of 10-6 M LCO to corn leaves increased photosynthetic rates (up to 36%).
Foliar application oflumichrome (l0-5 and 10-6 M), a breakdown product of riboflavin
produced by sorne rhizosphere bacteria, to corn (C4 plant) and soybean (C3 plant)
increased photosynthetic rates (up to 6%). Foliar application oflumichrome (10-5 M)
increased soybean leaf area and shoot dry weight. Foliar application of SA, acetyl
salicylic acid (ASA) and gentisic acid (GTA) aH enhanced photosynthesis in both
soybean and corn, however this resulted in increased leaf areas and shoot dry weights in
soybean and in corn only for GT A (10-3 M). Overall, the work reported here indicates
that a set of signaIs related to microbe-plant interactions can modifY plant physiology and
development leading to increased productivity.