# Laminar _Ulva_ Species: A Multi-Tool for Humankind?
## 2 The Biology of _Ulva_ sp.
### Phyto hormones  
Hormones are very small signal molecules that could control several physiological processes, such as helping steer the growth, cell elongation, adventitious bud formation, and development of the plant. Hormones are usually produced in one part of the plant and transferred to another part of the plant to initiate physiological changes. The most important hormones are auxins, cytokins, and gibberellins [19]. Auxins are small organic compounds constituting an aromatic ring and a side chain with a carboxylic acid group. They are also involved in multiple developmental processes in planta, such as key regulators of cell division and elongation, tissue differentiation, tropisms, apical dominance, senescence, abscission, and flowering. This is reported in bacteria, animal tissues, fungi, algae, and higher plants. Because of its versatile influence, auxin has been long recognized as \"the plant growth hormone\" [19]. Cytokins have a variety of activities in algae, including cell division, differentiation, and morphogenesis; rhizoid and thallus growth; chloroplast formation; photosynthesis control; metabolite generation; and stress tolerance. Authors like [PERSON] et al. [20], [PERSON] and [PERSON] [21], and [PERSON] et al. [22] reported that high levels of auxin, however, may inhibit cytokin biosynthesis via negative feedback, especially in mature algal cells or tissues, in higher plants. Gibberellins are involved in cell expansion, activation of cell division, increased metabolism, and the accumulation of pigments and lipids. Abscisic acid (ABA) is often referred to as a stress hormone or negative regulator whose concentrations increase under various biotic and abiotic stresses, controlling embryogenesis, seed dormancy, and senescence and stimulating physiological responses for stress adaptation such as stomatal closure. In algae, this can occur as two types: endogenous as a response to unfavorable environments such as salinity, pH, oxidative, temperature, and nitrogen-deficient stresses and exogenous downregulating auxin signaling and repressing growth by stimulating the transition from a vegetative phase to a cyst or reproductive stage, as observed by [PERSON] et al. [23] and by [PERSON] and [PERSON] [24] with _Haematococcus lacustris_ (formerly _H. pluvialis_) (Chlorophyta) and _Saccharina japonica_ (Phaeophyceae). It has already been proven in literature such as [PERSON] [25], [PERSON] [26], [PERSON] [27], and [PERSON] [28] that a callus may be formed from a single differentiated cell, and many callus cells are totipotent, meaning they can regenerate the entire plant. In other cases, callus cells contribute to somatic embryogenesis, a procedure in which adult somatic cells are used to create embryos. This is frequently due to a high ratio of auxin to cytokin or cytokin to auxin, which drive root and shoot regeneration, respectively, whereas a transitory auxin to cytokinin ratio favors callus formation. Other hormones, including abscisicic acid and brassinosteroids, can contribute to callus development and, in certain situations, replace auxin or cytokinin in the process. Auxin and cytokinin have been the most utilized and researched hormones for callus development, followed by organ regeneration [29]. As reported by [PERSON] [30], [PERSON] et al. [31], and [PERSON] [32], those auxins and cytokins, such as Indole-3-acetic acid (IAA) and 6-Benzylaminopurine (BAP), can be used in combination to accelerate the callus growing from species of the Gracilariaceae family. Another hormone, Kinetin (KIN), was also observed to cause callus development in _Gracilaria perplexa_ but had a detrimental impact on _Gracilaria tenusitipitata_ (Rhodophyta). [PERSON] and [PERSON] in 1998 [33] found that callus formation of the brown alga _Silvetia siliquosa_ (formerly _Pelvetia siliquosa_) is induced by up to 90% of IAA supplement in solid PES medium; however, at higher doses, the rate of induction decreases.