Plant betalains: Chemistry and biochemistry

Betalains are vacuolar pigments composed of a nitrogenous core structure, betalamic acid [4-(2-oxoethylidene)-
1,2,3,4-tetrahydropyridine-2,6-dicarboxylic acid]. Betalamic acid condenses with imino
compounds (cyclo-L-3,4-dihydroxy-phenylalanine/its glucosyl derivatives), or amino acids/derivatives
to form variety of betacyanins (violet) and betaxanthins (yellow), respectively. About 75 betalains have
been structurally unambiguously identified from plants of about 17 families (known till date) out of 34
families under the order Caryophyllales, wherein they serve as chemosystematic markers. In this review,
all the identified betalain structures are presented with relevant discussion. Also, an estimated annual
production potential of betalains has been computed for the first time. In addition, mutual exclusiveness
of anthocyanins and betalains has been discussed in the wake of new evidences. An inclusive list of betalain-
accumulating plants reported so far has been presented here to highlight pigment occurrence and
accumulation pattern. Betalain synthesis starts with hydroxylation of tyrosine to DOPA, and subsequent
cleavage of aromatic ring of DOPA resulting to betalamic acid formation. This pathway consists of two key
enzymes namely, bifunctional tyrosinase (hydroxylation and oxidation) and DOPA dioxygenase (O2-dependent
aromatic ring cleavage). Various spontaneous cyclisation, condensation and glucosylation steps
complement the extended pathway, which has been presented here comprehensively. The biosynthesis is
affected by various ecophysiological factors including biotic and abiotic elicitors that can be manipulated
to increase pigment production for commercial scale extraction. Betalains are completely safe to consume,
and contribute to health.